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Apophyllite-(KF)

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Apophyllite-(KF)

Fluorapophyllite
Maharashtra, India
General
Category Silicate mineral
Formula
(repeating unit)
KCa4Si8O20(F,OH)·8(H2O)
Strunz classification 09.EA.15
Crystal symmetry Tetragonal H–M Symbol 4/mmm (4/m 2/m 2/m) - ditetragonal dipyramidal
Unit cell a = 8.963(2) Å, c = 15.804(2) Å; Z = 2
Identification
Color Colorless, white, pink, pale, yellow, green; in thin section, colorless
Crystal habit Tabular to prismatic crystals; may be pseudocubic
Crystal system Tetragonal
Twinning Rare on {111}
Cleavage Perfect on {001}, imperfect on {110}
Fracture Uneven
Tenacity Brittle
Mohs scale hardness 4.5-5
Luster Vitreous, pearly on {001}
Diaphaneity Transparent to translucent
Specific gravity 2.33-2.37
Optical properties Uniaxial (+)
Refractive index nω = 1.530 - 1.536 nε = 1.532 - 1.538
Birefringence δ = 0.002
Dispersion High, may be anomalous
References [1][2]

Apophyllite-(KF) or fluorapophyllite is a mineral of the apophyllite group, with the chemical formula of KCa4Si8O20(F,OH)·8(H2O). It gets the first half of its name, "fluor", from containing more fluorine than hydroxide compared to the other minerals in the apophyllite group.

Fluorapophyllite crystallizes in the tetragonal crystal system.[3] Tetragonal minerals have three axes of different lengths and angles of 90 degrees. Fluorapophyllite is an anisotropic mineral and has low relief. This mineral belongs to the uniaxial (+) optical class, which means its indicatrix has a prolate sphenoid shape with a circular section, principal section, and one optic axis.[4]

Among the apophyllite group, fluorapophyllite is the most abundant compared to the other two minerals in the group, hydroxyapophyllite and natroapophyllite. It is popular among many mineral collectors because of the large, well-developed crystals they form and the multiple colors they come in. The most wanted variation of fluorapophyllite is the green colored variant, which is found in India. Fluorapophyllite is also found in New Jersey of the United States.[5] This mineral is found as a secondary mineral in vesicules in volcanic rocks such as basalt.

References

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