Apophyllite

Apophyllite
General
Category	Phyllosilicate
Formula (repeating unit)	(K,Na)Ca4Si8O20(F,OH)·8H2O
Crystal system	Tetragonal (Apophyllite-(NaF) is orthorhombic)
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)
References	[1]

The name apophyllite refers to a specific group of phyllosilicates, a class of minerals. Originally, the group name referred to a specific mineral, 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 apophyllite-(KF), apophyllite-(KOH), and apophyllite-(NaF). The name apophyllite is derived from the Greek ἀπόΦυλλίςο apophylliso, meaning "it flakes off", a reference to this class's tendency to flake apart when heated, due to water loss. These minerals are typically found as secondary minerals in vesicles in basalt or other volcanic rocks. A recent change 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.^[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.

Uses

Faceted Apophyllite, 0.60 ct, from India

Apophyllites are popular as collector's minerals. This popularity is due to a combination of factors, including their abundance, color variety, and well-defined crystals. Naturally forming pyramidal structures, they refract light in obvious rainbows, and can form "natural pyramids" when subjected to rock tumbling.

Species of Apophyllite

• Fluorapophyllite-(K) (formerly fluorapophyllite, apophyllite-(KF)), KCa₄Si₈O₂₀(F,OH)·8H₂O - white, colorless, yellow, green, violet^[3]
• Hydroxyapophyllite-(K) (formerly hydroxyapophyllite, apophyllite-(KOH)), KCa₄Si₈O₂₀(OH,F)·8H₂O - white, colorless^[4]
• Fluorapophyllite-(Na) (formerly natroapophyllite, apophyllite-(NaF)), Na₄Si₈O₂₀F·8H₂O - brown, yellow, colorless^[5]

References

• MinDat Listing
• Mineral Galleries
• Colville AA, Anderson CP, Black PM (1971). "Refinement of the crystal structure of apophyllite: I. X-ray diffraction and physical properties". American Mineralogist. 56: 1222–1233. 

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