Fluorapophyllite-(K)

Fluorapophyllite-(K), formerly known as apophyllite-(KF)^[3] and sometimes simply referred to as fluorapophyllite, is the most common mineral of the apophyllite group,^[4] with the chemical formula of KCa₄Si₈O₂₀(F,OH)·8(H₂O). It gets the first part of its name, "fluor", because it contains more fluorine than hydroxide compared to the other minerals in the apophyllite group.

Fluorapophyllite-(K)
General
Category	Phyllosilicate
Formula	KCa4Si8O20(F,OH)·8(H2O)
IMA symbol	Fapo-K[1]
Strunz classification	9.EA.15
Crystal system	Tetragonal
Crystal class	Ditetragonal dipyramidal (4/mmm) H–M symbol: (4/m 2/m 2/m)
Space group	P4/mnc
Unit cell	a = 8.963(2), 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
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	[2][3]

Fluorapophyllite crystallizes in the tetragonal crystal system.^[5] 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.^[6]

Fluorapophyllite 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.^[7] This mineral is found as a secondary mineral in vesicles in volcanic rocks such as basalt.

Images

• 
  Stilbite crystals surround a pastel green, translucent fluorapophyllite crystal
• 
  Isolated apophyllite cluster on contrasting matrix
• 
  A radial cluster of cubically-tipped apophyllite crystals
• 
  Light green form from Pune District, Maharashtra, India