Formula: KCa4Si8O20(OH,F).8H2O
Phyllosilicate (sheet silicate), apophyllite group, forms a series with fluorapophyllite-(K)
Specific gravity: 2.3 to 2.4
Hardness: 4 to 5
Streak: White
Colour: White, pink, green, yellow, violet. The green colour of some apophyllite has been attributed to vanadium Dana
Common impurities: Al,Na

Plutonic 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. Please check the apophyllite page for more information.
Hydroxyapophyllite-(K) is found chiefly as a late mineral in amygdules and druses in basalt, where it is associated with fluorapophyllite, datolite, pectolite, calcite and zeolites. Less frequently found in cavities in granite and syenite, in metamorphic rocks and limestone and calc-silicate rock, sometimes as an alteration of wollastonite, and as a late product in hydrothermal ore deposits (Dana, HOM).


In India all of the specimens from the major part of the Deccan basalts have been found on analysis to be fluorapophyllite, whereas those from the albite-rich basalts of Mumbai have been found to be hydroxylapophyllite-(K) (Minrec 34.1.30).

At Guanajuato, Guanajuato Municipality, Guanajuato, Mexico, many excellent hydroxyapophyllite-(K) exhibit specimens have been found (AM 63.196-202).

At the type locality, the Ore Knob mine, Ore Knob, Ashe county, North Carolina, USA, hydroxyapophyllite-(K) occurs as a 1.0 cm coating of multiply-twinned white crystals on a pyrrhotite and chalcopyrite ore (HOM).

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