| Grossular | |
|---|---|
 Grossulardodecahedron, 7 mm across, fromCoahuila, Mexico | |
| General | |
| Category | Nesosilicate |
| Formula | Ca3Al2(SiO4)3 |
| IMA symbol | Grs[1] |
| Strunz classification | 9.AD.25 |
| Crystal system | Cubic[2] |
| Crystal class | Hexoctahedral (m3m) H-M Symbol: (4/m3 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 |
| Ultravioletfluorescence | 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 inmarble from Mexico |
Grossular is acalcium-aluminium species of thegarnet group ofminerals. It has thechemical formula of Ca3Al2(SiO4)3 but the calcium may, in part, be replaced by ferrousiron and the aluminium by ferric iron. The name grossular is derived from thebotanical name for thegooseberry,grossularia, in reference to the green garnet of this composition that is found inSiberia. Other shades includecinnamon brown (cinnamon stone variety), red, and yellow. Grossular is agemstone.
In geological literature, grossular has often been calledgrossularite. Since 1971, however, use of the term grossularite for the mineral has been discouraged by theInternational Mineralogical Association.[4]
Hessonite or "cinnamon stone" is a common variety of grossular with the generalformula: Ca3Al2Si3O12. The name comes from theAncient Greek:ἣσσων (hēssōn), meaninginferior;[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 ofzircon. It was shown many years ago, by SirArthur Herbert Church, that manygemstones, especiallyengraved gems (commonly regarded as zircon), were actually hessonite. The difference is readily detected by thespecific 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 ofquartz (being about 7 on themohs scale), while the hardness of most garnet species is nearer 7.5.[5]
Hessonite comes chiefly fromSri Lanka and India, where it is found generally inplacer deposits, though its occurrence in its native matrix is not unknown.[5] It is also found inBrazil andCalifornia.
Grossular is found incontact metamorphosedlimestones withvesuvianite,diopside,wollastonite andwernerite.
A highly sought after variety of gem garnet is the fine green Grossular garnet fromKenya andTanzania calledtsavorite. This garnet was discovered in the 1960s in theTsavo 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 tovesuvianite, and there is confusion in terminology asviluite has long been used as a synonym forwiluite, asorosilicate of the vesuvianite group. This confusion in nomenclature dates back toJames Dwight Dana.[7] It comes from theVilyuy river area inSiberia. A similar green grossular garnet can be found in theWah Wah mountain range in Utah.[8]
Grossular is known by many other names, and also somemisnomers;[9]colophonite – coarse granules of garnet[10] (was later identified as a variety ofandradite),ernite,gooseberry-garnet – light green colored and translucent,[11]olyntholite/olytholite,romanzovite, andtellemarkite. Misnomers include[3]South African jade,garnet jade,Transvaal jade, andAfrican jade.
In 1991,Vermont named grossular garnet itsstate gemstone.[12]
One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Chisholm, Hugh, ed. (1911). "Cinnamon-stone".Encyclopædia Britannica. Vol. 6 (11th ed.). Cambridge University Press. p. 376.