Aheylite

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Aheylite
Aheylite-Cassiterite-177845.jpg
Size: 3.5×2.3×0.8 cm. Translucent ~0.2 cm spheres of aheylite are perched on dark cassiterite, together with elongated quartz crystals.
General
Category Phosphate minerals
Formula
(repeating unit)
(Fe2+,Zn)Al6(OH)8(PO4)4·4(H2O)
Strunz classification 08.DD.15
Crystal symmetry Triclinic pedial 1 or 1
Unit cell a = 7.885 Å, b = 10.199 Å, c = 7.672 Å; α = 110.84°, β = 115.12°, γ = 67.51°; Z = 1
Identification
Color Very pale blue, pale green, to blue-green
Crystal habit Interlocked crystals in felted and matted aggregates forming botryoidal, spherulitic masses; radiating, prismatic groups
Crystal system Triclinic
Cleavage Perfect {001}, {010} good
Fracture Hackly to splintery
Tenacity Brittle
Mohs scale hardness 5 to 5.5
Luster Porcelaneous to subvitreous
Streak White to greenish white
Diaphaneity Transparent in thin flakes
Specific gravity 2.84
Optical properties Biaxial (+)
Refractive index ~1.63
References [1][2][3]

Aheylite is a rare phosphate mineral with formula (Fe2+Zn)Al6[(OH)4|(PO4)2]2·4(H2O). It occurs as pale blue to pale green triclinic crystal masses.[1] Aheylite was made the newest member of the turquoise group in 1984 by International Mineralogical Association Commission on New Minerals and Mineral Names.

Composition[edit]

The turquoise group has a basic formula of A0-1B6(PO4)4-x(PO3OH)x(OH)8·4H2O. This group contains five other minerals. In addition to aheylite: planerite, turquoise, faustite, chalcosiderite, and an unnamed Fe2+-Fe3+ analogue. Aheylite is distinguished in this group by having Fe2+ dominant in the A-site. The ideal aheylite has a formula of Fe2+Al6(PO4)4(OH)8·4H2O. Its color is pale blue or green. With turquoise family the blue color is said to come from the octahedral coordination of Cu2+ in the absence of Fe3+.[4]

Name and discovery[edit]

It was first described for an occurrence in the Huanuni mine, Huanuni, Oruro Department, Bolivia, and named for Allen V. Heyl (1918–2008), an economic geologist for the United States Geological Survey.[2] It was discovered by Eugene Foord and Joseph Taggart.[4]

Occurrence[edit]

In addition to the type locality in Bolivia it has been reported from the Bali Lo prospect in the Capricorn Range, Western Australia[1] and the Les Montmins Mine, Auvergne, France.[2] It is a turquoise group mineral and occurs as a late hydrothermal phase in a tin deposit associated with variscite, vivianite, wavellite, cassiterite, sphalerite, pyrite and quartz in the type locality.[1][3]

Physical properties[edit]

It is found as an isolated mass of hemispheres and spheres clumped together. It has a vitreous to dull luster. It has a hackly to splintery fracture and it has a brittle tenacity. The hardness is about 5-5.5, and the specific gravity is 2.84. As far as optical properties, it had thin flakes; ipale blue, green to blue-green color; it streaks white, and has a subviterous luster.[4]

References[edit]