Grossular

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.

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Grossular
Grossular dodecahedron, 7 mm across, from Coahuila, Mexico
General
Category	Nesosilicate
Formula (repeating unit)	Ca3Al2(SiO4)3
IMA symbol	Grs[1]
Strunz classification	9.AD.25
Crystal system	Cubic[2]
Crystal class	Hexoctahedral (m3m) H-M Symbol: (4/m 3 2/m)
Space group	Ia3d
Identification
Color	light 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]
Cleavage	none
Fracture	conchoidal to uneven[2]
Mohs scale hardness	6.5 to 7[2]
Luster	greasy to vitreous[2]
Streak	Brown
Specific gravity	3.61 (+.15/−.04)
Polish luster	vitreous[2]
Optical properties	Single refractive, often anomalous double refractive[2]
Refractive index	1.740 (+.12/−.04)[2]
Birefringence	none
Pleochroism	none
Dispersion	.028
Ultraviolet fluorescence	near 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 spectra	Hessonite sometimes shows bands at 407 and 430 nm
Major varieties
Hessonite	yellow-red to reddish-orange
Tsavorite	intense green to yellowish green
Leuco-garnet	transparent and colorless[3]
Rosolite	translucent to opaque pink grossularite crystals in marble from Mexico

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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 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.

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.

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

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  Color range of grossular graphically displayed at the National Museum of Natural History Washington, D.C.
• 
  Group of Grossular crystals, largest 1.1 cm, from Val-des-Sources, Quebec

• Hydrogrossular