Grossular
Grossular, Ca3Al2(SiO4)3, is the calcium aluminium garnet: hardness 6.5 to 7, density 3.594, index 1.734. Tsavorite, hessonite, and where katoite begins.
Grossular is the calcium aluminium garnet, Ca3Al2(SiO4)3: cubic, hardness 6.5 to 7, density 3.594, one refractive index of 1.734. Tsavorite and hessonite are its gem varieties, per the International Gem Society. Hydroxyl can replace silica: worked from the 2013 IMA garnet report, past 13.04% water it is katoite.
In short
- One species, three series. The September 2026 IMA-CNMNC Master List lists grossular as Ca3Al2(SiO4)3, status A, 1962 s.p., Russia. The Handbook of Mineralogy sheet files it in the garnet group, in series with andradite, with hibschite and katoite, and with uvarovite. Grew et al., American Mineralogist 98 (2013) is the IMA garnet report.
- The 1.5 line. Grew et al. write the hydrous garnets as {Ca3}[Al2](Si3−x□x)O12−4x(OH)4x and name them by the dominant-constituent rule at the Z site: grossular while silicon outnumbers vacancies, that is x below 1.5; katoite, Ca3Al2(OH)12, beyond. Worked, the line falls at 13.04% H2O by mass; pure katoite is 28.57%.
- Hibschite, a documented discrepancy. The sheet still names a series with hibschite. Grew et al. discredited hibschite in favour of grossular, and the IMA list has no hibschite row, by script. The sheet’s Georgetown, California analysis carries 0.30% H2O: worked, x = 0.04, 2.5% of the way to the line.
- Tsavorite and hessonite. The International Gem Society calls tsavorite the emerald-green variety of grossular and hessonite the yellow-orange to reddish orange variety. Grew et al.’s list of old and trade names decodes both as grossular, and the IMA list has a row for neither. Tsavorite from Lualenyi, Kenya reads 1.743 and SG 3.61 (gem society): 14.46 g in water for 20.00 g.
- The Y-site ladder. By script, 6 IMA rows share the formula Ca3Y2(SiO4)3 with one element at Y: grossular (Al), andradite (Fe3+), uvarovite (Cr), goldmanite (V3+), rubinite (Ti3+) and eringaite (Sc). On their sheets grossular reads 1.734 and 14.44 g, andradite 1.887 and 14.74 to 14.87 g, uvarovite 1.865 and 14.69 to 14.75 g.
| Name | What it is (source) | IMA row? | H2O by mass | Refractive index | Density; 20.00 g piece in water |
|---|---|---|---|---|---|
| Grossular, Ca3Al2(SiO4)3 | End-member, x = 0 (Grew) | Yes: A, 1962 s.p. | 0 (ideal) | 1.734 (sheet) | 3.594; 14.44 g |
| Georgetown, California analysis | Sheet analysis (1); IGS gives colourless Georgetown grossular RI 1.737, SG 3.506 | As grossular | 0.30% printed; x = 0.04, worked | 1.737 (IGS) | 3.506 (IGS); 14.30 g |
| x = 1.5: the line | Silicon equals vacancies at Z: grossular below, katoite above (Grew) | - | 13.04%, worked | - | - |
| Katoite, Ca3Al2(OH)12 | Vacancy-dominant at Z (Grew) | Yes: A, 1982-080, Italy | 28.57%, worked | No sheet read | No sheet read |
| Hibschite | Discredited in favour of grossular (Grew); still named on the sheet’s series line | No | x below 1.5 (Grew) | - | - |
| Hydrogrossular | Grossular-katoite (Grew, Appendix 2); for appreciable OH of undetermined ratio (Dunn et al. 1985, quoted by Grew) | No | Not fixed | - | - |
| Transvaal jade | Grossular (Grew, Appendix 2); green, massive hydrogrossular from South Africa (IGS) | No | Not stated | 1.728 (IGS) | 3.488 (IGS); 14.27 g |
| Tsavorite | Emerald-green grossular (IGS); grossular (Grew, Appendix 2) | No | Not stated | 1.743 (IGS, Lualenyi) | 3.61 (IGS); 14.46 g |
| Hessonite; cinnamon stone | Yellow-orange to reddish orange grossular (IGS); both grossular (Grew, Appendix 2) | No | Not stated | 1.730 to 1.757 (IGS) | 3.4 to 3.71 (IGS); 14.12 to 14.61 g |
| Andradite, Ca3Fe3+2(SiO4)3 | Y = Fe3+; series with grossular (sheet) | Yes: G, 1868, Norway | 0 (ideal) | 1.887 (sheet) | 3.8 to 3.9; 14.74 to 14.87 g |
| Uvarovite, Ca3Cr2(SiO4)3 | Y = Cr; series with grossular (sheet) | Yes: A, 1967 s.p., Russia | 0 (ideal) | 1.865 (sheet) | 3.77 to 3.81; 14.69 to 14.75 g |
What is grossular? The calcium aluminium garnet
Grossular is the garnet with calcium at the X site and aluminium at the Y site, Ca3Al2(SiO4)3. The September 2026 IMA-CNMNC Master List gives it status A, 1962 s.p., with Russia as the country. On standard atomic weights the formula mass is 450.44 and the composition 37.35% CaO, 22.64% Al2O3 and 40.02% SiO2; the sheet’s ideal column prints 37.35, 22.63 and 40.02. GIA’s garnet description names grossular, also called grossularite, as one of five garnet species that are commercially important as gems. Grew et al.’s Appendix 2 decodes grossularite as grossular.
What the sheet records. Cubic, point group 4/m -3 2/m, commonly in dodecahedra or trapezohedra up to 15 cm with striated faces, and also granular, compact and massive. Parting on {110} is rarely observed; fracture uneven to conchoidal; brittle. Hardness 6.5 to 7; density 3.594 measured and calculated. Isotropic with weak strain birefringence, n = 1.734. Its colours run from yellow-green and green through golden yellow, pink, red and orange to colourless, white, grey and black, and may be sectored. The sheet says it may fluoresce weak golden yellow under longwave or shortwave UV; the International Gem Society says usually none. The lamp question is on fluorescent minerals explained.
Agreement with the other garnet pages. At 1.734 and 14.44 g in water for 20.00 g, grossular sits between pyrope (1.714, 14.42 g) and spessartine (1.800, 15.23 g); almandine reads 1.830 and 15.37 g. These are the same figures the how to tell if garnet is real page uses; its 1.71 line is not repeated here. The pink-grossular question is on is rhodonite a garnet.
Where does grossular end? The 1.5 line
The 1.5 line: a grossular in which hydroxyl groups have replaced half of the silicate groups is no longer grossular but katoite, and worked on the formula that point is 13.04% water by mass. Grew et al., American Mineralogist 98 (2013) write the hydrous calcium-aluminium garnets as {Ca3}[Al2](Si3−x□x)O12−4x(OH)4x, where □ is a vacancy and x lies between 0 and 3. Under the dominant-constituent rule a garnet is grossular while silicon exceeds vacancies at the Z site, so x below 1.5, and katoite, {Ca3}[Al2](□3)(OH)12, when vacancies dominate. They add that for the majority of these garnets x is below 1.5.
Worked. Each unit of x removes one SiO4 and adds (OH)4, two formula units of water. At x = 1.5 the formula Ca3Al2Si1.5O6(OH)6 has a mass of 414.36, of which 3 H2O is 54.05: 13.04%. At x = 0.5 the figure is 4.11%, at x = 1.0 8.45%, and pure katoite, x = 3, is 28.57%. The IMA list has a katoite row, Ca3Al2(OH)12, A, 1982-080, Italy.
Applied to the sheet. The Georgetown, Placer Co., California analysis prints 37.10% CaO and 0.30% H2O. Normalised to Ca 3.00 as the sheet’s formula is, that is 0.076 H2O per formula unit; if all of it sits as hydrogarnet groups, x = 0.04, 2.5% of the way to the line. The sheet’s own formula line places silicon 2.97 and aluminium 0.03 at Z and writes no hydrogen, so the worked x is an upper bound under that assumption, not a measured vacancy count. Georgetown grossular is grossular on either reading.
Hibschite and hydrogrossular: names the 2013 report retired
A documented discrepancy: the grossular sheet names a series with hibschite, a name the IMA garnet report discredited. Grew et al. record that hibschite was approved as a name for OH-bearing grossular with x below 1.5, because it had priority over plazolite and hydrogrossular; but that it is not distinct from grossular under the dominant-constituent rule, so hibschite is discredited in favour of grossular. A script over the September 2026 IMA list finds no row for hibschite, hydrogrossular or plazolite. We follow the report and print the sheet as it stands.
Hydrogrossular survives as a loose term. Grew et al. quote Dunn et al. (1985) as allowing the name hydrogrossular for members of the series with appreciable OH but undetermined SiO4/(OH)4 ratio, and their Appendix 2 decodes hydrogrossular as grossular-katoite. The International Gem Society’s hydrogrossular article writes the formula Ca3Al2(SiO4)3−x(OH)4x, says it is never transparent, and that it is often used as a jade substitute.
What a label should say. A garnet labelled hibschite is grossular if its x is below 1.5 and katoite if above; without an analysis, hydrogrossular is the honest word. Labelling practice is on how to label and catalogue a mineral collection.
Is tsavorite grossular? Tsavorite, hessonite and Transvaal jade
Tsavorite and hessonite are grossular under gem names. The International Gem Society tsavorite article calls tsavorite an emerald-green variety of grossular, named after Tsavo East National Park in Kenya and first discovered in Tanzania in 1967, and gives Lualenyi, Kenya material RI 1.743 and SG 3.61. Its hessonite article calls hessonite, also known as cinnamon stone, the yellow-orange to reddish orange variety of grossular, RI 1.730 to 1.757 and SG 3.4 to 3.71. Grew et al.’s Appendix 2 decodes tsavorite, tsavolite, hessonite, essonite and cinnamon stone as grossular, and cites Bordage et al. (2010) on a grossular of the tsavorite variety containing 0.14 V per formula unit. GIA says Campbell Bridges mined tsavorite in Kenya near Tsavo National Park.
Transvaal jade is a garnet, not jade. Hobbs, Gems and Gemology, Spring 1982 lists Transvaal jade and South African jade among the misleading jade names for grossularite, that is grossular, and Grew et al. decode Transvaal jade as grossular. The International Gem Society calls it green, massive hydrogrossular from South Africa, n = 1.728 and SG 3.488: 14.27 g in water for 20.00 g, 0.10 g above the heaviest jadeite on its sheet (3.43, 14.17 g). Its index is 0.036 above jadeite’s highest γ, 1.692, so a refractometer separates them where the balance barely does. Jade itself is on is jade a mineral, and its look-alikes on how to tell if jade is real.
Where the varieties come from, without guessing. The sheet’s Distribution names gems from the Merelani Hills, southeast of Arusha, Tanzania, and the Taita Hills, near Voi, Kenya; it does not say which variety each produced, and neither does this page. Colour causes for tsavorite and hessonite are not given here.
Grossular, andradite, uvarovite: the Y-site ladder
Six IMA rows share the formula Ca3Y2(SiO4)3 and differ only in the element at Y. A script over the September 2026 list finds grossular (Al), andradite (Fe3+), uvarovite (Cr), goldmanite (V3+), rubinite (Ti3+) and eringaite (Sc). The grossular sheet names series with two of them, andradite and uvarovite. On their sheets andradite reads n = 1.887 and density 3.8 to 3.9, 14.74 to 14.87 g in water for 20.00 g; uvarovite 1.865 and 3.77 to 3.81, 14.69 to 14.75 g.
Georgetown is near the aluminium end. The sheet’s Georgetown formula prints Al 1.92 and Fe3+ 0.05 at Y: aluminium is 97.5% of the site, worked. Naming by the dominant X-site cation is set out on the spessartine page; in these six rows X is calcium throughout, so the dominant Y cation is what differs between them.
Mali garnet sits between two of them. The International Gem Society says Mali garnet is a blend of grossular and andradite, predominantly grossular, with a refractive index of 1.77 in most specimens; Grew et al. decode Mali garnet and grandite as grossular-andradite. Which end a given stone is nearer needs an analysis; we do not interpolate a composition from its index.
Where grossular is found
The Occurrence field: in contact and regionally metamorphosed calcareous rocks, or rocks which have undergone calcium metasomatism; in some schists and serpentinites. Associates are calcite, dolomite, epidote, clinozoisite, wollastonite, scapolite, vesuvianite, diopside, tremolite and quartz.
The Distribution field says many localities, even for fine crystals and gem material, including: near Chernyshevsk, at the confluence of the Vilyui and Akhtaragda Rivers, Yakutia, Russia; the Ala Valley, Piedmont, Italy; the Zillertal, Tirol, Austria; Ocna de Fier, Romania; the Jeffrey mine, Asbestos, Quebec; Standish, Maine, and the Belvidere Mountain quarries, Lowell, Vermont; Crestmore, the Old Cosumnes copper mine, Mazourka Canyon and Santa Rosa, California, among others; Vesper Peak, Washington; large crystals from Xalostoc, Morelos, and the Sierra de la Cruces, Coahuila, Mexico; gems from the Merelani Hills, Tanzania; and the Taita Hills, near Voi, Kenya. No British locality is on the grossular sheet.
Cross-checks. The International Gem Society’s pink grossular in marble from Mexico, called xalostocite and landerite, matches the sheet’s Xalostoc, and Grew et al. decode both names as grossular. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.