| Aheylite | |
|---|---|
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) |
| IMA symbol | Ahe[1] |
| Strunz classification | 8.DD.15 |
| Crystal system | Triclinic Unknown space group |
| Unit cell | a = 7.885, b = 10.199 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 |
| 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 | [2][3][4] |
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.[2] Aheylite was made the newest member of the turquoise group in 1984 by International Mineralogical Association Commission on New Minerals and Mineral Names.
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+.[5]
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.[3] It was discovered by Eugene Foord and Joseph Taggart.[5]
In addition to the type locality in Bolivia it has been reported from the Bali Lo prospect in the Capricorn Range, Western Australia[2] and the Les Montmins Mine, Auvergne, France.[3] 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.[2][4]
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 subvitreous luster.[5]
