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.

Grossular
Grossular dodecahedron, 7 mm across, from Coahuila, Mexico
General
CategoryNesosilicate
Formula
(repeating unit)		Ca3Al2(SiO4)3
Strunz classification9.AD.25
Crystal systemCubic[1]
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[1]
Cleavagenone
Fractureconchoidal to uneven[1]
Mohs scale hardness6.5 to 7[1]
Lustergreasy to vitreous[1]
StreakBrown
Specific gravity3.61 (+.15 -.04)
Polish lustervitreous[1]
Optical propertiesSingle refractive, often anomalous double refractive[1]
Refractive index1.740 (+.12 -.04)[1]
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[1]
Absorption spectraHessonite sometimes shows bands at 407 and 430nm
Major varieties
Hessoniteyellow-red to reddish-orange
Tsavoriteintense green to yellowish green
Leuco-garnettransparent and colorless[2]
Rosolitetranslucent to opaque pink grossularite crystals in marble from Mexico

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.[3]

Hessonite
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;[4] an allusion to its lower hardness and lower density than most other garnet species varieties.[1]

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.

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. It is also found in Brazil and California.

Deposits

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.[5] 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.[6] It comes from the Vilyuy river area in Siberia.

Grossular is known by many other names, and also some misnomers;[7] colophonite – coarse granules of garnet,[8] ernite, gooseberry-garnet – light green colored and translucent,[9] olyntholite/olytholite, romanzovite, and tellemarkite. Misnomers include[2] South African jade, garnet jade, Transvaal jade, and African jade.

Cultural significance

Vermont has grossular garnet as its state gemstone.[10]

See also

References
  1. GIA Gem Reference Guide; Gemological Institute of America; 1995; ISBN 0-87311-019-6
  2. Grossular The Mineral and Gemstone Kingdom, accessed online January 25, 2007
  3. International Mineralogical Association (1971). "International Mineralogical Association: Commission on new minerals and mineral names" (PDF). Mineralogical Magazine. 38: 102–105. doi:10.1180/minmag.1971.038.293.14.
  4. "Cinnamon-stone" . Encyclopædia Britannica. 6 (11th ed.). 1911. p. 376.
  5. Viluite Mindat database
  6. The System of Mineralogy of James Dwight Dana. Descriptive Mineralogy By James Dwight Dana, Edward Salisbury Dana, 1892, p. 479-80
  7. Grossular Mindat mineral database, accessed January 25, 2007
  8. Colophonite The Free Dictionary, accessed online January 25, 2007
  9. Gooseberry Garnet WordWeb Online
  10. "Vermont Emblems". State of Vermont. Archived from the original on 2009-10-29. Retrieved 2009-11-12.
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