apophyllite

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A hydrous calcium-potassium silicate containing fluorine. The composition is variable but approximates to KFCa₄(Si₂O₅)₄ · 8H₂O. It resembles the zeolites, with which it is sometimes classified, but differs from most zeolites in having no aluminum. It exfoliates (swells) when heated, losing water, and is named from this characteristic; the water can be reabsorbed. It is essentially white, with a vitreous luster, but may show shades of green, yellow, or red. The symmetry is tetragonal, and the crystal structure contains sheets of linked SiO₄ groups; this accounts for the perfect basal cleavage of the mineral. It occurs as a secondary mineral in cavities in basic igneous rocks, commonly in association with zeolites. The specific gravity is about 2.3–2.4, the hardness 4.5–5 on Mohs scale, the mean refractive index about 1.535, and the birefringence 0.002. See also Silicate minerals; Zeolite.

Environment

Though technically not a zeolite, associated with them in traprocks and in some ore veins.

Practically always in crystals, frequently distinct individuals, varying from the common, short-prismatic habit with a more or less well-formed pyramidal truncation to simple, blunt, square prisms, pseudocubes. Tabular habit rare. Often over an inch or two (3-5 cm or more) across, or in 2-in.-long (5 cm) prisms. Prism faces have parallel vertical lines; base has a very pearly look.

Colorless, white, pale pink, pale to emerald green. Luster glassy and pearly; hardness 4Ɖ-5; specific gravity 2.3-2.4; fracture uneven; cleavage perfect basal. Brittle and a bit fragile from the ready cleavage; transparent to translucent.

Alkaline calcium, potassium fluosilicate (5.2% K ₂ O, 25.0% CaO, 53.7% SiO ₂ , 16.1% H ₂ O, with the fluorine replacing some of the oxygen). Sometimes variants are given the subspecies names hydroxyapophyllite, fluorapophyllite, and natroapophyllite.

Fuses easily, bubbling and swelling to a white vesicular enamel. The depths of the mass fluoresce weak greenish white after heating.

The different (pearly) luster on the basal (cleavage) face distinguishes apophyllite from some cubic minerals such as fluorite that it might otherwise suggest. The typical square pyramid and prism combination with the two lusters is easy to recognize. In case of doubt, the basal cleavage makes the identity certain. Distinguished from stilbite and heulandite by less exuberant swelling, quicker melting with boiling, and the fluorescence usual after heating.

Found in typical, well-formed white crystals in the traprocks of Paterson, New Jersey, and Bay of Fundy, Nova Scotia, beaches. Beautiful 2-3-in. (5-8 cm) crystals on prehnite were found near Washington, D.C., at Fairfax, Virginia. It is also sometimes found in cavities in pegmatitic dikes, where it is one of the last minerals to form, and in ore veins. Two notable occurrences of the latter are the delicate pink prismatic crystals at Guanajuato, Mexico, and once in the Harz Mountains of Germany. Excellent tabular crystal clusters and "roses" were found at French Creek and Cornwall, Pennsylvania. Unusually clear, tabular crystals occur in the Michigan Upper Peninsula copper mines, with anal cime and datolite. Particularly large, fine, pale green crystals were found in a railway cut (and now in numerous quarries) in Santa Catarina, Brazil. Very clear ones have come from Iceland.

• Mineral compounds

Apophyllite
General
Category	Silicate minerals
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
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.3 – 2.4
Refractive index	1.536
Pleochroism	Dichroic (colorless)

The name apophyllite refers to a specific group of phyllosilicates, 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 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 holes in basalt or other igneous rocks. They can also be called "fisheye stone".^[1] 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: 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.

Uses

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

• Apophyllite-(KF) (formerly fluorapophyllite), (K, Na)Ca₄Si₈O₂₀(F, OH) · 8H₂O - white, colorless, yellow, green, violet
• Apophyllite-(KOH) (formerly hydroxyapophyllite), KCa₄Si₈O₂₀(OH, F) · 8H₂O - white, colorless
• Apophyllite-(NaF) (formerly natroapophyllite), (K, Na)Ca₄Si₈O₂₀F · 8H₂O - brown, yellow, colorless

References

1. ^ http://www.maden.hacettepe.edu.tr/dmmrt/dmmrt50.html#d1040
2. ^ http://www.mindat.org/min-283.html

• 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. 

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