Stillwaterite Explained

Stillwaterite
Category:	Arsenide mineral
Formula:	Pd8As3
Imasymbol:	Slw^[1]
System:	Trigonal
Class:	Pyramidal (3) or rhombohedral H-M symbol: (3 or)
Symmetry:	P3 or P
Unit Cell:	a = 7.399(4) c = 10.311(15) [Å] V = 491.58 Å^3
Molweight:	1076.12 g/mol
Strunz:	2.AC.10a
Color:	light creamy gray
Habit:	microscopic crystals
Mohs:	4.5
Luster:	metallic
Opticalprop:	weakly anisotropic in air; distinctly anisotropic in oil; no bireflectance; uniaxial
Pleochroism:	no
Density:	10.96 g/cm^3
Diaphaneity:	opaque
Other:	non-radioactive
References:	^[2] ^[3] ^[4] ^[5] ^[6] ^[7] ^[8] ^[9] ^[10] ^[11]

Stillwaterite is a palladium arsenide mineral which has a general formula of Pd₈As₃. Stillwaterite was first discovered in the Banded and Upper zones of the Stillwater igneous complex in Montana, United States, 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.

Symmetry

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

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.

Significance

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.

Notes and References

1. Warr. L.N.. 2021. IMA–CNMNC approved mineral symbols. Mineralogical Magazine. 85. 3. 291–320. 10.1180/mgm.2021.43. 2021MinM...85..291W. 235729616. free.
2. Cabri, L.J., and LaFlamme, J.H.G. (1974) Rhodium, Platinum, and Gold Alloys from the Stillwater Complex. Canadian Mineralogist, 12, 399-403.
3. 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.
4. Fleischer, M., Pabst, A., Mandarino, J.A., and Chao, G.Y. (1977) New Mineral Names. American Mineralogist, 62, 1060.
5. 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.
6. 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.
7. Klein, C., Dutrow, B. (2007) The 23rd Edition of the Manual of Mineral Science. 675 p. John Wiley & Sons, Hoboken, NJ.
8. Watkinson, D.H., and Dunning, G. (1979) Geology and Platinum-Group Mineralization, Lac-des-Iles Complex, Northwestern Ontario. Canadian Mineralogist, 17, 453-462.
9. https://wwwn.cdc.gov/TSP/ToxFAQs/ToxFAQsLanding.aspx?id=19&tid=3 Centers for Disease Control and Prevention (CDC)
10. http://www.mindat.org/min-3787.html Mindat
11. http://webmineral.com/data/stillwaterite.shtml Webmineral