Andradite

minerals

garnet

demantoid

topazolite

Images

Formula: Ca3Fe3+2(SiO4)3
Nesosilicate (insular SiO4 groups), garnet group

Varieties

Demantoid is a green variety of andradite
Melanite is a black, titanium-bearing variety of andradite
Topazolite is a variety of andradite resembling topaz in colour and transparency

Specific gravity: 3.8 to 3.9
Hardness: 6½ to 7
Streak: White
Colour: Yellow, greenish yellow to emerald-green, dark green; brown, brownish red, brownish yellow; greyish black, or black
Solubility: Andradite is slightly soluble in hydrochloric acid
Common impurities: Ti,Cr,Al,Mg
Environments:

Pegmatites
Carbonatites
Metamorphic environments (typical)

Andradite typically occurs in contact or thermally metamorphosed impure calcium-rich sediments and particularly in skarn deposits, associated with hedenbergite and magnetite, or with diopside, vesuvianite, calcite and clintonite (Lauf p111) and maybe danburite. Andradite is also common in calc-silicate hornfels and marble (Lauf p112). The varieties demantoid and topazolite are formed primarily in serpentinite and chlorite schist (Lauf p115).

Localities

At Tange Ashin, Kengara, Afghanistan, andradite has been found with chlorite (Lauf p116).

At Serifos, Greece, andradite has been found on hedenbergite (Lauf p113).

At the Kurar quarry, Malad, Ward 38, Mumbai, Mumbai District, Maharashtra, India, just a few specimens of andradite to 0.5 mm have been found with prehnite, julgoldite and ilvaite (Minrec 34.1.30).

In Kerman province, Iran, demantoid is found in serpentinite (Lauf p115).

At Val Malenco, Italy, demantoid is associated with talc and chrysotile (Lauf p115).

At Monmand, Pakistan, andradite has been found with clinochlore (Lauf p116).

At the Rosario Mabel mine, Castrovirreyna district, Peru, andradite has been found in marble (Lauf p113).

At the Ocna de Fier mine, Resita, southwest Romania, andradite occurs in magnetite skarn associated with minor calcite (Lauf p111).

At the Wessels mine, Hotazel, South Africa, andradite is associated with hematite (Lauf p115).

At the N'Chwaning II mine, Kuruman, South Africa, andradite is associated with manganite, hausmannite and other manganese minerals (Lauf p115). Andradite pseudomorphs after rhodochrosite have been found here (KL p219).

At Coed-y-Brenin deposit, Ganllwyd, Gwynedd, Wales, UK, garnet is common but only occurs in the wider parts of the veins, where it was one of the last minerals to crystallise. It may occur on or in amphibole or chlorite. The garnet forms bands of crystals, exceptionally to 4 mm, which vary in colour from pale yellow through orange to deep red-brown and have inclusions of what is probably epidote. The garnet is most likely andradite, maybe with some grossular. Many crystals have orange cores and yellow outer zones, so it is possible that the cores are grossular and the outer zones andradite (JRS 21.116).

In San Benito county, California, USA, the titanium-rich variety melanite together with topazolite is found in serpentinite associated with perovskite, clinochlore, calcite, diopside and rare-earth-rich vesuvianite (Lauf p115).

Alteration

aegirine and CaO to andradite, quartz and Na2O
2NaFe3+Si2O6 + 3CaO → Ca3Fe3+2 (SiO4)3 + SiO2 + Na2O This is a high temperature process (DHZ 2A p508).

calcite, hematite and CO2 quartz to andradite and
3CaCO3 + Fe2O3 + 3SiO2 → Ca3Fe3+2Si3O12 + 3CO2

hematite, wüstite, quartz and calcite to andradite, hedenbergite, magnetite and CO2
2Fe2O3 + 2FeO + 5SiO2 + 4CaCO3 → Ca3Fe3+2(SiO4)3 + CaFe2+Si2O6 +Fe2+Fe3+2O4 +4CO2

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