Windhoekite

windhoekite

palygorskite

arisite

fluorapophyllite

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Formula: Ca2Fe3+3-x[Si8O20](OH)4.10H2O
Phyllosilicate (sheet silicate), palygorskite group
Crystal System: Monoclinic
Specific gravity: 2.62 measured, 2.630 calculated
Hardness: 2
Streak: Beige
Colour: Brown to yellowish brown
Environments

Hydrothermal envronments

Windhoekite is a hydrothermal mineral in miarolitic cavities of phonolite (Mindat).

Localities

At the type locality, the Aris Quarries, Aris, Windhoek Rural, Khomas Region, Namibia, windhoekite occurs as elongated, sectile prisms to 4 mm in length that form radiating clusters. Crystals are typically translucent and yellow-brown in colour with a silky to vitreous luster. Associated minerals include aegirine, arisite-(Ce), arisite-(La), fluorapophyllite-(K) and microcline. Windhoekite has been found with epitactic overgrowths of palygorskite. The palygorskite-group minerals (including windhoekite) are among the last to crystallise in the miarolitic cavities; the following order of crystallisation is suggested: microclineaegirinefluorapophyllitepalygorskite-group minerals → arisite.
The quarries exploit phonolite emplaced in schist. The host rock consists of aegirine, K-feldspar, nepheline and accessory minerals such as apatite, monazite-(Ce)/monazite-(La), zircon, albite, sanidine and sodalite (R&M 97.5.464-470).

At the Pervomaiskii quarry, Trudolyubovka, Bakhchysarai, Crimea, Ukraine, windhoekite consists of rosettes of goldish brown fine lamellas up to 2 m in size and is associated with calcite coated by black fine spherules of pumpellyite-(Fe) and laumontite (R&M 97.5.464-470).

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