Images
Formula: CaMg(PO4)F
Anhydrous phosphate containing halogen,
tilasite group
Crystal System: Monoclinic
Specific gravity: 3.15 to 3.27 measured, 3.29 calculated
Hardness: 4½ to 5
Streak: White
Colour: White, buff, pale pink
Environments
Localities
At Horní Slavkov, Sokolov District, Karlovy Vary Region, Czech Republic, isokite occurs as an alteration product
of triplite in a granite
pegmatite
(HOM).
In the Cubos-Mesquitela-Mangualde area, Mangualde, Mangualde, Viseu, Portugal, isokite occurs as an
alteration product
of triplite in a granite
pegmatite
(HOM).
At the Assunção Mine, Aldeia Nova, Ferreira de Aves, Sátão, Viseu, Portugal, the occurrence consists of a
granitic
pegmatite. The isokite is fine-grained and massive and,
with chlorite, replaces
primary triplite and
manganese-bearing fluorapatite
(AM 91: 1208 ).
At the Benson Mines, Benson, Star Lake, St. Lawrence county, New York, USA, isokite replaces
wagnerite in a pegmatite
dike in sillimanite grade gneiss
(HOM).
At the type locality, the Nkumbwa Hill Carbonatite, Isoka, Isoka District, Muchinga Mountains, Muchinga Province,
Zambia, the principal accessory minerals in the carbonatite are
apatite (including strontium-bearing
fluorapatite), isokite, pyrite,
magnetite, ilmenite,
baryte, monazite,
pyrochlore and rather rare sellaite.
The isokite has so far been found in intimate association with dolomite,
strontium-bearing
fluorapatite, monazite,
phlogopite, and pyrochlore.
Sellaite and isokite have not as yet been found in the same thin section,
although they occur together in the same bulk samples. Isokite appears to belong to a late stage of metasomatic
activity which enriched the intrusive dolomite plug with fluophosphates,
strontium, rare-earths and niobium.
Often isokite and monazite are intimately associated, both minerals
occurring as spherulites which replace coarsely crystalline dolomite along the
rhombohedral cleavage planes. The weathering of the Nkumbwa
carbonatite has been accompanied in places by extensive
silicification, and this has sometimes left isokite spherulites completely enveloped in
secondary quartz.
Isokite is a rare mineral, and there must be special circumstances that have favoured its occurrence at Nkumbwa. One
obvious condition for its formation is an abundance of fluorine, for isokite contains about three times as much
fluorine as fluorapatite. This was clearly present at Nkumbwa. At Nkumbwa,
dolomite, usually ankeritic, is
practically the only carbonate present, and what little calcite does occur appears
to be largely of supergene origin. This relative paucity of calcium
and abundance of phosphate probably also account for the absence of fluorite at
Nkumbwa, despite the abundance of fluorine
(MM 30.681-690).
Other associated minerals include daqingshanite-(Ce) and
strontianite
(HOM).
Back to Minerals