Isokite

isokite

daqingshanite-(Ce)

monazite

sellaite

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

Pegmatites
Carbonatites

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).

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