The Stillwater Complex chromitites: The response of chromite crystal chemistry to magma injection


  • D. LENAZ Department of Mathematics and Geosciences. Via Weiss 8, 34127 Trieste, Italy.
  • G. GARUTI Department of applied Geosciences and Geophysics. Peter Tunner Strasse 5, 8700 Leoben, Austria
  • F. ZACCARINI Department of applied Geosciences and Geophysics. Peter Tunner Strasse 5, 8700 Leoben, Austria
  • R.W. COOPER Department of GeologyLamar University, 77710 Beaumont TX, USA
  • F. PRINCIVALLE Department of Mathematics and GeosciencesVia Weiss 8, 34127 Trieste, Italy.



X-ray single crystal diffraction, Structural refinement, Cr-spinel, Stillwater


Nineteen chromite crystals from the A, B, E, G, H, J and K chromitite layers of the Peridotite Zone of the Stillwater Complex (Montana, USA) have been studied by means of X-ray single crystal diffraction and microprobe analyses. The results show that samples from the basal A layer are quite different from the others showing very high oxygen positional parameter u (0.2633-0.2635) and Ti- contents (0.059-0.067apfu). Mg# values are within the range 0.21-0.23 while for the other chromites it is in the range 0.45-0.47. Moreover, for the other samples, according to the structural parameters, two groups have been identified. The first one comprises samples of layers B, E and G, the second includes H, J and K layer samples. It is supposed that high Fe2+ and Ti contents of A layer samples are due to the post-crystallization reaction with interstitial liquid. This fact allowed a very slow cooling rate as evidenced by the high u values. The fractionation of evolved magma from within the intrusion and pulse of a new magma bringing more chromium into the chamber lead to Cr- and Fe3+ -rich compositions and consequently to the increase of the cell edges. The decrease of u values seems to be related to the Cr+Fe3+ and/or Al contents.


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