Apophyllite

The name apophyllite refers to a specific group of phyllosilicates, a class of minerals. Originally, the group name referred to a specific mineral,[3] but was redefined in 1978 to stand for a class of minerals of similar chemical makeup that comprise a solid solution series, and includes the members fluorapophyllite-(K), fluorapophyllite-(Na), hydroxyapophyllite-(K). The name apophyllite is derived from the Greek ἀποφυλλίζω apophylliso, meaning "it flakes off", a reference to this class's tendency to flake apart when heated, due to water loss. Exfoliation of apophyllite is also possible by treating it with acids or simply by rubbing it. These minerals are typically found as secondary minerals in vesicles in basalt or other volcanic rocks. A recent change (2008) in the nomenclature system used for this group was approved by the International Mineralogical Association, removing the prefixes from the species names and using suffixes to designate the species.[4] A subsequent nomenclature change approved by the International Mineralogical Association in 2013 renamed the minerals to include both suffixes and prefixes, as shown above.[5]

Apophyllite
General
Category	Phyllosilicate
Formula (repeating unit)	(K,Na)Ca4Si8O20(F,OH)·8H2O
Crystal system	P4/mnc (no. 128)
Identification
Color	Usually white, colorless; also blue, green, brown, yellow, pink, violet
Crystal habit	Prismatic, tabular, massive
Cleavage	Perfect on (001)
Fracture	Uneven
Mohs scale hardness	4.5 - 5
Luster	Vitreous; pearly
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.3 – 2.4
Refractive index	1.536
Pleochroism	Dichroic (colorless)
Other characteristics	Radioactive 4.37% (K)
References	[1][2]

Though relatively unfamiliar to the general public, apophyllites are fairly prevalent around the world, with specimens coming from some of the world's most well-known mineral localities. These localities include: Jalgaon, India; the Harz Mountains of Germany, Mont Saint-Hilaire in Canada, and Kongsberg, Norway, with other locations in Scotland, Ireland, Brazil, Japan, and throughout the United States.