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Category Arsenide mineral
(repeating unit)
Strunz classification 2.AC.10a
Crystal system Trigonal
Rhombohedral class
Unit cell a = 7.399(4)
c = 10.311(15) [Å]
V = 491.58 Å3
Formula mass 1076.12 g/mol
Color light creamy gray
Crystal habit microscopic crystals
Crystal symmetry Trigonal
H-M symbol: (3 or 3)
Space group: P3 or P3
Mohs scale hardness 4.5
Luster metallic
Diaphaneity opaque
Density 10.96 g/cm3
Optical properties weakly anisotropic in air; distinctly anisotropic in oil; no bireflectance; uniaxial
Pleochroism no
Other characteristics non-radioactive

[1] [2] [3] [4] [5] [6] [7] [8]

[9] [10]

The palladium arsenide mineral Stillwaterite has a general formula of Pd8As3. Stillwaterite was first discovered in the Banded and Upper zones of the Stillwater igneous complex in Montana, US, and has been reported in the Lac-des-Iles area of Ontario, Canada. Outside of North America, this rare mineral has been found in northern Finland.


Stillwaterite has hexagonal symmetry, meaning that it has four crystallographic axes: three are horizontal axes of equal length, and the fourth is a vertical axis of different length.

Optical properties[edit]

Because it is opaque, stillwaterite is most commonly viewed under reflected light, appearing light creamy gray in color. It is weakly anisotropic in air, displaying dark gray to brownish gray color. In oil immersion, it shows distinct anisotropy with brownish black color and a blue to yellow-brown tinge. Hexagonal minerals such as stillwaterite are referred to as uniaxial crystals because they have only one direction, along the optic axis, in which light is not reoriented.


Since its discovery in the mid-1970s, Stillwaterite has been studied to better understand the characteristics of complex mineral assemblages of gold, silicates, and other palladium arsenides with which it is associated. Minerals containing palladium can have economic significance, and are mined for use in industrial and commercial applications.


  1. ^ Cabri, L.J., and LaFlamme, J.H.G. (1974) Rhodium, Platinum, and Gold Alloys from the Stillwater Complex. Canadian Mineralogist, 12, 399-403.
  2. ^ CaBri, L.J., LaFlamme, J.H.G., Stewart, J.M., Rowland, J.F., and Chen, T.T. (1975) New data on some palladium arsenides and antimonides. Canadian Mineralogist, 13, 321-335.
  3. ^ Fleischer, M., Pabst, A., Mandarino, J.A., and Chao, G.Y. (1977) New Mineral Names. American Mineralogist, 62, 1060.
  4. ^ Hänninen, E., Törnroos, R., and Lahti, S.I. (1986) Stillwaterite and associated platinum group minerals from the Siikakämä layered mafic intrusion, northern Finland. Lithos, 19, 87-93.
  5. ^ Jones, W.R., Peoples, J.W., and Howland, A.L. (1960) Igneous and Tectonic Structures of the Stillwater Complex, Montana. Geological Survey Bulletin, 1071-H, 281-333.
  6. ^ Klein, C., Dutrow, B. (2007) The 23rd Edition of the Manual of Mineral Science. 675 p. John Wiley & Sons, Hoboken, NJ.
  7. ^ Watkinson, D.H., and Dunning, G. (1979) Geology and Platinum-Group Mineralization, Lac-des-Iles Complex, Northwestern Ontario. Canadian Mineralogist, 17, 453-462.
  8. ^ Centers for Disease Control and Prevention (CDC) Accessed 19 November 2010.
  9. ^ Mindat Accessed 8 September 2010.
  10. ^ Webmineral Accessed 24 September 2010.