Grayite

Grayite, ThPO4 • (H2O), is a thorium phosphate mineral of the Rabdophane group first discovered in 1957 by S.H.U. Bowie in Rhodesia. It is of moderate hardness occurring occasionally in aggregates of hexagonal crystals occasionally but more commonly in microgranular/cryptocrystalline masses. Due to its thorium content, grayite displays some radioactivity although it is only moderate and the mineral displays powder XRD peaks without any metamict-like effects. The color of grayite is most commonly observed as a light to dark reddish brown but has also been observed as lighter yellows with grayish tints. It has a low to moderate hardness with a Mohs hardness of 3-4 and has a specific gravity of 3.7-4.3. It has been found in both intrusive igneous and sedimentary environments.[3][4]

Grayite
General
CategoryPhosphate minerals
Rhabdophane group
Formula
(repeating unit)		(Th,Pb,Ca)PO
4·(H
2O)
Strunz classification8.C.J.45
Dana classification40.04.07.04
Crystal systemHexagonal
Crystal classTrapezohedral (622)
(same H-M symbol)
Space groupP6222
Unit cella = 6.957
c = 6.396 [Å]; Z = 3
Identification
ColorPale yellow, yellow, yellowish gray, often a dark reddish brown
Crystal habitCryptocrystalline Aggregate
Mohs scale hardness3-4
LusterResinous to waxy to greasy
DiaphaneitySemitransparent
Density6.41 g/cm3 (calculated)
Optical propertiesUniaxial
Refractive indexnα = 1.66
nβ = 1.69
BirefringenceModerate with max at δ = 1.660
Ultraviolet fluorescenceMay fluoresce green or yellow under short wave UV
Other characteristics Radioactive
References[1][2]

Occurrence

Formations including grayite were originally documented in Rhodesia (now Zimbabwe) in 1957 and subsequently around the globe. Some of these locales include the states of Wyoming and Colorado as well as Madagascar. Grayite has often been found in pegmatitic environments amongst other thorium minerals, particularly monazite ((Ce,La)PO
4). Recent work has shown widespread occurrences in Wisconsin pegmatitic environments. Other notable finds of pegmatitic grayite occur in Bulgaria.[4][5] Grayite has also been found in sedimentary environments with an observation of high concentrations in cracks raising the possibility of the mineral as a precipitate from fluid mobilized ions. Formation of grayite and other rhabdophane minerals in this context has been documented in literature.[6][7]

As member of rhabdophane group

Grayite is isostructural with members of the Rhabdophane group such as brockite and rhabdrophane. While previous work has identified grayite as a pseudohexagonal orthorhombic member of the rhabdophane group along with ningyoite, more contemporary work seems to maintain a hexagonal crystal structure.[8] These hydrated phosphate minerals often include radioactive elements such as thorium, uranium, and cerium. Powder XRD analysis produces peaks matching those of rhabdophane.[7]

Relationship with monazite

In the identification of new hydrated phosphate minerals related to rhabdophane XRD peak information is usually recorded through different sample preparation methods. Besides standard powder XRD, samples are often heated to ~850 °C so that the structure changes. The peak information is analyzed again and upon doing this hydrated thorium phosphate minerals will show a monazite-like structure indicating a possible alteration relationship.[7][4]

See also

References
  1. Grayite in Handbook of Mineralogy (PDF) . Retrieved on 2015-11-17.
  2. Grayite at Mindat.org
  3. [Bowie, S.H.U. (1957) Summary of progress for 1956, Geologic Survey of Great Britain, pg. 67]
  4. [Nikola Zidarov and Ognyan Petrov, 2011, "Grayite From Sredna Gora Pegmatites -- First Find in Bulgaria", Comptes rendus de l’Acade'mie bulgare des Sciences, Vol 64, No12, pp.1719-1726]
  5. ["Thirty-Sixth Rochester Mineralogical Symposium: Contributed Papers In Specimen Mineralogy—Part 1." Rocks & Minerals 85.2 (2010): 160-164. EBSCO. 1 Dec. 2015.]
  6. [Alfons Berger, Edwin Gnos, Emilie Janots, Alain Fernandez, Jörg Giese, Formation and composition of rhabdophane, bastnäsite and hydrated thorium minerals during alteration: Implications for geochronology and low-temperature processes, Chemical Geology, Volume 254, Issues 3–4, 15 September 2008, Pages 238-248 in Chemical Geology]
  7. J.R. Dooley, Jr., and John C. Hathaway, 1961, Two Occurrences of Thorium-bearing Minerals with Rhabdophane-like Structures, USGS Professional Paper 424-C, pg. 339-341
  8. [Frances G. Fisher and Robert Meyrowitz, 1961, "Brockite, a New Calcium Thorium Phosphate from the Wet Mountains, Colorado", American Mineralogist, Vol. 47, pg 1336-1335.]
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