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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
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:
microcline → aegirine →
fluorapophyllite →
palygorskite-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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