What is Hydrogrossular?
Hydrogrossular is a hydrated grossular-range garnet material, commonly massive and coloured white, green, pink or grey.
Mineralogy and structure
Reference formula: Ca3Al2(SiO4)3-x(OH)4x. Hydrogrossular is a compositionally hydrated member of the grossular range rather than a duplicate of grossular. The formula expresses a solid solution, so water and hydroxyl content can vary. Hydrogrossular belongs to the silicates and has a cubic crystal system. Typical hardness is 6–7 on the Mohs scale and specific gravity is approximately 3.3–3.6.
Colour and appearance
Typical colours include white, green, pink, grey. The usual granular, massive, botryoidal habit and vitreous, resinous lustre are useful descriptive features, but colour alone is not diagnostic.
Formation and localities
Hydrogrossular forms in or is associated with serpentinites, skarns and calcium-rich metamorphic rocks. Important documented localities include South Africa, Canada, the United States, Kenya, Tanzania and Russia. A precise locality and host rock are more useful than a broad colour label.
Collecting notes
Hydrogrossular should be labelled as a confirmed species, variety or group-level reference according to the evidence available for the specimen. Record colour, transparency, crystal form, matrix, locality and any treatment history.
