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Formula: NaKBeZr2(PO4)4.2H2O
Hydrated phosphate, gainesite group,
beryllium- and zirconium- bearing
mineral
Crystal System: Tetragonal
Specific gravity: 2.94 measured, 3.08 calculated
Hardness: 4
Streak: Pale lavender
Colour: Deep purplish blue
Luminescence: Not fluorescent under long wave or short wave UV
Common impurities: Rb,Ca,Sr,Ba,Mg,Mn,Fe,Cs,Ce,Hf,P,Si,F
Environments
Although it was approved in 1993, to date (May 2024) selwynite has been reported only from the type locality.
Localities
The type locality, the Wycheproof granite quarry, Wycheproof, Buloke Shire, Victoria, Australia, is on a low
granite knoll that consists of a pale grey, medium-grained,
muscovite-bearing, granite of
Devonian age (419.2 to 358.9 million years ago). It projects through flat-lying Tertiary (65.5 to 2.6 million years ago)
sediments. Pegmatite veins were found in boulders, close to
being in situ, at the base of the southeastern face of the quarry. Quartz,
orthoclase, albite,
muscovite and schorl form the veins,
which contain miarolitic and interstitial cavities that may be lined with crystals of
eosphorite or filled with other phosphate minerals, including
wardite, cyrilovite,
rockbridgeite,
leucophosphite, a
kidwellite-like mineral and saleeite,
as well as the zirconium-bearing species. Some cavities appear to have formed
by dissolution of primary phosphates, including
fluorapatite, and are now occupied by skeletal or powdery aggregates
consisting of quartz, muscovite,
eosphorite and cyrilovite.
Selwynite has been found in only three cavities. On the type specimen, it forms the infilling of an interstitial
cavity 8 mm across in feldspar and
quartz. Eosphorite and cream-coloured
montmorillonite are associated minerals. Selwynite forms
irregular infillings that consist of intergrowths of indistinct radiating crystals. No single crystals or crystal faces
have been observed. Its most striking feature is the deep purplish blue colour, although in one cavity, weathering has
reduced the selwynite to a granular pale lavender translucent aggregate mixed with
limonite and clay. Unweathered
selwynite is transparent with a vitreous lustre
(CM 33.1.55-58).
Selwynite from Wycheproof
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