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Formula: Ca2Zn(PO4)2.2H2O
Hydrated phosphate, fairfieldite group, forms a solid solution with
collinsite
Crystal System: Triclinic
Specific gravity: 3.16 measured
Hardness: 3½
Streak: White
Colour: Colourless to greenish grey
Solubility: Very slowly soluble in hydrochloric acid
Environments
Localities
The type locality is Reaphook Hill, Martins Well, South Flinders Ranges, South Australia, and to date (January 2021) this is the only locality where
hillite has been identified. It has been found as doubly terminated euhedral crystals, less than 50 μm long, and as concentric zones up to 200 μm
in thickness within zinc-bearing collinsite. Hillite occurs
on the prism faces of scholzite, as concentric zones within
collinsite, and as clusters of crystals on crusts of collinsite
in a small gossan on argillaceous siltstone. Other
phosphates in the gossan include tarbuttite,
parahopeite, switzerite and
phosphophyllite
(CM 41.981-988).
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