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Formula:
Ca3Al7.7Si3P4O22.9(OH)13.3F2.8H2O
Hydrated phosphate, perhamite group
Crystal system: Trigonal
Specific gravity: 2.48 measured, 2.476 calculated for the empirical formula
Hardness: 5
Streak: White
Colour: Snow white, pale yellowish white, white with a greenish tint
Luminescence: Not fluorescent under long wave or short wave UV
Environments
Localities
There are two co-type localities, Huber stock, Krásno, Sokolov District, Karlovy Vary Region, Czech Republic and
the Silver Coin Mine, Valmy, Iron Point Mining District, Humboldt County, Nevada, USA.
At Huber stock, Krásno, Sokolov District, Karlovy Vary Region, Czech Republic, samples of krásnoite were found
at the abandoned Huber open pit in strongly altered areas, which range in size from 1 cm to 10 cm across.
Accumulations of krásnoite up to 3.5 cm across are present; however, the mineral is always intergrown with
white to pale yellow supergene
fluorapatite. Fluorine-rich
crandallite, whitish crystals of
kolbeckite and whitish earthy aggregates of a younger generation of
isokite are also found with some specimens of krásnoite. Two
different mineral assemblages commonly host krásnoite:
(1) plimerite, kolbeckite,
pharmacosiderite and minerals of the
chalcosiderite–turqouise
series.
(2) fluellite and milky-coloured
fluorapatite in cavities in a
quartz gangue.
Krásnoite belongs to the last stage of supergene
mineralisation at the Huber open pit, where it forms from the breakdown of
fluorapatite, triplite and
isokite in the presence of acidic ground waters.
Krásnoite occurs as compact to finely crystalline aggregates, generally about 0.1 mm across, which are
further intergrown, forming millimetre-sized clusters. The crystals are platy, have a hexagonal outline and can reach
0.1 mm on edge; isolated crystals have not been found. Krásnoite varies in colour from snow white (most
common) to pale yellowish white and white with a greenish tint. Crystals are partly transparent with a pearly to
greasy lustre
(MM 76.3.625-634).
Krásnoite from Huber Stock - Image
At the Silver Coin Mine, Valmy, Iron Point Mining District, Humboldt County, Nevada, USA, krásnoite occurs as
part of a rich secondary phosphate assemblage, which formed
on phosphatic argillite wall rocks of the abandoned
lead - zinc -
silver mine. Krásnoite occurs very late in a paragenetic sequence that
includes (in approximate order from early to late) quartz,
baryte, fluorapatite,
goethite, rockbridgeite,
cacoxenite, alunite,
wardite,
turquoise/chalcosiderite,
leucophosphite,
lipscombite/zinclipscombite,
kidwellite,
strengite/variscite,
iangreyite, krásnoite,
meurigite-Na and jarosite.
The krásnoite is likely to have formed under acidic conditions mainly from the breakdown of
primary
fluorapatite found within the phosphatic
argillite.
At Silver Coin, krásnoite forms as off-white coloured balls and rosette-like clusters up to about 1 mm
across. Within these clusters, hexagonal plates may reach 0.4 mm across, but are exceedingly thin (less than
1 micron thick) and always intergrown. The crystals have a dull to pearly lustre
(MM 76.3.625-634).
Krásnoite from the Silver Coin Mine -
Image
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