Tarbuttite

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
Tarbuttite from Broken Hill mine (Kabwe mine) in Central Province, Zambia
General
CategoryPhosphate minerals
Formula
(repeating unit)		Zn2(PO4)(OH)
Strunz classification8.BB.35
Dana classification41.6.7.1
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1
Unit cella = 5.400 Å, b = 5.654 Å
c = 6.465 Å, α = 102.51°
β = 102.46°, γ = 86.50°
Z = 2[1]
Identification
ColorWhite, colorless, yellow, red, green, or brown
Crystal habitEquant to short prismatic [001], sheaf-like aggregates, crusts, individual crystals rounded and deeply striated
CleavagePerfect on {010}
FractureIrregular, uneven
Mohs scale hardness3.5
LusterVitreous, pearly on cleavages[1]
StreakWhite
DiaphaneityTransparent, translucent
Specific gravity4.12; 4.19 (calc.)
Density4.12 g/cm3 (measured)
Optical propertiesBiaxial (-)
Refractive indexnα = 1.660
nβ = 1.705
nγ = 1.713
Birefringenceδ = 0.053
2V angle50° (measured)
DispersionWeak,[2] strong[1]
Ultraviolet fluorescenceNon-fluorescent[3]
References[2]

Description and habit

Tarbuttite is white, yellow, red, green, brown, or colorless; in transmitted light it is colorless.[2] 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.[4]

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.[1] Individual crystals are commonly rounded and striated.[2]

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).[5]

History

Tarbuttite was discovered in 1907 in Broken Hill, Northern Rhodesia, (now Kabwe, Zambia).[2][6] The mineral was described from specimens in cellular limonite in the largest hill of the group, Kopje No. 2.[7][8] In a cave discovered in Kopje No. 1 by boring a tunnel,[6] tarbuttite was also found in association with hopeite as obscure crystals and crystals smaller than 1⁄16 mm (0.0025 in)[8] and as an encrustation on some bones.[9] Several specimens of the mineral were collected by Percy Coventry Tarbutt, a director of the Broken Hill Exploration Company.[4][7] In 1907, the name tarbuttite was proposed by L.J. Spencer in the journal Nature in honor of Tarbutt.[4]

When the International Mineralogical Association was founded, tarbuttite was grandfathered as a valid mineral species.[2]

Occurrence

Tarbuttite has been found in Algeria, Angola, Australia, Canada, China, Namibia, the United States, and Zambia.[1][2]

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.[1]

References
  1. "Tarbuttite" (PDF). Handbook of Mineralogy. Mineral Data Publishing. Retrieved July 13, 2012.
  2. "Tarbuttite". Mindat. Retrieved July 13, 2012.
  3. "Tarbuttite". Webmineral. Retrieved July 13, 2012.
  4. Spencer, p. 22.
  5. Cocco, p. 321.
  6. Spencer, p. 1.
  7. Spencer, p. 2.
  8. Spencer, p. 30.
  9. Spencer, p. 31.

Bibliography

Further reading
  • Richmond, Wallace E. (December 1938). "Tarbuttite" (PDF). American Mineralogist. Mineralogical Society of America. 23 (12): 881–893.

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