Hydrated normal arsenate, vivianite group, forms series with annabergite and with erythrite
Specific gravity: 2.57 to 2.73 measured, 2.57 calculated
Colour: White; colourless in transmitted light
Solubility: Soluble in acids
Hörnesite occurs in oxidised arsenic ore deposits, on fractures in
serpentine, in marble
xenoliths and as post-mine efflorescence on tunnel walls or dump rocks
At the Fiano tuff quarries, Nocera Inferiore, Salerno, Campania, Italy, hörnesite occurs in limestone blocks, thermally metamorphosed, in a volcanic tuff (Webmin).
At the type locality, the Ciclova deposit, Ciclova Română, Caraş-Severin County, Romania, hörnesite occurs in the oxidised copper-molybdenum deposit. Associated Minerals include garnet and calcite (Mindat).
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