Apophyllite

Apophyllite
General
Category	Phyllosilicate
Formula (repeating unit)	(K,Na)Ca4Si8O20(F,OH)·8H2O
IMA symbol	Apo
Crystal system	Tetragonal
Space group	P4/mnc
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
Birefringence	0.000–0.003
Pleochroism	Dichroic (colorless)
Other characteristics	Radioactive 4.37% (K)
References

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 fluorapophyllite-(K), fluorapophyllite-(Na), hydroxyapophyllite-(K). The name apophyllite is derived from the Greek apophyllízo (ἀποφυλλίζω), 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. A subsequent nomenclature change approved by the International Mineralogical Association in 2013 renamed the minerals to include both suffixes and prefixes, as shown above.

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.

Structure

Apophyllite has an unusual structure for a phyllosilicate. Whereas most phyllosilicates have a T layer (silica backbone) consisting of interlocked 6-fold rings of silica tetrahedra, with pseudohexagonal symmetry, the T layer in apophyllite consists of interlocked 4-fold and 8-fold rings of silica tetrahedra with true tetragonal symmetry.

• 
  T layer (silica backbone layer) of apophyllite
• 
  Structure of apophyllite viewed in the {100} direction, parallel to layering
• 
  Unit cell of apophyllite

Species

• Fluorapophyllite-(K) (formerly fluorapophyllite, apophyllite-(KF)), KCa₄Si₈O₂₀(F,OH)·8H₂O – white, colorless, yellow, green, violet
• Hydroxyapophyllite-(K) (formerly hydroxyapophyllite, apophyllite-(KOH)), KCa₄Si₈O₂₀(OH,F)·8H₂O – white, colorless
• Fluorapophyllite-(Na) (formerly natroapophyllite, apophyllite-(NaF)), NaCa₄Si₈O₂₀F·8H₂O – brown, yellow, colorless
• Fluorapophyllit-(Cs) (new) CsCa₄(Si₈O₂₀)F·8H₂O
• Fluorapophyllit-(NH4) (new) NH₄Ca₄(Si₈O₂₀)F⋅8H₂O

• 
  Isolated Fluorapophyllite-(K) cluster on contrasting matrix
• 
  Hydroxyapophyllite
• 
  Fluorapophyllite-(Na)

See also

• Carbonatite
• Gyrolite
• Zeolite
• List of minerals

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.
• Spencer, Leonard James (1911). "Apophyllite" . In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 2 (11th ed.). Cambridge University Press. p. 195. This describes the older definition as a specific mineral.