Apophyllite

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Apophyllite
General
Category	Mineral
Chemical formula	(K,Na)Ca4Si8O20(F,OH) • 8H2O
Identification
Color	Usually white, colorless; also blue, green, brown, yellow, pink, violet
Crystal habit	prismatic, tabular, massive
Crystal system	Tetragonal; Orthorhombic (natroapophyllite)
Cleavage	[001] Perfect
Fracture	Uneven
Mohs Scale hardness	4.5 - 5
Luster	Vitreous; Pearly
Refractive index	1.536
Pleochroism	Dichroic (colorless)
Streak	White
Specific gravity	2.3 - 2.4
Diaphaneity	Transparent to translucent

The name apophyllite refers to a specific group of phyllosilicate minerals, a class of minerals that also includes the micas. 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 fluorapophyllite, hydroxyapophyllite, natroapophyllite, and the very rare carletonite. The name apophyllite is derived from the Greek apophylliso, meaning "it flakes off," a reference to this class's tendency flake apart when heated, due to water loss. These minerals are typically found as secondary minerals in basalt.

Though relatively unfamiliar to the general public, apophyllites are fairly prevalent around the world, with specimens coming from some of the worlds most well-known mineral localities. These localities include: Poona, 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.

Apophyllites are almost exclusively used as collector's minerals, a role in which they are very popular. This popularity is due to a combination of factors, including their abundance, color variety, and well-defined crystals.

[edit] Varieties of apophyllite

• fluorapophyllite, (K, Na)Ca₄Si₈O₂₀(F, OH) · 8H₂O - white, colorless, yellow, green, violet
• hydroxyapophyllite, KCa₄Si₈O₂₀(OH, F) · 8H₂O - white, colorless
• natroapophyllite, (K, Na)Ca₄Si₈O₂₀F · 8H₂O - brown, yellow, colorless
• carletonite, KNa₄Ca₄(CO₃)₃Si₈O₁₈(F, OH) · 1H₂O - blue, pink, white, colorless

[edit] References

• WebMineral Listing
• 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.