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Vermiculite
Vermiculite
Vermiculite
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Vermiculite
General
CategoryPhyllosilicates
Formula(Mg,Fe2+,Fe3+)3[(Al,Si)4O10](OH)2·4H2O
IMA symbolVrm[1]
Strunz classification9.EC.50
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/m
Unit cella = 5.24 Å, b = 9.17 Å
c = 28.6 Å; β = 94.6°; Z = 2
Identification
ColorColorless, white, yellow, green, brown, black
Crystal habitAs large crystalline plates to clay-sized particles; lamellar to scaly
CleavagePerfect on {001}
TenacityPliable
Mohs scale hardness1.5–2
LusterGreasy or vitreous (pearly at cleavage planes)
StreakWhite or yellowish, translucent, shiny, light-brown or greenish in color, in some cases. For example, palabora vermuculite.
DiaphaneityTranslucent
Specific gravity2.4–2.7 (0.065–0.130 when exfoliated)
Optical propertiesBiaxial (−)
Refractive indexnα = 1.525 – 1.561 nβ = 1.545 – 1.581 nγ = 1.545 – 1.581
Birefringenceδ = 0.020
PleochroismX in paler shades than Y and Z
References[2][3][4]

Vermiculite is a hydrous phyllosilicate mineral which undergoes significant expansion when heated. Exfoliation occurs when the mineral is heated sufficiently; commercial furnaces can routinely produce this effect. Vermiculite forms by the weathering or hydrothermal alteration of biotite or phlogopite.[3] Large commercial vermiculite mines exist in the United States, Russia, South Africa, China, and Brazil.

Occurrence

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Vermiculite was first described in 1824 for an occurrence in Millbury, Massachusetts. Its name is from the Latin vermiculare, "to breed worms", for the manner in which it exfoliates when heated.[5][3]

It typically occurs as an alteration product at the contact between felsic and mafic or ultramafic rocks such as pyroxenites and dunites. It also occurs in carbonatites and metamorphosed magnesium-rich limestone. Associated mineral phases include: corundum, apatite, serpentine, and talc. It occurs interlayered with chlorite, biotite and phlogopite.[2]

Structure

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Vermiculite is a 2:1 clay, meaning it has two tetrahedral sheets for every one octahedral sheet. It is a limited-expansion clay with a medium shrink–swell capacity. Vermiculite has a high cation-exchange capacity (CEC) at 100–150 meq/100 g. Vermiculite clays are weathered micas in which the potassium ions between the molecular sheets are replaced by magnesium and iron ions.

Commercial uses

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Vermiculite from Paakkila, Tuusniemi, Eastern Finland

Molded shapes

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This process involves mixing exfoliated vermiculite with inorganic bonding agents such as sodium silicate, cement (specific quantities), and other compounds, such as those containing potassium,[clarification needed] to produce an 'earth damp' mixture. This material is then hydraulically pressed into shape in a mold and then heat cured at temperatures up to 180 °C for up to 24 hours, depending upon the thickness of the moulded part. Such parts can withstand service temperatures of up to 1150 °C and are often used in the aluminium smelting industry as back-up insulation behind the carbon cathode in the pot cells which contain the molten mixture of cryolite and alumina. The moulded shapes and boards are used in:

Calcium silicate boards

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Exfoliated vermiculite is added to a calcium silicate slurry. This is then dewatered by pressing or by using one of the Fourdriner/Magnani/Hatschek processes to form a flat board which is then heat cured under pressure (typically 10–15 bar) for periods of up to 24 hours.

Brake linings

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Finer grades of exfoliated vermiculite are being used in brake linings primarily for the automotive market. The properties of vermiculite that make it an appropriate choice for use in brake linings include its thermal resistance, ease of addition to other raw materials to achieve a homogeneous mix, and its shape and surface characteristics.[citation needed]

Roof and floor screeds and insulating concretes

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Anthophyllite, vermiculite

Exfoliated vermiculite (typically the finer grades) can be added at site to Portland cement and other aggregates, rheological aids, and water to produce roof and floor concrete screeds (i.e. top layer screed coats) which are lightweight and insulating. In many cases, vermiculite-based roof screeds are used in conjunction with other insulation materials, such as polystyrene board, to form a total roofing system. A bituminous binder can also be used with exfoliated vermiculite to produce a dry, lightweight roof screed which has the advantages of low thermal conductivity, low moisture content, and ease of placement (by pouring from the bag and then tamping).[citation needed]

Soilless growing medium

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Exfoliated vermiculite is combined with other materials such as peat or composted pine bark to produce soilless growing medium for the professional horticulturalist and for the home gardener. These mixes promote faster root growth and give quick anchorage to young roots. The mixture helps retain air, fertilizer, and moisture, releasing them as the plant requires them.[6] These mixes were pioneered by Boodley and Sheldrake. Exfoliated vermiculite is also used as a growing medium for hydroponics.[citation needed]

Seed germination

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Vermiculite, alone or mixed with soil or peat, is used to germinate seeds; very little watering is required. When vermiculite is used alone, seedlings should be fed with a weak fertilizer solution when the first true leaves appear, e.g. with one teaspoon of 5-10-5 soluble fertilizer per US gallon of water (1:768 ratio), gradually increased to one tablespoon (1:256 ratio) when transplanting.[7]

Root crop storage

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Pour vermiculite around bulbs placed in container. If clumps are dug, allow to dry for a few hours in the sun and then place in cartons or bushel baskets and cover with vermiculite. The absorptive power of vermiculite acts as a regulator that prevents mildew and moisture fluctuation during the storage period. It will not absorb moisture from the inside of stored tubers, but it does take up free water from the outside, preventing storage rot.

Soil conditioner

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Where the native soil is heavy or sticky, gentle mixing of vermiculite as a soil conditioner—up to one-half the volume of the soil—is recommended. This creates air channels and allows the soil mix to breathe. Mixing vermiculite in flower and vegetable gardens or in potted plants will provide the necessary air to maintain vigorous plant growth. Where soils are sandy, mixing of vermiculite into the soil will allow the soil to hold the water and air needed for growth.

As loose-fill insulation

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Exfoliated vermiculite treated with a water repellent is used to fill the pores and cavities of masonry construction and hollow blockwork to enhance fire ratings (e.g. Underwriters Laboratories Wall and Partition designs), thermal insulation, and acoustic performance. Expanded vermiculite has also been used as thermal insulation in the attics and walls of houses and in water heaters, fire safes, stoves, furnaces, and refrigerators.[8]

Refractory/insulation gunning and castable mixes

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Exfoliated vermiculite can be combined with high alumina (also known as calcium aluminate) cements and other aggregates such as expanded shale, clay, and slate or sodium silicate to produce refractory/insulation concretes and mortars. In the early days of their use, these products were batched at or very close to the place of installation. This continues to be the case in some limited circumstances; however, more and more use is being made of pre-batched, proprietary mixes. Mixes containing vermiculite are used in areas where strength and corrosion/abrasion resistance are of secondary importance, the most important factor being the insulation performance of the in-place refractory lining. These mixes are used in industries including iron/steel, cement, and hydrocarbon processing.

Fire protection

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Vermiculite board used for ductwork fireproofing

Vermiculite is used as an additive to fireproof wallboard.

High temperature coating

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Vermiculite dispersions are typically either chemically or physically very finely delaminated vermiculite in a fluid medium. These dispersions can be used to make vermiculite 'paper' sheets by pouring them onto a piece of smooth, low surface-energy plastic, and allowing to dry. The resulting sheet can then be peeled off the plastic. Typical end-uses for vermiculite dispersions include inclusion in high temperature coatings or binders for construction materials, gaskets, specialty papers/textiles, oxidation-resistant coating on carbon based composites, and as barrier coatings for films.

Waste treatment

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The cation exchange capacity (up to 1,000 milliequivalents per kg) of vermiculite allows it to be used in fluid purification processes for waste water, chemical processing, and the pollution-control of air in mines and gases in industrial processes. In addition to its ion exchange properties, exfoliated vermiculite can retain liquids within the inter-laminar voids of the individual particles, as well as between the particles themselves.

Others

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  • As a packing material, valued for its high absorbency.
  • As a cooling substrate in blacksmithing.
  • As a substrate for various animals and/or a medium for incubation of eggs.
  • As a lightweight aggregate for plaster, proprietary concrete compounds, firestop mortar, and cementitious spray fireproofing: Exfoliated vermiculite is used in both hand and spray-applied general building plasters to improve coverage, ease of handling, adhesion to a wide variety of substrates, fire resistance, and resistance to chipping/cracking/shrinkage.
  • As a component of the interior fill for firestop pillows, along with graphite.
  • As a carrier for dry handling and slow release of agricultural chemicals.
  • As a hot topping: both exfoliated and crude vermiculite have been used for hot topping in the steel industry. When poured onto molten metal, crude vermiculite exfoliates immediately and forms an insulating layer, allowing the material to be transported to the next production process without losing too much heat.
  • Used to permit slow cooling of hot pieces in glassblowing, lampwork, steelwork, and glass beadmaking.
  • Used in in-ground swimming pools to provide a smooth pool base: Finer grades of exfoliated vermiculite plus Portland cement may be combined either on-site or in a factory premix to provide a base for swimming pool vinyl liners. These mixes are pumped into place using a rotor stator pump, or hand poured.
  • Used in commercial hand warmers.
  • Used in AGA cookers as insulation.
  • Used in explosives storage as a blast mitigant.
  • Used to absorb hazardous liquids for solid disposal.
  • Used in gas fireplaces to simulate embers.
  • Used as part of a substrate for cultivation of fungi.

Commercial manufacture of exfoliated vermiculite

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Vermiculite output in 2005

In 2014, South Africa, Brazil, the US, and China were the top producers of mined, concentrated and unexfoliated vermiculite, with about 90% world share. South Africa's production is decreasing, while Brazil's is significantly increasing.

While some end processors and exfoliators of vermiculite specialize, with proprietary products sold in a wide variety of industries, some have more varied end products, with less stringent technical requirements. Some vermiculite exfoliators blend with lower-cost perlite also. Vermiculite exfoliators have an international trade association called The Vermiculite Association to represent the industry's interests and to exchange information.

Asbestos contamination

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Although not all vermiculite contains asbestos, some products were made with vermiculite that contained asbestos until the early 1990s.[9] Vermiculite mines throughout the world are now regularly tested for it and are supposed to sell products that contain no asbestos. The former vermiculite mine in Libby, Montana, did have tremolite asbestos as well as winchite and richterite (both fibrous amphiboles)—in fact, it was formed underground through essentially the same geologic processes as the contaminants.

Pure vermiculite does not contain asbestos and is non-toxic. Impure vermiculite may contain, apart from asbestos, also minor diopside or remnants of the precursor minerals biotite or phlogopite.

Controversy over health risks

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The largest and oldest vermiculite mine in the United States was started in the 1920s, at Libby, Montana, and the vermiculite was sold under the commercial name Zonolite. The Zonolite brand and the mine were acquired by the W. R. Grace and Company in 1963. Mining operations at the Libby site stopped in 1990 in response to asbestos contamination. While in operation, the Libby mine may have produced 80% of the world's supply of vermiculite.[10]

The United States government estimates that vermiculite was used in more than 35 million homes, but does not recommend its removal. Nevertheless, homes or structures containing vermiculite or vermiculite insulation dating from before the mid-1990s—and especially those known to contain the "Zonolite" brand—may contain asbestos, and therefore may be a health concern.

An article published in The Salt Lake Tribune on December 3, 2006, reported that vermiculite and Zonolite had been found to contain asbestos, which had led to cancers such as those found in asbestos-related cases. The article stated that there had been a cover-up by W. R. Grace and Company and others regarding the health risks associated with vermiculite and that several sites in the Salt Lake Valley had been remediated by the EPA when they were shown to be contaminated with asbestos. W. R. Grace and Company has vigorously denied these charges.[11]

The vermiculite deposit at the mine in Libby, Montana, was (and is) heavily contaminated with asbestos.[12] Numerous people were knowingly[13] exposed to the harmful dust of vermiculite that contained asbestos. The mine had been operating since the 1920s, and environmental and industrial controls were virtually non-existent until the mine was purchased by the W. R. Grace and Company in 1963. Yet, knowing the human health risks, the mining company still continued to operate there until 1990. Consequently, many of the former miners and residents of Libby have been affected and continue to suffer health problems. Over 400 people in the town have died from asbestos-related disease due to contamination from vermiculite mining from nearby Zonolite Mountain, where soil samples were found to be loaded with fibrous tremolite (known to be a very hazardous form of asbestos), and countless others there who insulated their homes with Zonolite have succumbed to asbestos-related diseases, most of whom never were employed in environments where asbestos was an issue.[14]

After a 1999 Seattle Post-Intelligencer story claimed that asbestos-related disease was common in the town, the EPA, in response to political pressure, made cleanup of the site a priority and called Libby the worst case of community-wide exposure to a toxic substance in U.S. history.[15][16] The EPA has spent $120 million in Superfund money on cleanup.[16] In October 2006, W. R. Grace and Company tried to appeal the fines ($54.5 million) levied on them from the EPA, but the Supreme Court rejected the appeal.[17] The United States government pursued criminal charges against several former executives and managers of the mine for allegedly disregarding and covering up health risks to employees.[17] They were also accused of wire fraud, and of obstructing the government's cleanup efforts. As of the indictment date, about 1,200 residents of the Libby area had been identified as suffering from some kind of asbestos-related abnormality.[18] The case ended in acquittals on May 8, 2009.[19][20][21] On June 17, 2009, the EPA issued a public health emergency in and near Libby, thereby allowing federal agencies to provide funding for health care, and for removal of contaminated insulation from affected homes.[22]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Vermiculite

View on Grokipedia
from Grokipedia
Vermiculite is a hydrous magnesium-aluminum-iron silicate mineral of the phyllosilicate group, structurally similar to mica, containing interlayer water molecules that cause it to expand dramatically—up to 30 times its volume—when heated to 800–1100°C, yielding lightweight, porous, and fire-resistant flakes. These expanded flakes exhibit low density (0.065–0.20 g/cm³), high cation-exchange capacity, and excellent absorption properties, enabling applications in thermal insulation, lightweight construction materials, and soil aeration. In horticulture, vermiculite improves soil drainage while retaining moisture and nutrients, often comprising 10–20% of potting mixes; industrially, it serves as a carrier for fertilizers, pesticides, and animal feed additives due to its inertness and anti-caking effects. A major controversy surrounds vermiculite mined from the Libby, Montana deposit (operated 1920–1990), which supplied about 80% of U.S. production and was intergrown with tremolite-actinolite asbestos, resulting in widespread contamination of commercial products, thousands of asbestos-related illnesses and deaths among exposed populations, and ongoing Superfund cleanup by the EPA since 2002.

Geological Occurrence and Formation

Natural Deposits and Global Distribution

Vermiculite occurs primarily as a secondary alteration product in metamorphic environments associated with ultramafic rocks rich in magnesium , such as pyroxenites and dunites, where or undergoes hydration and expansion under low-grade metamorphic conditions. Deposits typically form along shear zones or contacts between these ultramafic bodies and surrounding metamorphic rocks, yielding lenticular or planar ore bodies with flake sizes varying from fine to coarse depending on the host rock's abundance and alteration intensity. Commercial viability hinges on the presence of large, expandable crystals in igneous or metamorphic hosts, with extraction feasibility determined by grade, flake quality, and minimal deleterious impurities. Historically, the deposit in , , dominated global supply, accounting for approximately 80% of world vermiculite production from the until its closure in due to operational and quality challenges associated with intergrowths. Post-Libby, U.S. deposits shifted to sites in (Enoree district), (Louisa), and smaller occurrences in , , , , and , with identified resources estimated at 2–3 million tons. These non-Libby U.S. ores generally exhibit higher purity and lower contamination risks compared to Libby, facilitating ongoing extraction feasibility. Globally, South Africa's Phalaborwa (Palabora) deposit, part of a carbonatite-pyroxenite complex, remains a cornerstone, with reserves estimated at 14 million tons supporting annual production exceeding 160,000 tons as of 2023. holds reserves of 6.6 million tons, including the Catalão mine yielding predominantly super-fine grade ore at up to 60,000 tons per year, while China's reserves stand at 2.9 million tons amid growing domestic output. Additional significant deposits occur in , (45,000 tons production in 2023), , and , though comprehensive reserve data varies due to disparate reporting standards; U.S. reserves are pegged at 25 million tons, underscoring potential for sustained extraction across regions with favorable geological settings. Ore quality differs regionally, with South African and Brazilian sources often providing coarser, expandable flakes suitable for insulation, contrasted by finer grades in some Asian deposits.
CountryEstimated Reserves (million tons)Key Deposits/Notes
25Post-Libby sites in SC, VA; resources 2–3 Mt
14; leading producer
6.6Catalão; fine-grade focus
2.9Emerging production

Formation Processes and Associated Minerals

Vermiculite originates as a secondary mineral through the chemical alteration of primary phyllosilicates, predominantly or , in magnesium-rich environments. This transformation entails the leaching of interlayer cations and their replacement by hydrated magnesium ions, expanding the interlayer spacing to approximately 14.3 Å and imparting the mineral's distinctive flaky, expandable morphology. The process is driven by either hydrothermal fluids at elevated temperatures (typically 100–300°C) or under surface conditions, where circulating waters facilitate in ultramafic or host rocks. Hydrothermal alteration predominates in many deposits, involving metasomatic reactions that convert trioctahedral micas into vermiculite while preserving the 2:1 layer structure but altering octahedral occupancy with iron and magnesium. Weathering pathways, conversely, proceed via progressive hydration and oxidation in soils derived from granitic or metamorphic terrains, often yielding residual flake vermiculite from schists. Petrological recognition of these mechanisms dates to mid-20th-century studies, such as those on U.S. deposits in the –, which linked vermiculite to post-metamorphic fluid interactions rather than primary . Subsequent isotopic analyses, including oxygen and stable isotopes, have corroborated metamorphic precursors followed by low-temperature alteration, distinguishing vermiculite formation from direct igneous processes. Vermiculite commonly associates with -group minerals, such as -actinolite series members, due to shared origins in altered mafic-ultramafic protoliths where or decomposes concurrently. In specific locales like the deposit, amphiboles including winchite, richterite, and asbestiform contaminate ore at levels up to 5–10% by volume, arising from parallel metasomatic overprints rather than direct genetic linkage to vermiculite. Empirical mineralogical surveys across global deposits reveal variable amphibole presence, with non-Libby sites (e.g., in or ) exhibiting negligible contamination, underscoring that hazardous co-minerals stem from localized , not universal vermiculite genesis. This variability has been quantified through diffraction and microscopy in studies since the , emphasizing deposit-specific over blanket assumptions.

Physical and Chemical Properties

Crystal Structure and Composition

Vermiculite is a member of the phyllosilicate group, specifically a 2:1 trioctahedral layered with a general of (Mg,Fe²⁺,Al)₃(Al,Si)₄O₁₀(OH)₂·4H₂O, where interlayer molecules and exchangeable cations such as Mg²⁺, Ca²⁺, or Na⁺ occupy the space between the layers. The structure features two inward-facing tetrahedral sheets of silica tetrahedra sandwiching a central octahedral sheet dominated by magnesium, with substitutions of iron and aluminum in both tetrahedral and octahedral positions contributing to layer charge imbalances typically ranging from 0.6 to 0.9 per . This charge is compensated by hydrated interlayer cations, enabling the mineral's distinctive hydration properties without the continuous swelling seen in lower-charge smectites. X-ray diffraction analysis reveals a characteristic basal spacing (d₀₀₁) of approximately 14.4 Å in the fully hydrated state, corresponding to a trihydrate layer configuration where molecules form discrete layers coordinated to interlayer cations; partial leads to contractions to about 11.8 Å (dihydrate) or 9.8 Å (monohydrate), with complete loss of interlayer resulting in a collapsed structure near 10 Å. Variations in composition, particularly iron content from Fe²⁺ or Fe³⁺ substitutions in the octahedral sheet (up to 20-30 mol% in some samples), influence optical properties, yielding colors from pale yellow or greenish in low-iron varieties to bronze-brown or golden in iron-rich ones, as confirmed by empirical spectroscopic data. In contrast to , a dioctahedral with lower layer charge (0.2-0.4 per unit) primarily from tetrahedral substitutions and thus higher relative to its charge but prone to osmotic swelling with unrestricted uptake, vermiculite exhibits more fixed hydration states due to its higher charge originating from octahedral deficiencies, leading to distinct interlayer collapse behaviors under thermal or chemical stress. This structural in vermiculite, with cation exchange capacities often exceeding 150 meq/100 g in Mg-dominated forms, underscores its differentiation from montmorillonite's more expandable interlayers, as evidenced by comparative patterns showing vermiculite's resistance to full interlayer expansion beyond discrete hydrate phases.

Thermal Expansion and Exfoliation Mechanism

Vermiculite's arises from the rapid heating of its layered phyllosilicate structure, which contains interlayer molecules bound between 2:1 sheets. When heated quickly to 800–1100°C, this undergoes , converting to that generates sufficient internal to overcome interlayer bonding forces, thereby separating the layers and causing exfoliation. The process is endothermic, involving initial of hydration shells around interlayer cations, followed by explosive release of free as superheated vapor, with the rate of heating critical to prevent gradual and ensure pressure buildup. This mechanism yields volume expansions typically ranging from 8 to 15 times the original, with optimal conditions achieving up to 30-fold increases for fine, high-purity concentrates, resulting in a low-density ( 0.1–0.2 g/cm³) porous material resembling an . The expanded structure exhibits high , often exceeding 100 m²/g as measured by BET nitrogen adsorption isotherms, due to the creation of interlamellar voids. Expansion efficiency depends on factors including (finer grains expand more due to higher surface-to-volume ratios), interlayer (ideally 5–10 wt%), and cation type (e.g., magnesium-rich varieties expand better than iron-substituted ones, as cations modulate hydration and bonding strength). The exfoliation phenomenon was first documented in 1824 by Thomas H. Webb, who observed a sample from a talc deposit expanding into worm-like forms when heated over a flame, coining the name "vermiculite" from the Latin vermicularis for its resemblance to writhing larvae. Subsequent replications in the early , including controlled furnace tests, verified the steam-pressure model through observations of gas and structural analysis via , confirming layer separation without altering the fundamental tetrahedral-octahedral-tetrahedral (TOT) sheet composition. These empirical validations distinguished vermiculite's behavior from mere thermal dilation in other micas, highlighting the role of hydrated interlayer chemistry in enabling reversible yet explosive delamination.

Historical Development

Early Discovery and Initial Uses

Vermiculite was first described as a distinct in by Thomas H. Webb, who identified it in a deposit near , specifically in Millbury. Webb named the mineral "vermiculite" from the Latin vermiculare, meaning "to breed worms," referring to its characteristic expansion into worm-like forms when heated, a property he observed through empirical experimentation. Initially regarded as a variety of , vermiculite attracted attention as a geological curiosity due to this thermal behavior, with early 19th-century records limited primarily to North American localities and lacking systematic classification. By the early , interest shifted toward practical applications, with non-commercial experiments in the exploring its potential for fireproofing and insulation based on its low thermal conductivity and expansion properties. These tests demonstrated vermiculite's ability to form , heat-resistant aggregates, though adoption remained sporadic without large-scale production. In 1915, the Denver Mining & Manufacturing Company initiated the first documented marketing of expanded vermiculite from a deposit discovered in 1913 near Hecla, , promoting it for insulation uses amid growing industrial demand, yet sales were limited and the venture short-lived. Widespread awareness of vermiculite's utility did not emerge until after , driven by postwar reconstruction needs for lightweight materials, but pre-1920s records show no evidence of systematic or studies, with focus confined to its physical expansion and basic qualities rather than biological effects.

Commercialization and the Libby Mine Era

The commercialization of vermiculite accelerated in the early with the recognition of its exfoliation properties for lightweight insulation, driven by expanding industrial and construction demands. In 1919, E.N. Alley acquired mining claims in the Rainy Creek district near , establishing the Zonolite Company, which began commercial extraction of vermiculite ore in the . This operation capitalized on the deposit's high-quality ore, which expanded dramatically upon heating, making it suitable for applications like fireproofing and packing materials. The Zonolite mine quickly became a , supplying ore that was processed into products such as attic insulation under the Zonolite brand, with initial shipments supporting nascent markets in the . Demand surged during due to needs for in military applications, including and aircraft, where vermiculite's low density and heat resistance provided causal advantages over heavier alternatives like alone. Postwar housing booms further propelled output, as the material's efficacy in aggregates and insulation aligned with economic incentives for cost-effective building materials. By the mid-20th century, the Libby operation under Zonolite achieved dominance, accounting for approximately 80% of global vermiculite supply through efficient scaling and limited competition from other deposits. U.S. Geological Survey data indicate that between the 1920s and 1990, Libby produced millions of tons of ore, with annual peaks in the hundreds of thousands of tons during high-demand eras, underscoring the mine's role in meeting verifiable industrial volumes. In 1963, W.R. Grace & Company acquired the Zonolite Company, integrating the mine into its portfolio and intensifying production through expanded processing facilities. Under Grace's management, output peaked, with the mine yielding over 200,000 tons annually in the 1970s and 1980s, sustained by economic imperatives including low extraction costs and the ore's unique expandability that outperformed substitutes in volume efficiency. Geological surveys as early as the had identified associated minerals, including tremolite-actinolite , in the vermiculite ore, with U.S. Bureau of Mines reports in the confirming fibrous contaminants during routine assays. Operations continued unabated, however, as contemporaneous lacked causal evidence linking low-concentration exposures to widespread risks, prioritizing the mineral's proven utility amid regulatory voids and competitive market dynamics; was then standard in insulation, with Libby ore's vermiculite fraction deemed the value driver. The mine closed in 1990 following and shifting environmental scrutiny, marking the end of this era's dominance.

Mining and Production

Major Mining Operations and Reserves

The Libby Mine in , , operated from the until its closure in 1990, serving as the world's largest vermiculite producer for much of the with annual outputs exceeding 200,000 metric tons in peak years. Following its shutdown due to operational and contamination issues, global production shifted primarily to deposits in , , and , where lower asbestos content facilitated continued extraction. This transition aligned with rising demand for coarser-grade vermiculite suitable for insulation and horticultural applications, prompting exploration efforts in these regions as documented in U.S. Geological Survey assessments. In , the (Palabora) complex dominates vermiculite , accounting for the bulk of the country's 170,000 metric tons produced in 2023 through open-pit methods adjacent to primary operations. Vermiculite here occurs as golden-brown flakes intergrown with , , and minerals, necessitating selective blasting and separation to yield five classified grades for export. South Africa's total reserves stand at 14 million metric tons, supporting long-term viability via low-cost despite increasing pit depths exceeding 800 meters in associated operations. Brazil's vermiculite output reached 53,000 metric tons in 2023, drawn from deposits in , , and states, with reserves estimated at 6.6 million metric tons. Key operations, such as those near Catalão in , employ open-pit extraction targeting medium- to large-flake varieties, though co-occurring ultramafic minerals require careful sorting to maintain quality. A notable example is the São Luís de Montes Belos deposit, which holds of 286,000 metric tons of , underscoring Brazil's role in supplying finer exfoliation-grade material amid global demand growth. China's reported of 33,000 metric tons in 2023 likely understates actual output due to limited data transparency, with reserves at 2.9 million metric tons concentrated in and other western provinces. Operations there involve of deposits like Qieganbulake, featuring large flake sizes but challenged by variable purity and export-oriented processing. contributes around 29,000 metric tons annually from Siberian sites, while smaller producers (100,000 metric tons in 2023, mainly from ) and other nations fill niche roles. Globally, reserves total over 60 million metric tons, with economic extraction favoring shallow open-pit sites amenable to mechanical separation over deeper or complexly mineralized zones. U.S. reliance on imports, totaling 60,000 metric tons in 2024 primarily from and , has persisted since 1990, reflecting the closure of domestic large-scale sites. Global vermiculite mine production totaled an estimated 516,000 metric tons in 2022, decreasing slightly to 500,000 tons in 2023 and 460,000 tons in 2024, reflecting stable but marginally declining output amid consistent demand. led production with approximately 170,000 tons in 2024, followed by the at 100,000 tons, at 60,000 tons, and smaller contributions from , , , and . U.S. output has held steady at around 100,000 metric tons annually since 2020, derived from operations in and , while imports rose to an estimated 60,000 tons in 2024, mainly from (51%) and (42%). The closure of the Libby mine in in 1990, which had accounted for up to 80% of global supply but yielded contaminated with tremolite-actinolite asbestos, prompted a rapid shift to alternative sources in , , and , where deposits are routinely tested and confirmed asbestos-free, enabling verifiable elimination of contaminated material from commercial channels. This transition supported sustained production levels without the health liabilities of prior U.S.-sourced , though it increased reliance on foreign suppliers. Key challenges include the energy demands of and thermal exfoliation, which require significant to expand the , alongside post-2020 supply chain disruptions from COVID-19-related lockdowns and bottlenecks that constrained crude availability for processors. Geopolitical vulnerabilities in dominant producers like and , combined with production concentration, expose the sector to risks from regional instability, export restrictions, and trade tensions, as evidenced by fluctuating import volumes. The market value, reflecting these dynamics, grew from $0.19 billion in 2024 to a projected $0.2 billion in 2025 at a 4.5% CAGR, buoyed by needs despite supply constraints.

Processing and Manufacturing

Exfoliation Techniques

Exfoliation of vermiculite primarily involves rapid thermal treatment to vaporize interlayer , causing the layered structure to expand into flakes. The traditional method employs vertical gas- or oil-fired furnaces where crude , pre-dried and sized to 1-5 mm particles, is introduced at the top and heated to 800-1100°C for a of less than 8 seconds. This flash heating induces steam pressure between layers, yielding expansion ratios of 5-30 times the original volume, with optimal flake sizes achieved at 900-1000°C depending on composition. Prior to exfoliation, integration of mechanical sorting—such as crushing, screening, and air classification—removes impurities like minerals and asbestos-bearing fragments, ensuring uniform heating and minimizing defects in expanded product quality. Post-1990 advancements in furnace design, including improved insulation and automated feed controls, have reduced by 20-30% through better heat recovery and shorter cycle times, though fossil fuel dependency persists. Emerging microwave-based techniques, such as continuous tunnel processing, offer alternatives by selectively heating via dielectric absorption, achieving more uniform expansion at lower bulk temperatures (around 700-900°C) and reduced residence times of seconds to minutes. A 2025 study demonstrated that exfoliation enhances structural integrity and flake uniformity compared to conventional methods, with efficiencies improved due to volumetric heating that avoids surface overheating. These methods integrate pre-sorting to further limit impurities, as heterogeneous can cause uneven penetration and suboptimal expansion.

Quality Control and Purification Methods

Quality control protocols for vermiculite prioritize empirical fiber analysis to verify absence of asbestos contaminants, employing (PCM) for rapid screening and (TEM) for precise identification and quantification of fibers. These techniques align with ISO 22262-1 for sampling and qualitative assessment of bulk materials, followed by ISO 22262-2 for quantitative determination, enabling detection sensitivities down to trace levels in commercial products. ASTM practices, such as E2356 for comprehensive surveys, supplement these by standardizing bulk sampling strategies to represent variability. Following the closure of the asbestos-associated mine in , regulatory frameworks enforce contamination thresholds below 1% by weight for non-hazardous classification, with EPA Method 600/R-93/116 specifically adapted for vermiculite insulation through dispersion and to isolate potential fibers. Producers of post-Libby vermiculite, sourced from deposits in , , and , implement routine TEM-verified testing at multiple processing stages, confirming compliance and distinguishing clean material from legacy risks. Purification commences with physical separation post-crushing and screening to eliminate minerals. Magnetic separators remove iron-bearing impurities, improving concentrate grades in phlogopite-vermiculite ores, while high-intensity variants target weakly magnetic contaminants. Flotation exploits vermiculite's hydrophobic surface for selective recovery, and gravity-based methods, including water dispersion, segregate denser particles like potential aggregates from floating vermiculite flakes, achieving effective impurity rejection adaptable from analytical protocols. Electrostatic separation further differentiates based on conductivity, as ed processes demonstrate for rejection without chemical alteration. Chemical treatments remain secondary, with mild acid leaching applied selectively to dissolve siliceous residues while preserving interlayer cations essential for exfoliation. Integrated systems in contemporary facilities yield high-purity exfoliates from verified low-contaminant ores, with operational data underscoring mechanical efficacy in maintaining standards absent historical impurities.

Applications and Uses

Insulation and Construction Materials

Expanded vermiculite has been widely used as loose-fill insulation in attics and wall cavities of residential and commercial buildings, particularly those constructed from the through the . This application leverages its lightweight, granular structure, which yields an R-value of approximately 2.1 to 2.5 per inch, providing effective but moderate compared to modern alternatives like or foam. The material's inherent properties, including low thermal conductivity (around 0.065 ) and non-combustibility, enhance resistance in assemblies. Empirical tests on vermiculite-filled walls (8 to 12 inches thick) demonstrate endurance ratings of up to 3 hours, as it resists flame spread and maintains structural integrity under elevated temperatures by releasing bound . In lightweight concretes and plasters, vermiculite acts as an aggregate to reduce while preserving and insulating performance, suitable for roof decks, floor underlays, and coatings. Despite these advantages, vermiculite loose-fill insulation is susceptible to settling over decades, which can reduce installed depth and effective R-value by 10-20% in poorly compacted installations, necessitating over-installation or supplemental barriers per building evaluations. Certified asbestos-free vermiculite remains permissible under current building codes, such as those aligned with ASTM standards for insulation materials, provided purity verification. Market analyses project continued growth in construction applications, with global vermiculite demand reaching approximately $430 million in 2025, driven by needs for sustainable, lightweight thermal barriers in expanding urban infrastructure.

Agriculture, Horticulture, and Soil Amendment

Vermiculite serves as a key component in potting soil and soilless growing media, acting like a mineral sponge to retain water and nutrients and release them gradually to roots, making it suitable for plants requiring consistently moist but not waterlogged conditions, such as ferns or tropical houseplants. Its ability to enhance , retain water up to three to four times its weight, and provide a sterile environment that minimizes risks supports and early development, typically incorporated at 10-20% in mixes. These properties promote uniform moisture distribution and oxygen availability to roots, supporting higher rates compared to traditional mixes, as observed in practices where fine-grade vermiculite is layered over seeds or mixed at ratios of 1:1 with or . Additionally, its sterility reduces damping-off diseases, making it suitable for propagating cuttings and starting sensitive crops. In root storage applications, dry vermiculite is used to surround bulbs, tubers, or harvested roots, absorbing excess moisture to prevent rot while maintaining levels that inhibit during overwinter storage. This method has been empirically effective for crops like onions and potatoes, preserving viability for months without chemical preservatives. An EPA assessment of consumer garden products containing vermiculite, conducted around 2000, concluded that exposures pose minimal health risks when using properly processed material. As a soil conditioner, vermiculite improves drainage and in heavy clay by loosening compacted structure and facilitating water percolation, which counters waterlogging common in such substrates. Field applications at incorporation rates of 10-20% by volume have shown enhanced penetration and establishment in clay-heavy fields. In controlled cultivation using vermiculite-based soilless media, such as cocopeat-vermiculite blends, plants reached 50% flowering 28 days after and first harvest at 58 days, indicating accelerated growth cycles that contribute to overall yield improvements through better uptake and reduced transplant stress. Pure vermiculite exhibits no inherent , relying on its composition for inert performance in these roles. Compared to , vermiculite offers greater cost-effectiveness for moisture-dependent applications in , as its superior and nutrient retention reduces irrigation frequency and leaching, particularly in or setups where perlite's drainage focus demands more frequent inputs. However, vermiculite tends to compact over time, so it is often combined with perlite to maintain aeration and prevent settling. While perlite excels in arid conditions for alone, vermiculite's multifunctionality in balanced mixes provides economic advantages for sustained crop productivity without added amendments.

Industrial and Other Specialized Uses

Exfoliated vermiculite is employed in linings and high-temperature insulation due to its thermal stability, which allows it to withstand temperatures up to approximately 1100°C without significant degradation, stemming from the mineral's layered structure that expands controllably under heat while maintaining structural integrity. This property facilitates its use in producing insulation shapes combined with sodium or binders for applications in furnaces and . In , vermiculite integrates into coatings and mixes for facilities and tunnel linings, where its low thermal conductivity and fire resistance prevent flame spread and heat transfer. Vermiculite serves as a key component in materials, including brake pads and clutch linings, acting as a safe substitute owing to its thermal resistance, which dissipates heat during , and its ability to blend seamlessly with resins and fillers for enhanced durability. Manufacturers incorporate fine-grade exfoliated vermiculite to improve abrasion resistance and fluid sealing in automotive and components exposed to high temperatures. Expanded vermiculite functions as an absorbent for hazardous spills, leveraging its porous and high surface area to adsorb oils and chemicals, with studies demonstrating effective uptake of crude when the material is or hydrophobized, achieving capacities superior to hydrated forms. Its chemical inertness and thermal stability make it suitable for containing reactive spills without secondary hazards. In shipping, vermiculite cushions hazardous materials packaging, complying with IATA regulations due to its absorbent rate that encapsulates liquids and prevents leakage during transport.

Health and Safety Considerations

Asbestos Contamination in Specific Deposits

The primary source of asbestos contamination in vermiculite ore has been the deposit near , where raw ore contained up to 21-26% tremolite-actinolite asbestos by weight, with processed concentrates ranging from 0.3-7%. This contamination consisted mainly of fibrous minerals such as , , winchite, and richterite, intergrown with the vermiculite flakes due to the specific of the host rock formation. The asbestos was not an intrinsic component of vermiculite's mineral structure—a hydrous magnesium aluminum sheet —but rather a co-occurring impurity from the enclosing metamorphic and igneous rocks in that locality. Geological surveys confirm that such contamination is not characteristic of vermiculite deposits globally or even within the ; asbestos minerals are absent or negligible in most other known reserves, including major producers in , Brazil, , and , which account for the bulk of current production. The U.S. Geological Survey's reconnaissance of domestic deposits, for instance, found no widespread presence of asbestos-forming amphiboles, attributing rare occurrences to localized host rock associations rather than the vermiculite genesis process itself. Only a limited number of ore bodies worldwide exhibit this issue, with EPA assessments emphasizing that vermiculite ores typically contain a range of accessory minerals but as a contaminant in just isolated cases. Historically, the Libby mine supplied over 70% of the vermiculite used in the United States from to its closure in 1990, including much of the loose-fill insulation installed in homes during that period, resulting in widespread distribution of contaminated material across processing sites and end products. Subsequent sourcing from asbestos-free deposits has eliminated this in modern vermiculite products, as verified through routine mineralogical testing of active global mines.

Empirical Evidence on Health Risks

Pure vermiculite, absent asbestos contamination, has not been associated with adverse effects in empirical studies, with observed risks in contaminated materials attributable solely to the fibers rather than the vermiculite itself. Peer-reviewed reviews confirm no serious risks from exposure to vermiculite alone, citing its chemical durability and lack of fibrogenic or carcinogenic properties independent of contaminants. Cohort studies of workers at the Libby, Montana vermiculite mine, where ore was heavily contaminated with tremolite-actinolite asbestos, document elevated risks of mesothelioma (standardized mortality ratios of 3-10 times baseline) and other asbestos-related diseases, but these are linked to high-intensity occupational exposures during mining, milling, and processing rather than downstream consumer or end-use applications. Mesothelioma incidence among Libby miners and processors exceeded that of nearby residents by factors of 10-15, underscoring dose-dependent effects from cumulative fiber burdens exceeding 25-100 fiber-years per milliliter in occupational settings. In contrast, non-occupational exposures from commercial expansion or handling of Libby-derived vermiculite showed lower but detectable risks only in prolonged manufacturing contexts, with standardized mortality ratios around 10.5 for mesothelioma among processors. Quantitative exposure assessments indicate minimal airborne fiber release from undisturbed vermiculite insulation, typically below 0.1 fibers per cubic centimeter (f/cc), falling short of occupational thresholds that trigger effects. For consumer gardening applications, a 2000 EPA analysis of 38 vermiculite-containing products found trace in some but concluded very low lifetime cancer risks (e.g., 1 in 7.7 million for half-hour annual use over 10 years), affirming minimal risks from intermittent or use. Epidemiological dose-response models for , including types in vermiculite contaminants, demonstrate a supralinear relationship where rarity persists at low cumulative exposures (<10-25 fiber-years), countering claims of zero safe thresholds by highlighting no observed cases in population studies of para-occupational or environmental levels below heavy industrial benchmarks. Industry monitoring and regulatory data align with these findings, showing safe exposure thresholds (e.g., 0.1 f/cc permissible limits) upheld by absence of excess disease in controlled low-dose scenarios, distinct from activist assertions lacking quantitative support.

Regulatory Responses and Mitigation Strategies

In response to asbestos contamination in vermiculite from the mine, the U.S. Environmental Protection Agency (EPA) designated the site as a location in October 2002, initiating extensive cleanup operations that have incurred costs exceeding $500 million through settlements, reimbursements, and ongoing remediation efforts as of the mid-2010s. Under the Toxic Substances Control Act (TSCA), the EPA implemented import certification requirements in 2000 mandating that suppliers of expanded vermiculite attest to its asbestos-free status, with U.S. Customs and Border Protection conducting inspections and testing to enforce compliance and prevent contaminated material from entering the market. For residential settings, EPA guidelines emphasize that undisturbed vermiculite attic insulation poses minimal airborne risk, advising homeowners to avoid disturbance and hire certified professionals for any necessary removal to prevent fiber release during handling. To identify vermiculite insulation, which may be distinguished from cellulose insulation—a common alternative—note that vermiculite appears as loose, pebble-like granules or rocky gravel, often grayish-brown, silvery-gold, or tan with a metallic sheen or shiny mica flakes; particles are lightweight, rough, and accordion-shaped. Cellulose insulation, by contrast, appears as fluffy, soft, shredded gray paper or grayish-brown pulp-like material, fibrous with no shine, resembling blown-in recycled newspaper. In attic inspections, cellulose forms a uniform fluffy gray layer with paper-like texture and no distinct pebbles, whereas vermiculite exhibits scattered shiny, irregular granules with reflective surfaces, appearing more granular and less uniform. Vermiculite insulation, especially from pre-1990 installations, may contain asbestos; if suspected, do not disturb the material and seek professional testing. Encapsulation—sealing the material in place with barriers—is permitted as a lower-disturbance strategy when full removal is impractical, though empirical studies indicate that the primary exposure vector is active disruption rather than passive presence, with fiber liberation occurring mainly through mechanical agitation like or vacuuming. These measures have facilitated the safe commercialization of asbestos-free vermiculite sources, such as those from , under strict testing protocols that align with low observed risks in uncontaminated products; however, critics argue that precautionary assumptions treating all legacy vermiculite as uniformly hazardous may impose disproportionate economic burdens, as cost-benefit evaluations reveal negligible health increments from undisturbed insulation compared to the high costs and potential exposure spikes from mandated interventions.

Economic and Environmental Impacts

Market Dynamics and Economic Value

The global vermiculite market, valued at $260.48 million in 2021, is forecasted to reach $355.8 million by the end of 2025, reflecting a (CAGR) of approximately 8%. This expansion is driven primarily by demand in and sectors, where vermiculite serves as a , thermally stable filler and insulator, with accounting for a substantial share of consumption due to its role in and retention. applications, including insulation and concrete, further bolster growth amid rising adoption in energy-efficient building practices. Major exporters include and , which dominate global trade flows; exported vermiculite, , and related unexpanded minerals valued at $25.36 million in 2023, while ranks as a primary supplier to markets like the . Worldwide exports of unexpanded vermiculite exceeded $224 million in 2023, underscoring its role in international commodity chains. Economically, vermiculite generates localized employment in resource-rich areas, such as South Africa's region, where operations prioritize hiring from surrounding communities to support . As a naturally abundant, low-cost alternative to synthetic insulators and aggregates, it enhances value creation by reducing material expenses in end-use industries without compromising performance. However, legacy asbestos contamination in certain deposits has imposed economic constraints, particularly in the United States, where historical production—once significant—has declined sharply following the closure of the mine in 1990 and subsequent liabilities, limiting domestic output to about 100,000 tons annually and increasing reliance on imports. This has depressed U.S. mining activity despite global demand, with imports reaching an estimated 50,000 tons valued at $26 million in 2023, primarily from and . Counterbalancing these challenges, growth is sustained by demand for certified, asbestos-free vermiculite in certifications (e.g., standards) and regulated supply chains, which mitigate health-related risks while enabling premium pricing for verified sources.

Environmental Effects of Mining and Use

Vermiculite extraction primarily occurs through , which disrupts local habitats by removing overburden and vegetation, and generates dust emissions during ore processing and transport. At contaminated sites like the deposit, operational dust and waste piles containing amphibole fibers led to widespread soil and sediment contamination, affecting aquatic and terrestrial ecosystems through deposition and runoff. The U.S. Environmental Protection Agency (EPA) designated the Libby site a location in 2002, initiating remediation that has removed millions of cubic yards of contaminated material, including from over 2,600 residential, commercial, and public properties by November 2018, with subsequent deletions of operable units from the demonstrating partial ecological restoration through soil capping and revegetation. Modern vermiculite mining at asbestos-free deposits, such as those in operational since the 1940s and expanded post-1990, employs dust suppression and progressive reclamation to mitigate habitat loss, with regulatory requirements ensuring backfilling and replanting that support recovery comparable to other non-toxic operations. Vermiculite itself exhibits no inherent environmental , as the is chemically stable, inert, and non-leaching, preventing or absent site-specific contaminants like . Environmental critiques, often from advocacy groups, emphasize historical operator in control at early 20th-century U.S. sites, where lax practices allowed persistent deposition despite awareness of risks by the ; however, empirical monitoring post-remediation shows contained airborne and concentrations below ecological thresholds in treated zones, underscoring the distinction between legacy and the mineral's neutral properties. In applications, expanded vermiculite's low —typically reducing unit weight by 20-40% when used as an aggregate—lowers the volume of material transported to sites, thereby decreasing consumption and associated carbon emissions from haulage trucks. Life cycle assessments of incorporating vermiculite aggregates indicate reduced compared to standard mixes, with benefits accruing from minimized structural dead loads that enable smaller foundations and less overall material use, though comparative data against polymeric foams remain limited and site-dependent. Unlike leachable additives, vermiculite's stability ensures no ongoing release of pollutants during use or end-of-life disposal, contributing to net environmental advantages in durable over heavier alternatives.

Recent Developments and Future Prospects

Technological Innovations

Continuous tunnel expansion represents a key post-2020 in vermiculite , enabling higher and improved mechanical properties over conventional methods. A 2025 study in RSC Advances detailed a -assisted process that achieves uniform expansion while reducing and enhancing the structural of the resulting material, addressing longstanding limitations in expansion yield and durability. This technique leverages heating to rapidly exfoliate interlayer water, producing expanded vermiculite suitable for insulation with superior stability. Nano-vermiculite composites have advanced insulation applications through integration with like and MXene, yielding materials with enhanced fire retardancy and electrical . published in February 2025 highlighted vermiculite's role in two-dimensional nanocomposites for fire-retardant barriers and energy-efficient barriers, capitalizing on its natural layered structure for superior heat dissipation. Similarly, a 2023 method produced expanded vermiculite/xonotlite composites exhibiting low thermal conductivity (approximately 0.05 W/m·K) and high , demonstrating empirical improvements in performance. Sustainable integration of vermiculite includes its use as a partial replacement for fine aggregates in heat-resistant mortars, promoting without compromising mechanical integrity. A 2025 experimental study found that incorporating exfoliated vermiculite at up to 20% substitution levels in Portland slag cement-based mortars resulted in viable sustainable alternatives with enhanced resistance, supported by microstructural confirming interlayer expansion contributions to . These developments align with broader efforts to minimize environmental impacts through low-energy processing and waste-derived enhancements.

Emerging Markets and Sustainability Efforts

The global vermiculite market is forecasted to expand from USD 430 million in 2025 to USD 878 million by 2032, reflecting a of 10.7%, with significant contributions from emerging applications in , , and . In the region, demand is projected to grow at a CAGR of approximately 6.5% through 2030, led by which accounts for over 50% of regional consumption, driven by urbanization and horticultural uses including soil amendments for where vermiculite improves aeration and nutrient retention without chemical additives. and other South American producers are scaling output to supply expanding agricultural sectors, leveraging vermiculite's role in sustainable soil conditioning that aligns with standards. A key growth driver in emerging markets is vermiculite's utility in (EV) infrastructure, particularly as a non-combustible absorbent and insulator for packaging and . Aqueous vermiculite dispersions have been developed and certified for rapidly cooling battery fires by forming oxygen barriers, addressing challenges in EV storage and transport amid rising global adoption. This application supports green certifications in battery , as vermiculite's stability reduces reliance on synthetic alternatives, with demand tied to EV market expansion in . Sustainability efforts focus on resource stewardship and reduced environmental footprint, including USGS-documented of medium- and large-grade deposits in regions like and to offset depletion from current production levels estimated at 500,000 metric tons annually worldwide. Initiatives for closed-loop target exfoliated vermiculite from , enabling reuse in or aggregates to minimize virgin , though implementation remains nascent and varies by . Asbestos-free vermiculite sources, verified through rigorous testing protocols, underscore the material's ongoing viability, with empirical data on its inert properties countering potential regulatory overreach that could hinder access despite proven benefits in low-impact applications.

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