Tarbuttite
Phosphate mineral
From Wikipedia, the free encyclopedia
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 Coventry Tarbutt.
| Tarbuttite | |
|---|---|
Tarbuttite from Broken Hill mine (Kabwe mine) in Central Province, Zambia | |
| General | |
| Category | Phosphate minerals |
| Formula | Zn2(PO4)(OH) |
| IMA symbol | Tbt[1] |
| 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[2] |
| Identification | |
| Color | White, colorless, yellow, red, green, or brown |
| Crystal habit | Equant to short prismatic [001], sheaf-like aggregates, crusts, individual crystals rounded and deeply striated |
| Cleavage | Perfect on {010} |
| Fracture | Irregular, uneven |
| Mohs scale hardness | 3.5 |
| Luster | Vitreous, pearly on cleavages[2] |
| Streak | White |
| Diaphaneity | Transparent, translucent |
| Specific gravity | 4.12; 4.19 (calc.) |
| 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,[3] strong[2] |
| Ultraviolet fluorescence | Non-fluorescent[4] |
| References | [3] |
Description and habit
Tarbuttite is white, yellow, red, green, brown, or colorless; in transmitted light it is colorless.[3] 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.[5]
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.[2] Individual crystals are commonly rounded and striated.[3]
Structure
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).[6]
History
Tarbuttite was discovered in 1907 in Broken Hill mine, Northern Rhodesia, (now Kabwe, Zambia).[3][7] The mineral was described from specimens in cellular limonite in the largest hill of the group, Kopje No. 2.[8][9] In a cave discovered in Kopje No. 1 by boring a tunnel,[7] tarbuttite was also found in association with hopeite as obscure crystals and crystals smaller than 1⁄16 mm (0.0025 in)[9] and as an encrustation on some bones.[10] Several specimens of the mineral were collected by Percy Coventry Tarbutt, a director of the Broken Hill Exploration Company.[5][8] In 1907, the name tarbuttite was proposed by L.J. Spencer in the journal Nature in honor of Tarbutt.[5]
When the International Mineralogical Association was founded, tarbuttite was grandfathered as a valid mineral species.[3]
Occurrence
Tarbuttite has been found in Algeria, Angola, Australia, Canada, China, Namibia, the United States, and Zambia.[2][3]
Tarbuttite forms as secondary mineral in oxidized zinc deposits. It has been found in association with cerussite, descloizite, hemimorphite, hopeite, hydrozincite, "limonite", parahopeite, pyromorphite, scholzite, smithsonite, and vanadinite.[2]
