Tarbuttite
| Tarbuttite | |
|---|---|
Tarbuttite from Broken Hill mine (Kabwe mine) in Central Province, Zambia
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| General | |
| Category | Phosphate minerals |
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Formula (repeating unit) |
Zn2(PO4)(OH) |
| Strunz classification | 8.BB.35 |
| Dana classification | 41.6.7.1 |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal (1) (same H-M symbol) |
| Space group | P1 |
| Unit cell | a = 5.400 Å, b = 5.654 Å c = 6.465 Å, α = 102.51° β = 102.46°, γ = 86.50° Z = 2 |
| Identification | |
| Color | White, colorless, yellow, red, green, or brown |
| Cleavage | Perfect on {010} |
| Fracture | Irregular, uneven |
| Mohs scale hardness | 3.5 |
| Luster | Vitreous, pearly on cleavages |
| Streak | White |
| Diaphaneity | Transparent, translucent |
| Density | 4.12 g/cm3 (measured) |
| Optical properties | Biaxial (-) |
| Refractive index | nα = 1.660 nβ = 1.705 nγ = 1.713 |
| Birefringence | δ = 0.053 |
| 2V angle | 50° (measured) |
| Dispersion | Weak, strong |
| Ultraviolet fluorescence | Non-fluorescent |
| References | |
Tarbuttite is a rare phosphate mineral with formula Zn2(PO4)(OH). It was discovered in 1907 in what is now Zambia and named for Percy Tarbutt.
Tarbuttite is white, yellow, red, green, brown, or colorless; in transmitted light it is colorless. Traces of copper cause green coloring, while iron hydroxides cause the other colors. Colorless crystals tend to be transparent while colored specimens have varying degrees of transparency.
The mineral occurs as equant to short prismatic crystals up to 2 cm (0.79 in), in sheaf-like or saddle-shaped aggregates, or as crusts. Individual crystals are commonly rounded and striated.
Zinc ions are surrounded by oxygen in a nearly perfect trigonal bipyramid and phosphate groups are tetrahedral. The crystal structure consists of zig-zag chains of Zn1 polyhedra linked by phosphate groups and pairs of Zn2 polyhedra. In each unit cell are two formula units of Zn2(PO4)(OH).
Tarbuttite was discovered in 1907 in Broken Hill, Northern Rhodesia, (now Kabwe, Zambia). The mineral was described from specimens in cellular limonite in the largest hill of the group, Kopje No. 2. In a cave discovered in Kopje No. 1 by boring a tunnel, tarbuttite was also found in association with hopeite as obscure crystals and crystals smaller than 1⁄16 mm (0.0025 in) and as an encrustation on some bones. Several specimens of the mineral were collected by Percy Coventry Tarbutt, a director of the Broken Hill Exploration Company. In 1907, the name tarbuttite was proposed by L.J. Spencer in the journal Nature in honor of Tarbutt.
When the International Mineralogical Association was founded, tarbuttite was as a valid mineral species.
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