Apophyllite-(KF)

Fluorapophyllite
Apophyllite-(KF), 3.3 x 2.4 x 2.2 cm. Pune District, Maharashtra, India
General
Category	Phyllosilicate
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

1. Handbook of Mineralogy
2. Fluorapophyllite on Mindat.org
3. Johnsen, O. (2002) Photographic Guide to Minerals of the World. 439 p. Oxford University Press, Great Clarendon Street, Oxford.
4. http://www.webpages.uidaho.edu/~mgunter/opt_min/article.pdf
5. Cook, Robert B. (1995) Fluorapophyllite. Rocks and Minerals, 70. 394-398.