Variscite is a rare hydrated aluminum-bearing phosphate mineral (AlPO4·2H2O).
The mineral is usually pastel shade of green. It resembles turquoise but is usually greener (turquoise has more bluish tone). These two minerals are also chemically related (both are hydrated Al-bearing phosphates) and may occur together. Copper content is the principal difference between them. Copper is an essential constituent of turquoise, but it is absent in variscite. Both minerals are valued semi-precious stones.
Variscite occurs in host rocks as a cementing material filling cracks or in nodular form. Most important deposits are known from Utah and Nevada. It occurs also in Australia, Brazil, Germany, etc.
Variscite forms veins in shattered rocks, but it is not a hydrothermal mineral. It was groundwater that carried phosphatic material to the deposition place in contact with Al-rich rocks. Variscite precipitated out of water at near-surface temperatures. The source rocks were therefore located above from the site of precipitation, not below. Phosphatic groundwater is common in areas that contain phosphatic deposits (mostly sedimentary phosphorites). Aluminum is from silicate minerals.
Filling the cavities in a brecciated fine-grained sandstone. Width of sample 11 cm. Queensland, Australia. TUG 1608-5222
Leave a Reply