Acanthite is a mineral very similar to argentite, differing only in internal structure; the crystal of Acanthite being monoclinic - acanthite can be turned into argentite by heating. Acanthite is an important ore of silver, and forms spiky-looking crystals
Actinolite is a mineral that commonly occurs in crystalline schists, often being the chief constituent of green-coloured schists and greenstones. It is used somewhat as an asbestos material. It has the formulae Ca2(Mg,Fe)5Si8O22(OH)2 (the same as nephrite, but being less attractive and harder to cut and polish is less valuable than nephrite) and a relative hardness of 6.
Adamite is a rare mineral with a brilliant fluorescence, it is of scientific interest and is much desired by collectors for its bright colours and general attractiveness, but has no industrial use. It has the formulae Zn2(AsO4)(OH) and a relative hardness of 4. Adamite was discovered by the French mineralogist Gilbert-Joseph Adam at Chanarcillo, Chile during the 19th century, and the mineral was confirmed as a distinct species in 1886 and named after its discoverer.
Adularia (Moonstone) is a semi-precious, clear, transparent, glassy form of potash feldspar which is found mainly in the crevices of crystalline schists and gneisses, often in beautifully perfect crystals. It has sometimes a pearly, opalescent reflection or play of colours. It derives its name from Adula, a mountain peak in Switzerland, where fine specimens are found.
Aegirite (Aegirine) is a rock-forming mineral of the pyroxenes group, distinguished by it's long needle-like crystals, found mainly in rocks rich in soda and poor in silica. Named after Aegir, Icelandic god of the sea. It has the formulae NaFe(Si2O6) and a relative hardness of 7.
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