Andradite

andradite

garnet

demantoid

topazolite

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

Varieties

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

Crystal System: Isometric
Specific gravity: 3.8 to 3.9 measured, 3.859 calculated
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)
Hydrothermal 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).

The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia, are skarn deposits at the contact between granodiorite and volcanic rocks. Andradite is a primary mineral occurring as crystals up to 2.5 cm across, associated with actinolite and sometimes with talc (AJM 22.1.33).

The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned iron mine, with both underground and open cast workings. The iron ores contain magnetite as the ore mineral and occur predominantly as masses of all sizes enclosed in a large skarn body formed by contact metasomatism of dolomitic limestone at the margins of a granite intrusion. In parts of the underground workings magnetite is also found in marble in contact with the granite. The skarn rocks consist mainly of tremolite, actinolite, diopside and garnet.
Andradite is common in the skarn and iron ore. It usually occurs as scattered small grains intimately associated with tremolite-actinolite, diopside, biotite, fluorite and magnetite (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)

At the Shijiang Shan-Shalonggou mining area, Inner Mongolia, China, the mineral deposits occur predominantly in veins of hydrothermal origin in skarn. Andradite is a typical skarn mineral that in this deposit is associated with clinochlore, olshanskyite and other boron minerals. The samples suggest a late hydrothermal alteration of the andradite that in turn crystallised to yield secondary crystals on borates (R&M 96.5.398).

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 Belqeys Mountain, Takab County, West Azerbaijan Province, Iran, skarn mineralisation includes fine demantoid specimens. Traces of chromium (Cr³⁺) commonly control the deep green colour. Additionally Fe²⁺-Ti⁴⁺ intervalence charge transfer as well as Fe²⁺-Fe³⁺ interactions can play a secondary role in the tint. In the Belqeys Mountains, demantoid mineralisation occurs in limestone layers within metamorphosed amphibolite and ophiolitic rocks (serpentinite, dolerite and basalts). An intrusion with granodiorite composition is believed to be the source of hydrothermal fluids that formed skarn mineralisation. Demantoid mineralisation occurs in both pockets and veinlets in limestone within pyrite, magnetite, quartz, calcite and diopside. Matrix is mostly composed of limestone and marble covered by fine crystals of diopside to 10 mm in size. Large demantoid crystals to 8 cm dominated by trapezohedral and dodecahedral faces, with exceptional deep green colour and concentric zoning have been found here (R&M 97.5.448-449).

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).

At the Luck Fairfax Quarry, Centreville, Culpeper Basin, Fairfax county, Virginia, USA, microcrystals of andradite to 1mm in association with feldspar have been found (R&M 98.2.124).

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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