Fluorapophyllite-(K)

fluorapophyllite-(K)

apophyllite

hydroxyapophyllite-(K)

fluorapophyllite-(Cs)

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Formula: KCa4Si8O20F.8H2O
Phyllosilicate (sheet silicate), apophyllite group, forms a series with hydroxyapophyllite-(K)
Crystal System: Tetragonal
Specific gravity: 2.33 to 2.37
Hardness: 4½ to 5
Streak: White
Colour: Colourless, pale green, aquamarine, white, pink, yellow
Common impurities: Al,Na
Environments

Igneous environments
Metamorphic environments
Hydrothermal environments
Basaltic cavities

Historically all the apophyllite group minerals were reported simply as "apophyllite", and indeed it requires analysis to differentiate between them, so there is comparatively little information about the individual members of the group, except in recent times. See the apophyllite page for more information.
Fluorapophyllite-(K) is a secondary mineral in cavities in basalt, cavities in granite, in tactites and in some hydrothermal veins (Mindat).

Localities

At the Basalt Quarry, Ambariomiambana, Ambohimalaza Commune, Sambava District, Sava, Madagascar, fluorapophyllite-(K) crystals to 3 cm have been found in small cavities in the main quarry wall (Min Rec 41-3.242).

At the Wiwatersrand goldfield, South Africa, fluorapophyllite-(K) is found rarely, and always with quartz, which it postdates. Specimens have been recovered from the Stilfontein and Mponeng mines, coloured colourless through white to pale green, the last caused by secondary mineral inclusions (R&M 96.4.324-325).

At the Dara-i-Pioz Glacier, Districts of Republican Subordination, Tajikistan, Fluorapophyllite-(K) is an associate of fluorapophyllite-(Cs) (CM 57.965–971).

The Central Mine, Central, Keweenaw county, Michigan, USA, initially targeted a series of sub-parallel mineralised fissure veins where the most copper-rich portion of the vein was close to the base of the main greenstone flow.
Although not particularly common, some fine apophyllite-(K) specimens from the Central mine do exist (MinRec 54.1.53-81).

The Cliff Mine, Phoenix, Keweenaw county, Michigan, USA, is situated at the base of a roughly 70-metre basalt cliff. A curious feature of the impressive thickness of the greenstone flow here is that it contains zones of “pegmatoid”: areas where slow cooling in the core of the lava flow allowed for large feldspar crystals exceeding 1 cm to grow. Such features are normally only observed in intrusive igneous rocks and are almost unheard of in basalt flows.
The Cliff mine primarily exploited rich copper mineralisation in the Cliff fissure (vein). Although mineralised with copper to some extent along its entire length, the part of the vein just below the greenstone flow carried the richest copper mineralisation by far. A significant amount of the copper recovered at the Cliff mine came from amygdaloids in the tops of 13 basalt flows which were cut by the Cliff vein. The discovery and mining of this vein proved that the veins were the source of the large masses of float copper that were already well known, and proved that the primary ore mineral in the district was native copper, not sulphides, as had been suspected earlier.
Fluorapophyllite is much less common at the Cliff mine than at other fissure mines in the area. It occurs as small, colourless and transparent crystals to about 1 cm, mostly in the vein itself (MinRec 54.1.25-49).

At Fanwood Quarry, Watchung, Somerset county, New Jersey, USA, only a handful of specimens of fluorapophyllite-(K) are known to exist. Glassy crystals up to 1 cm on datolite have been collected in the middle amygdaloid (Minrec 41-2.166).

At the Braen Quarry, Haledon, Passaic county, New Jersey, USA, fluorapophyllite-(K) is relatively uncommon and occurs as isolated crystals on datolite, prehnite or calcite or directly attached to the walls of breccia cavities (Min Rec 40-6.516).

At Wind Mountain, Cornudas Mountains, Otero county, New Mexico, USA, fluorapophyllite-(K) is an associate of windmountainite (CM 58.477-509).

At New York City, New York, USA, lustrous white fluorapophyllite-(K) crystals are found filling seams in schist and gneiss. Beneath Roosevelt Island, plates to 21 cm across of colourless, transparent to translucent fluorapophyllite-(K) crystals to 12 mm have been. Tabular, translucent, greenish fluorapophyllite-(K) crystals to 18 mm across have been recovered from the tunnel in areas under Queens and Manhattan (R&M 84.3.226).

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