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Willemite
Willemite
Willemite
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Willemite
Willemite from Namibia
General
CategorySilicate mineral
FormulaZn2SiO4
IMA symbolWlm[1]
Strunz classification9.AA.05 (10 ed)
8/A.01-20 (8 ed)
Dana classification51.1.1.2
Crystal systemTrigonal
Crystal classRhombohedral (3)
(same H-M symbol)
Space groupR3
Identification
ColorColorless to white, gray, black, flesh-red, burgundy-red, pink, brown, dark brown, mahogany-brown, honey-yellow, yellow, apple-green, blue, pastel green, light blue, azure-blue
Crystal habitFibrous, botryoidal to massive
Cleavage{0001}, {1120} – imperfect
FractureIrregular to conchoidal
Mohs scale hardness5.5
LusterVitreous to resinous
DiaphaneityTransparent to opaque
Specific gravity3.9 – 4.2
Optical propertiesUniaxial (+)
Refractive indexnω = 1.691 – 1.694 nε = 1.719 – 1.725
Birefringenceδ = 0.028
Other characteristicsStrongly fluorescent; may be phosphorescent
References[2][3][4]
Major varieties
troostitezinc is partly replaced by manganese

Willemite is a zinc silicate mineral (Zn2SiO4) and a minor ore of zinc. It is highly fluorescent (green) under shortwave ultraviolet light. It occurs in a variety of colors in daylight, in fibrous masses and apple-green gemmy masses. Troostite is a variant in which part of the zinc is partly replaced by manganese, it occurs in solid brown masses.

It was discovered in 1829 in the Belgian Vieille-Montagne mine. Armand Lévy was shown samples by a student at the university where he was teaching. Lévy named it after William I of the Netherlands[5] (it is occasionally spelled villemite).[6][7][8] The troostite variety is named after Dutch-American mineralogist Gerard Troost.[9]

Occurrence

[edit]
Willemite variety troostite from New Jersey

Willemite is usually formed as an alteration of previously existing sphalerite ore bodies, and is usually associated with limestone. It is also found in marble and may be the result of a metamorphism of earlier hemimorphite or smithsonite.[10] Crystals have the form of hexagonal prisms terminated by rhombohedral planes: there are distinct cleavages parallel to the prism-faces and to the base. Granular and cleavage masses are of more common occurrence.[11] It occurs in many places, but is best known from Arizona and the zinc, iron, manganese deposits at Franklin and Sterling Hill Mines in New Jersey. It often occurs with red zincite (zinc oxide) and franklinite (Fe,Mn,Zn)(Fe,Mn)2O4 (an iron rich zinc mineral occurring in sharp black isometric octahedral crystals and masses). Franklinite and zincite are not fluorescent.

Uses

[edit]

Artificial willemite was used as the basis of first-generation fluorescent tube phosphors. When doped with manganese ions, it fluoresces with a broad white emission band. Some versions had some of the zinc replaced with beryllium. In the 1940s it was largely replaced by second-generation halophosphors based on fluorapatite. These, in turn have been replaced by the third-generation TriPhosphors.[12][13]

Crystal structure of willemite
In natural and in ultraviolet light

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Willemite

View on Grokipedia
from Grokipedia
Willemite is a with the Zn₂SiO₄, crystallizing in the hexagonal system and renowned for its bright green under shortwave . It typically forms as secondary crystals in oxidized zones of ore deposits, often appearing as prismatic, , or granular masses in colors ranging from colorless and white to green, yellow, red, and brown. The mineral exhibits a vitreous to resinous luster, a Mohs hardness of 5.5, and a specific gravity between 3.89 and 4.19, making it brittle with indistinct cleavage. Optically, it is uniaxial positive with refractive indices ω = 1.691–1.694 and ε = 1.719–1.725, and it may show after excitation. Structurally, willemite consists of corner-sharing ZnO₄ and SiO₄ tetrahedra forming tunnel-like channels along the c-axis, with R3 and parameters a = 13.948 , c = 9.315 . Willemite occurs worldwide in lead-zinc deposits, particularly in limestones, as an alteration product of , and is associated with minerals like , , and hemimorphite. Notable localities include the Franklin and Sterling Hill mines in , , where it forms massive ore bodies and displays exceptional , as well as sites in , , and . As a significant source of , it serves as an ore mineral, while its unique optical properties make it popular among collectors; additionally, synthetic willemite finds applications in ceramics due to its thermal and mechanical stability. Named in 1830 after , willemite was first described from but gained prominence through studies of North American deposits.

Etymology and History

Discovery

Willemite was first observed in by American geologists Lardner Vanuxem and William H. Keating, who described specimens from a zinc deposit in Franklin, New Jersey, as "siliceous of ." In their analysis, published in the Journal of the Academy of Natural Sciences of , they noted the mineral's composition rich in and silica, with approximate proportions indicating about 70% and 30% silica after , setting it apart from previously known compounds. The formal description of willemite as a distinct came in 1830 from French mineralogist Armand Lévy, based on specimens from the Vieille-Montagne mine near Moresnet, (then part of the ). Lévy's work in the Annales des Mines (series 3, volume 7, pages 361-364) provided detailed crystallographic and chemical observations, confirming it as a through quantitative assays showing a composition consistent with Zn₂SiO₄. This distinguished willemite from other zinc-bearing minerals like (ZnS), a , by its nature and hexagonal . Early chemical analyses by Lévy and contemporaries, including tests and ignition experiments, further verified the , emphasizing its role as a secondary formed through oxidation processes. In the , amid the Industrial Revolution's surge in demand for and alloys, mineralogists such as James D. Dana played key roles in classifying willemite within groups. This period saw increased scrutiny of ores, with willemite recognized as a viable source in metamorphic deposits, contributing to expanded mining efforts in and starting around 1850.

Naming and Early Recognition

Willemite was named in 1830 by the French mineralogist Serve-Dieu Abailard Lévy after King William I of the (known as Willem in Dutch), honoring the monarch's support for scientific endeavors, including in the Belgian territories under his rule at the time. The type locality for the mineral was the Vieille-Montagne mine near Moresnet in what is now , then part of the . The mineral gained recognition as a distinct species shortly after its naming, appearing in early mineralogical handbooks such as James Dwight Dana's A System of Mineralogy (first edition, 1837), where it was classified among the based on its and crystallographic properties. This inclusion helped solidify willemite's place in systematic , distinguishing it from related compounds. A variety of willemite was earlier referred to as "troostite," a name introduced in 1832 by Charles Upham Shepard for what he described as a silicate from North American localities; it was later reassigned to the manganese-bearing variant of willemite in honor of the American mineralogist and geologist Gerard Troost (1776–1850). Early descriptions often confused willemite with other silicates, such as labeling it a "siliceous of " in analyses by Vanuxem and Keating (1824), reflecting initial uncertainties in its composition before Lévy's formal identification. These confusions were resolved in the first editions of major classification systems, where willemite was properly categorized as a .

Physical and Optical Properties

Appearance and Morphology

Willemite exhibits a range of colors, including colorless, white, , , , , and , with variations often influenced by impurities. These hues can appear as pastel , apple-green, honey-, flesh-, or mahogany- in different specimens. In terms of crystal habits, willemite typically forms hexagonal prisms in the hexagonal system, appearing as stout or slender prisms terminated by rhombohedra, sometimes reaching up to 10 cm in length. However, it more commonly occurs in massive, granular, fibrous, or aggregates, with tiny hexagonal prisms or radial tufts of acicular needles noted in many deposits. Willemite displays distinct cleavage on {11\overline{2}0}, with a poor cleavage on {0001}. Its is uneven to subconchoidal or irregular. The has a vitreous to resinous luster and is transparent to translucent, though it can appear opaque in denser aggregates. In deposits, willemite grains typically range from microscopic sizes to several centimeters, particularly in geodes where larger prismatic crystals develop. Under ultraviolet light, its can enhance visibility of these forms, often glowing green.

Fluorescence and Luminescence

Willemite exhibits intense green under shortwave light at 254 nm, primarily activated by trace amounts of (Mn²⁺) substituting for in its crystal lattice. This response peaks at approximately 525 nm and is particularly vivid in specimens from zinc ore deposits, where concentrations around 1% yield maximum intensity. Under ultraviolet light at 365 nm, the is weaker and less commonly observed, often appearing at medium intensity only in select samples. follows excitation, manifesting as a that is variable in duration but typically short-lived in many specimens. Some willemite specimens display , emitting faint greenish-white light when mechanically stressed, such as by rubbing. Fluorescence variations occur among subtypes; for instance, troostite, a manganese-rich variety, produces an orange-red glow under shortwave UV. Overall intensity depends on specimen purity and internal , with higher levels and lower iron impurities enhancing brightness, while can result in differential responses across crystal zones. The of willemite was notably observed in the 1920s at the Franklin Mine in , where it sparked early scientific investigations into mechanisms.

Chemical Composition and Crystal Structure

Formula and Composition

Willemite is a zinc silicate mineral with the ideal chemical formula \ceZn2SiO4\ce{Zn2SiO4}. This composition corresponds to a molecular weight of 222.86 g/mol, consisting of 58.68 wt% zinc (Zn), 12.60 wt% silicon (Si), and 28.72 wt% oxygen (O), or equivalently 73.04 wt% ZnO and 26.96 wt% SiO₂ by oxide weight. Minor substitutions commonly occur in the zinc sites, where divalent cations such as Fe²⁺, Mn²⁺, or Mg²⁺ can replace up to 10-15% of the Zn, leading to variations in the formula such as (Zn,Fe,Mg)₂SiO₄. A notable manganese-rich variety known as troostite has the formula (Zn,Mn)₂SiO₄, typically with MnO contents exceeding 3 wt%. Trace impurities including calcium (Ca), lead (Pb), and occasionally arsenic (As) are also present, influencing the mineral's color variations from colorless to green or brown. These compositional variations affect the mineral's , which ranges from 3.89 to 4.19 g/cm³ depending on the extent of substitutions and impurities. In modern studies, willemite's formula and composition are routinely confirmed using analytical techniques such as (XRF) spectroscopy for bulk elemental analysis and electron microprobe analysis (EMPA) for precise in-situ measurements of major and minor elements.

Structural Details

Willemite crystallizes in the trigonal , exhibiting a hexagonal appearance due to its prismatic . The is R\overline{3} (No. 148), with parameters of a = 13.948 , c = 9.315 , and Z = 18. This arrangement reflects the mineral's framework nature, where all cations occupy tetrahedral sites. The atomic structure consists of chains of edge-sharing ZnO₄ tetrahedra aligned parallel to the c-axis, forming double strands that are cross-linked by corner-sharing SiO₄ tetrahedra to create a three-dimensional framework. The Zn atoms occupy two distinct tetrahedral sites: one type shares edges with adjacent ZnO₄ tetrahedra to build the chains, while the other connects these chains via corner-sharing with both ZnO₄ and SiO₄ units. The SiO₄ tetrahedra are isolated and link eight surrounding ZnO₄ tetrahedra through their corners, resulting in open tunnels along the c-direction with a diameter of approximately 5.73 Å. This tetrahedral framework imparts stability to the structure, with average bond lengths of Zn-O around 1.94 Å and Si-O around 1.62 Å. Willemite (α-Zn₂SiO₄) is the thermodynamically polymorph featuring the described rhombohedral framework with edge-sharing ZnO₄ chains. A metastable β-Zn₂SiO₄ phase, possessing an orthorhombic , can be synthesized at lower temperatures (e.g., ~650 °C), but it transforms to the α-phase upon heating above approximately 1000 °C and is not observed in natural samples. Twinning in willemite is rare, typically occurring on {10\overline{1}0}, but polysynthetic twinning can appear in fibrous varieties, leading to lamellar or sector growth patterns. Such twinning is infrequently reported and does not significantly alter the overall structural framework.

Geological Occurrence

Formation Processes

Willemite primarily forms through metasomatic processes in contact metamorphic zones, where zinc-bearing fluids interact with or , leading to the replacement of primary sulfide minerals such as . This occurs in environments like skarns, where igneous intrusions provide and silica-rich fluids that alter host rocks, precipitating willemite at temperatures typically ranging from 150°C to 300°C. The reaction is driven by water-rock interactions and fluid mixing, often involving pH increases from acidic hydrothermal fluids reacting with carbonates, favoring willemite stability over under oxidizing conditions. Secondary formation of willemite takes place via oxidation and enrichment in zinc deposits, particularly in the zones above primary ores. These processes involve meteoric waters or low-temperature hydrothermal fluids (<100°C) that leach and reprecipitate , often in association with silica, forming willemite in nonsulfide assemblages. In such settings, willemite develops through the alteration of pre-existing minerals like or zincite under surface or near-surface conditions, contributing to economic concentrations in arid or semi-arid regions. Willemite commonly forms in paragenesis with other zinc minerals, including zincite, , hemimorphite, and (a historical term for hemimorphite), reflecting shared hydrothermal or oxidative origins in carbonate-hosted deposits. These associations are prominent in high-temperature hypogene environments, where willemite coexists with and zincite in metamorphic aureoles, or in zones alongside hemimorphite. Although predominantly metamorphic or hydrothermal, willemite has rare igneous occurrences, such as in pegmatites linked to alkaline , where it forms through late-stage fluid differentiation in intrusive complexes.

Principal Localities

The type locality for willemite is the Altenberg mine (also known as Vieille-Montagne or La Calamine) near in the Plombières-Vieille Montagne district, , , where it was first identified and described in 1830 from zinc-rich deposits in . This historic site, part of early 19th-century operations, yielded massive and crystalline willemite associated with other minerals, establishing its role as a key secondary in oxidized zones. Among the premier global deposits, the Franklin and Sterling Hill mines in Sussex County, New Jersey, USA, stand out as the most renowned for producing exceptional fluorescent willemite varieties, including green-glowing massive and hexagonal prismatic crystals under ultraviolet light. These unique Precambrian deposits, mined extensively from 1897 until the Franklin closure in 1954 and Sterling Hill in 1986, supplied significant zinc ore and remain iconic for their mineral diversity, with willemite occurring in vugs and veins alongside franklinite and calcite. Other major sites include the Tsumeb Mine in the Oshikoto Region, Namibia, which has produced gemmy yellow to green cadmian willemite crystals in dolomite-hosted veins since the early 20th century. The Mammoth-Saint Anthony Mine (also called the Mammoth Mine) in the Tiger district, Pinal County, Arizona, USA, is notable for blue to green willemite crystals in oxidized zinc-copper deposits, often intergrown with wulfenite. Similarly, the Broken Hill Mine in Yancowinna County, New South Wales, Australia, hosts willemite in a world-class lead-zinc-silver deposit, typically as botryoidal masses in supergene zones. Lesser but noteworthy occurrences include the Tighza Mine in the Midelt Province, Drâa-Tafilalet Region, Morocco, where willemite appears as a in contact zones of carbonate-hosted lead- deposits. In , willemite is reported from the Mestersvig lead- deposit in the Scoresby Land region, forming minor secondary phases in sedimentary-hosted ores. Nonsulfide deposits rich in willemite continue to attract exploration interest globally, such as in the Bongará district, , for potential future as of 2024. As of 2024, willemite is primarily of interest for mineral collecting rather than active commercial , with global production mainly from ores.

Uses and Applications

Industrial and Economic Uses

Willemite functions as a minor of , primarily smelted to produce metal, with the containing approximately 58% by weight. This extraction process involves and leaching to separate from the matrix, though it is generally less efficient than processing due to higher energy requirements for reduction. Historically, synthetic manganese-doped willemite served as a key material in fluorescent lamps during and , prior to the advent of halophosphate phosphors, where it emitted green light at approximately 525 nm under excitation. In modern applications, willemite provides silicates essential for ceramics glazes and pigments, enabling the formation of crystalline structures that enhance aesthetic and functional properties in . Synthetic willemite also finds emerging uses in optoelectronic devices, such as samarium-doped for luminescent applications, and in , including willemite-incorporated scaffolds for tissue regeneration, as of 2023–2025. Economically, willemite deposits hold value in high-grade locales like the Franklin and Sterling Hill mines in , where zinc concentrations exceeded 20-30% in bodies, supporting historical operations despite relatively low total tonnage compared to global reserves. Today, it is typically recovered as a in broader activities rather than targeted extraction. Environmental considerations in willemite processing include management of from leaching, which generate silica-rich residues; nonsulfide ores like willemite produce less than sulfides, though heavy metal mobilization in requires containment to mitigate soil and .

Collectibility and Other Applications

Willemite is highly sought after by mineral collectors due to its vivid green under shortwave light, with specimens from the historic Franklin and Sterling Hill mines in being particularly prized for their intense glow and classic associations with and . These fluorescent examples often command prices ranging from $100 to over $1,000 at auctions and mineral shows, depending on , aesthetic , and luminescence strength, as seen in sales of cabinet-sized pieces from these localities. In , transparent willemite crystals are rarely faceted into small gems up to about 10 carats, prized for their vitreous luster and strong dispersion of approximately 0.027, though the mineral's brittleness (Mohs hardness 5.5) limits durability for jewelry wear. More commonly, it is cut into cabochons to showcase its color and while accommodating its fragility, with refractive indices ranging from 1.691 to 1.725 and of 0.028. Scientifically, willemite serves as a valuable subject in , particularly as a potential Rb-Sr geochronometer for nonsulfide deposits, enabling direct age determination of mineralization events. It also functions as a reference material in studies of mineral and crystal chemistry, where its consistent fluorescent response under UV light aids research on structures and formation processes. Culturally, willemite features prominently in educational displays and museum exhibits focused on mineral , such as those at the Franklin Mineral Museum and , where it illustrates the phenomenon of in ores and engages visitors in interactive UV-light demonstrations. Synthetic willemite, produced for laboratory use, supports ongoing research in these areas without relying on rare natural samples.

References

  1. https://rruff.geo.arizona.edu/doclib/hom/willemite.pdf
  2. https://www.mindat.org/min-4292.html
  3. https://d-nb.info/1269672126/34
  4. https://archive.org/details/systemofmineralo01danarich
  5. https://classicgems.net/gem_willemite.htm
  6. https://www.gemologyproject.com/wiki/index.php?title=Willemite
  7. https://www.fomsnj.org/PDF/Willemite_Dunn.pdf
  8. https://www.fluomin.org/uk/fiche.php?id=199
  9. https://rruff.geo.arizona.edu/doclib/MinMag/Volume_21/21-119-388.pdf
  10. https://www.kgg.org.uk/f-377h.html
  11. http://webmineral.com/data/Willemite.shtml
  12. https://classicgems.net/gem_troostite.htm
  13. https://www.mdpi.com/2075-163X/6/1/20
  14. https://pubs.usgs.gov/of/2013/1309/pdf/of2013-1309.pdf
  15. https://materials.springer.com/isp/crystallographic/docs/sd_0545098
  16. https://link.springer.com/content/pdf/10.1007/s40145-022-0607-1.pdf
  17. https://www.sciencedirect.com/science/article/abs/pii/S0925838816323015
  18. https://geologyscience.com/gemstone/willemite/
  19. https://www.sciencedirect.com/science/article/abs/pii/S0169136819307140
  20. https://pubs.geoscienceworld.org/segweb/economicgeology/article-abstract/98/4/819/22341/Formation-of-Willemite-in-Hydrothermal
  21. https://pubs.usgs.gov/bul/1135a/report.pdf
  22. http://www.geo.arizona.edu/~mdbarton/MDB_papers_pdf/Barton02_BeRiMG050.pdf
  23. https://www.sterlinghillminingmuseum.org/origin-of-the-ore-body
  24. https://link.springer.com/article/10.1007/s00126-017-0781-1
  25. https://pubs.usgs.gov/periodicals/mcs2024/mcs2024-zinc.pdf
  26. https://pubmed.ncbi.nlm.nih.gov/22302740/
  27. https://digitalfire.com/material/willemite
  28. https://www.sciencedirect.com/science/article/abs/pii/S0925346725009000
  29. https://www.sciencedirect.com/science/article/abs/pii/S1751616123003491
  30. https://pubs.usgs.gov/pp/0180/report.pdf
  31. https://dep.nj.gov/wp-content/uploads/njgws/techincal-publications-and-reports/bulletins-and-reports/bulletins/bulletin65.pdf
  32. https://pubs.geoscienceworld.org/ejm/eurjmin/article/31/5-6/983/571928/Distribution-of-trace-elements-in-willemite-from
  33. https://openresearch-repository.anu.edu.au/items/cc3a0b5b-13e5-4fce-b6cb-de32dc3454ea
  34. https://www.gemsociety.org/article/willemite-jewelry-and-gemstone-information/
  35. https://www.sterlinghillminingmuseum.org/fluorescence
  36. https://www.irocks.com/minerals/species/buy-willemite-fine-mineral-specimens-photos
  37. http://gemologyproject.com/wiki/index.php?title=Willemite
  38. https://www.sciencedirect.com/science/article/abs/pii/S016913680700090X
  39. https://fiercelynxdesigns.com/blogs/articles/willemite-the-fluorescent-zinc-silicate-mineral
  40. https://www.franklinmineralmuseum.org/fluorescent-minerals/
  41. https://www.gemrockauctions.com/learn/a-z-of-gemstones/willemite
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