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Grossular
Grossular
Grossular
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Grossular
Grossular dodecahedron, 7 mm across, from Coahuila, Mexico
General
CategoryNesosilicate
FormulaCa3Al2(SiO4)3
IMA symbolGrs[1]
Strunz classification9.AD.25
Crystal systemCubic[2]
Crystal classHexoctahedral (m3m)
H-M Symbol: (4/m 3 2/m)
Space groupIa3d
Identification
Colorlight to dark green, light to dark yellow to reddish brown, brown, orange, red, yellow, green, white, occasionally translucent to opaque pink. It is also but rarely found in colorless form[2]
Cleavagenone
Fractureconchoidal to uneven[2]
Mohs scale hardness6.5 to 7[2]
Lustergreasy to vitreous[2]
StreakBrown
Specific gravity3.61 (+.15/−.04)
Polish lustervitreous[2]
Optical propertiesSingle refractive, often anomalous double refractive[2]
Refractive index1.740 (+.12/−.04)[2]
Birefringencenone
Pleochroismnone
Dispersion.028
Ultraviolet fluorescencenear colorless to light green – inert to weak orange in longwave and weak yellow-orange in shortwave; yellow – inert to weak orange in longwave and shortwave[2]
Absorption spectraHessonite sometimes shows bands at 407 and 430 nm
Major varieties
Hessoniteyellow-red to reddish-orange
Tsavoriteintense green to yellowish green
Leuco-garnettransparent and colorless[3]
Rosolitetranslucent to opaque pink grossularite crystals in marble from Mexico

Grossular is a calcium-aluminium species of the garnet group of minerals. It has the chemical formula of Ca3Al2(SiO4)3 but the calcium may, in part, be replaced by ferrous iron and the aluminium by ferric iron. The name grossular is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia. Other shades include cinnamon brown (cinnamon stone variety), red, and yellow. Grossular is a gemstone.

In geological literature, grossular has often been called grossularite. Since 1971, however, use of the term grossularite for the mineral has been discouraged by the International Mineralogical Association.[4]

Hessonite

[edit]
Striated crystals of hessonite, a variety of the grossular species

Hessonite or "cinnamon stone" is a common variety of grossular with the general formula: Ca3Al2Si3O12. The name comes from the Ancient Greek: ἣσσων (hēssōn), meaning inferior;[5] an allusion to its lower hardness and lower density than most other garnet species varieties.[2]

It has a characteristic red color, inclining to orange or yellow, much like that of zircon. It was shown many years ago, by Sir Arthur Herbert Church, that many gemstones, especially engraved gems (commonly regarded as zircon), were actually hessonite. The difference is readily detected by the specific gravity, that of hessonite being 3.64 to 3.69, while that of zircon is about 4.6. Hessonite has a similar hardness to that of quartz (being about 7 on the mohs scale), while the hardness of most garnet species is nearer 7.5.[5]

Hessonite comes chiefly from Sri Lanka and India, where it is found generally in placer deposits, though its occurrence in its native matrix is not unknown.[5] It is also found in Brazil and California.

Deposits

[edit]

Grossular is found in contact metamorphosed limestones with vesuvianite, diopside, wollastonite and wernerite.

A highly sought after variety of gem garnet is the fine green Grossular garnet from Kenya and Tanzania called tsavorite. This garnet was discovered in the 1960s in the Tsavo area of Kenya, from which the gem takes its name.

Viluite is a variety name of grossular; that is not a recognized mineral species.[6] It is usually olive green though sometimes brownish or reddish, brought about by impurities in the crystal. Viluite is found associated with and is similar in appearance to vesuvianite, and there is confusion in terminology as viluite has long been used as a synonym for wiluite, a sorosilicate of the vesuvianite group. This confusion in nomenclature dates back to James Dwight Dana.[7] It comes from the Vilyuy river area in Siberia. A similar green grossular garnet can be found in the Wah Wah mountain range in Utah.[8]

Grossular is known by many other names, and also some misnomers;[9] colophonite – coarse granules of garnet[10] (was later identified as a variety of andradite), ernite, gooseberry-garnet – light green colored and translucent,[11] olyntholite/olytholite, romanzovite, and tellemarkite. Misnomers include[3] South African jade, garnet jade, Transvaal jade, and African jade.

Cultural significance

[edit]

In 1991, Vermont named grossular garnet its state gemstone.[12]

See also

[edit]

References

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

Grossular

View on Grokipedia
from Grokipedia
Grossular is a calcium-aluminum belonging to the group, with the Ca₃Al₂(SiO₄)₃. It crystallizes in the cubic system, typically forming dodecahedral or trapezohedral crystals, and exhibits a vitreous to resinous luster, a Mohs of 6.5–7, and a specific of approximately 3.59. Grossular is notable for its wide color range, from colorless and white (when pure) to shades of green, yellow, orange, red, brown, pink, and even black, influenced by trace impurities such as iron, , and ; it shows the broadest color spectrum among species. As a member of the series, grossular forms complete solid solutions with (Ca₃Fe₂Si₃O₁₂) and partial ones with and , but it is end-member pure in many specimens. It lacks true cleavage, instead parting irregularly on {110}, and displays an uneven to , with brittle tenacity. Optically isotropic with a of 1.734, grossular is transparent to opaque and may fluoresce weakly golden yellow under light. Grossular primarily occurs in contact and regional metamorphic environments, including marbles, calc-silicate rocks, schists, and serpentinites derived from calcium-rich protoliths, often associated with minerals like , dolomite, , , , and . Notable gem varieties include tsavorite, a vivid form colored by and , prized for its emerald-like hue and sourced mainly from East African deposits in and ; and hessonite, the orange to brownish variety due to iron, historically known as "cinnamon stone." These varieties, along with colorless or pale types, are valued in jewelry for their and brilliance, though grossular has limited industrial use compared to other garnets like .

Nomenclature and varieties

Etymology

The name grossular derives from the Latin grossularia, the botanical name for the (Ribes grossularia), owing to the greenish hue of its typical variety that resembles the fruit. German mineralogist first described the mineral in 1803 from specimens collected in , initially naming the brownish-orange form Kanelstein ( stone) in reference to its color. In 1808, Werner renamed it grossularite to encompass the green specimens more accurately, later shortened to grossular. This naming distinguished it within the broader group—named from Latin granatus for seeds due to crystal shape— as the primary calcium-aluminum end-member among related . Historical variations like "cinnamon stone" persisted for the hessonite-like variety before formal classification, highlighting early recognition of its color diversity. Later, specific varieties such as received names tied to their discovery sites, like Tsavo National Park in 1967.

Principal varieties

Grossular, a calcium-aluminum in the group, exhibits a wide range of colors due to trace impurities, leading to several principal varieties distinguished by their hues, transparency, and gemological characteristics. These varieties include hessonite, , hydrogrossular, and lesser-known forms such as leucogarnet, rosolite, and yellowish types, each colored primarily by iron, , , or substitutions. Unlike magnesium-rich garnets like , which are typically deep red and found in kimberlites, grossular varieties are calcium-dominant and occur in metamorphic or environments, avoiding confusion with due to their distinct chemistry and lighter tones. Hessonite, also known as "cinnamon stone," is the orange-brown variety of grossular, characterized by its translucent to transparent appearance and warm, honey-like glow often described as resembling "whiskey in water." The color arises from iron (Fe³⁺) impurities, with higher iron content increasing the reddish tones. It has a Mohs hardness of approximately 7, slightly softer than some other garnets, and a ranging from 1.74 to 1.75, making it suitable for into gems despite occasional inclusions. Historically prized in ancient jewelry, hessonite's value depends on color saturation, with vivid orange specimens commanding higher prices. Tsavorite represents the vivid green, transparent variety of grossular, prized for its emerald-like brilliance and high clarity. The intense green hue results from substitutions of and , with vanadium often predominant (V₂O₃:Cr₂O₃ ratio >1:1). Discovered in near Komolo in Tanzania's Lelatema Hills and later in , it was named after Tsavo National Park by . With a of 6.5–7.5 and of 1.731–1.754, is durable for jewelry and highly valued, often fetching $400–$2,000 or more per carat for fine, eye-clean stones over 1 carat. Hydrogrossular is the massive, opaque to translucent green variety, featuring hydroxyl (OH) substitutions in its structure that distinguish it within the grossular-katoite series. Its green color stems from iron or traces, often with black inclusions giving a speckled appearance resembling . Primarily sourced from , it is too opaque for faceting but valued for carvings, beads, and cabochons due to its 6.5–7 hardness and affordability. Other notable varieties include leucogarnet, a colorless form representing nearly pure grossular without significant impurities, ideal for transparent gems; rosolite, a rare pinkish variety colored by ; and yellowish types, sometimes referred to as Merelani grossular, resulting from minor iron content. These less common forms highlight grossular's color spectrum from iron (yellow-orange), chromium-vanadium (green), to (pink), but they are generally lower in demand compared to hessonite and .

Crystal structure and properties

Crystal structure

Grossular crystallizes in the isometric ( with Ia\overline{3}d (No. 230). The unit cell is cubic with a lattice parameter a ≈ 11.85 and contains eight units (Z = ). In this structure, calcium cations occupy 24 dodecahedral sites, aluminum cations fill 16 octahedral sites forming [AlO6] polyhedra, and silicon occupies 24 tetrahedral sites as [SiO4] tetrahedra, all interconnected by shared oxygen anions to form a three-dimensional framework. A notable variation occurs in hydrogrossular, where partial substitution of [SiO4]4- by [(OH)4]4- (or H4O4) disrupts the regular tetrahedral arrangement, often resulting in lower symmetry and a massive, non-crystalline habit rather than well-formed crystals. Grossular typically exhibits dodecahedral or trapezohedral crystal habits, though it can also form granular masses or compact aggregates. This structural arrangement contributes to its characteristic hardness of 6.5–7 on the Mohs scale.

Physical and chemical properties

Grossular is a member of the group with the ideal Ca₃Al₂(SiO₄)₃, representing the calcium-aluminum end-member composition. Minor substitutions, such as Fe³⁺ replacing Al³⁺ at the octahedral site, can occur in natural specimens, though these are typically limited and do not significantly alter the core structure. The mineral exhibits a Mohs of 6.5 to 7, which, combined with its good toughness, renders it suitable for applications despite its brittleness. Its specific gravity ranges from 3.59 to 3.64, reflecting the dense packing of its isometric crystal structure. Grossular displays a vitreous to resinous luster and produces a white streak when tested on an unglazed plate. Grossular lacks true cleavage, instead fracturing conchoidally to unevenly under stress. It demonstrates high stability. Chemically, grossular is insoluble in common acids such as , though hydrogrossular varieties, which incorporate hydroxyl groups, show increased reactivity due to their hydrated structure.

Optical properties

Grossular, a member of the group, exhibits isotropic optical behavior typical of cubic crystals, with no under standard conditions, though weak strain birefringence may occur due to internal stresses. Its refractive index ranges from 1.731 to 1.754, varying by composition and variety; for instance, the chromium-bearing variety often shows values around 1.743, slightly higher due to the influence of Cr³⁺ ions. The dispersion of grossular is moderate at 0.028, contributing to subtle in faceted stones but less pronounced than in other garnets. is generally absent, but weak effects can appear in colored varieties, such as a slight yellow-green shift in green when viewed along different axes. Absorption spectra in grossular depend on trace elements: green varieties like display characteristic chromium-related bands near 440 nm, causing broad absorption below approximately 490 nm, while iron-influenced brown specimens show bands around 530 nm. is typically inert or weak, producing a golden yellow to orange-red glow under long-wave or short-wave light in some specimens, particularly those with or impurities. In gemological testing, grossular is distinguished from similar green garnets like demantoid (andradite variety) by its lower dispersion (0.028 versus 0.057), resulting in reduced fiery scintillation, alongside confirmatory and lack of characteristic horsetail inclusions.

Geological occurrence

Formation

Grossular primarily forms through contact or regional of impure limestones or siliceous sediments, where calcium-rich protoliths react with silica and alumina under elevated temperatures and pressures. In these settings, particularly in skarns developed at the margins of igneous intrusions, grossular crystallizes as part of calc-silicate assemblages, often alongside minerals such as , , , and . These processes typically occur at temperatures between 500 and 700°C and low to medium pressures (generally below 7 kbar), with hydrothermal fluids playing a key role in facilitating metasomatic reactions that concentrate calcium and aluminum. Secondarily, grossular can form in bodies or altered basic rocks through , where fluids rich in calcium and silica interact with ultramafic or protoliths during serpentinization or rodingitization. As the calcium-aluminum end-member of the group, grossular is characteristic of these calc-silicate environments, stabilizing in assemblages that reflect the availability of CaO and Al₂O₃ under the prevailing conditions. Varieties like hydrogrossular may develop in such altered, low-temperature hydrothermal settings.

Major deposits

Grossular is primarily extracted from metamorphic deposits worldwide, with significant concentrations in regions of contact metamorphism. The most prominent sources of gem-quality grossular, particularly the vibrant green variety, are located in , especially and . Mining in the Merelani Hills of northern Tanzania began in the 1970s following the discovery of tsavorite in 1967 near Tsavo in Kenya, and these areas remain the epicenter of production. Kenya's tsavorite output from sites like the Mwatate region drives much of the global supply. In , the Jeffrey Mine in (now ), , has historically yielded exceptional grossular specimens, including hydrogrossular and green varieties, from its asbestos operations that ceased in 2001 but continue to provide collector material. The mine's skarn-hosted deposits produced lustrous, gemmy crystals up to several centimeters, renowned for their clarity and color range from peach to deep green. South Africa's Transvaal region, particularly around , is a key locality for hydrogrossular, a massive, translucent variety often marketed as "Transvaal jade" due to its jade-like appearance in green and gray-green hues. These deposits, formed in serpentinized ultramafic rocks, have been exploited since the early for ornamental stone. Other notable deposits include hessonite grossular from Sri Lanka's gem gravels and India's and regions, where cinnamon-brown crystals are recovered from alluvial sources. In , the Vilui River Basin in produces the olive- viluite variety, a less common grossular type linked to alkaline intrusions. In the United States, occurrences in California's Inyo County and Vermont's Belvidere Mountain Quarries yield diverse grossular forms, including transparent and orange crystals from asbestos-associated skarns. As of 2025, sustainable mining initiatives in East Africa's districts continue to gain momentum, including efforts by organizations like Pact to improve artisanal practices, reduce environmental impact, and enhance community health in and Tanzania's operations.

Applications and cultural significance

Gemstone and industrial uses

Grossular garnet, particularly its transparent varieties such as and hessonite, is highly valued in jewelry for faceting into gems that showcase their vibrant colors and brilliance. , with its intense green hue, is typically cut into faceted shapes like ovals and cushions to maximize light return, making it suitable for high-end pieces such as engagement rings and necklaces. Hessonite, known for its warm orange to reddish tones, is also faceted but often exhibits a distinctive "cinnamon stone" effect due to inclusions, which adds character without detracting from its appeal in earrings and pendants. The value of grossular gems is primarily determined by color intensity, clarity, and size, with premium examples commanding higher prices. For , vivid green shades free of brownish tones are most desirable, while clarity is generally good in stones under 2 carats, though inclusions like pockets can appear in larger specimens; prices typically range from $400 to $2,000 per carat for faceted stones of 1-2 carats, escalating significantly for rarer sizes over 2 carats. Hessonite values emphasize rich, unbrownish orange hues and moderate clarity, as eye-visible inclusions are common but acceptable, with prices generally lower than due to greater availability. Opaque varieties, such as those with or translucency, are often cut as cabochons to highlight surface effects, suitable for bold statement jewelry. In industrial applications, grossular garnet contributes to abrasives and , leveraging its of 6.5-7.5 on the for effective material removal without excessive wear. Since the late 19th century, grossular has been used alongside other -group minerals in abrasive powders for grinding and polishing metals and , though remains more dominant. In , abrasives, including grossular varieties, are propelled with high-pressure water to precisely slice metals, stone, and composites in , prized for their durability and low dust production. Grossular also finds niche use in refractories for , where its calcium-aluminum composition provides thermal stability in high-temperature environments like furnace linings. Ornamental applications highlight hydrogrossular, the massive, translucent to opaque variety often resembling , which is carved into decorative objects such as spheres, skulls, and figurines to exploit its mottled green appearance and mid-range hardness. These carvings, sourced mainly from , serve as affordable jade substitutes in sculptures and inlays. Hydrogrossular is also fashioned into beads for necklaces and bracelets, valued for their earthy tones and durability in strung jewelry; historically, grossular varieties have been used in intaglios and cameos for seals and adornments since ancient times. Market trends for grossular reflect growing demand in the colored gem sector, with the global market projected to expand from USD 737 million in to USD 1.07 billion by 2033 at a CAGR of about 4.8%, driven by jewelry and industrial needs. , as a premium grossular variety, sees heightened interest in ethical sourcing from East African mines, where initiatives promote traceability to ensure conflict-free origins and community benefits. technology is increasingly adopted for supply chains, including , to provide verifiable provenance from mine to market, enhancing trust and supporting sustainable practices. Treatments for grossular are rare, with most gems entering the market untreated to preserve their natural appeal. , applied sparingly to hessonite, involves controlled heating to intensify orange hues and reduce inclusions, resulting in more vibrant colors that are stable and undetectable without advanced testing. Such enhancements are uncommon for , which is prized for its untreated, vivid green.

Cultural significance

Grossular garnet has been utilized in historical artifacts, particularly in ancient . Archaeological evidence from reveals worked , including varieties of grossular such as hessonite, used in beads and seals as part of Indian Ocean maritime trade networks dating back to antiquity. Similarly, grossular sourced from Indian and deposits featured in Hellenistic-Roman jewelry and carvings, highlighting its early cultural value in decorative and trade contexts. During the , hessonite grossular appeared in European jewelry, often set in gold rings, while the yellow variety was known as "hyacinth" or "cinnamon stone" for its warm hues reminiscent of spices from its origins. In modern times, grossular holds symbolic status as the state gem of , , designated in 1991 to honor local green varieties from deposits near Eden Mills, symbolizing the region's geological heritage. In crystal healing practices, grossular is associated with abundance, manifestation, and emotional balance, particularly through its connection to the ; green forms like are linked to prosperity and renewal. , the vivid green grossular from , evokes national pride in and as a rare local treasure discovered in the , often incorporated into contemporary jewelry reflecting regional identity. Grossular's collectibility extends to museums worldwide, where specimens showcase its color diversity, from the transparent crystals at the Smithsonian to those highlighting Vermont's rodingite-hosted varieties. Unlike or other red garnets with deep mythological associations in ancient lore, grossular lacks prominent ties to legends, emphasizing instead its role in practical and symbolic human endeavors. In 2025 bridal trends, garnets appear in sustainable, colored-stone designs, blending tradition with modern elegance for engagements and weddings.

References

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  11. https://www.handbookofmineralogy.org/pdfs/grossular.pdf
  12. https://www.mdpi.com/2075-163X/9/11/691
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