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 Volume 10 - Issue 1 (2012) |
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The Stillwater Complex chromitites: The response of chromite crystal chemistry to magma injection
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D. LENAZ, G. GARUTI, F. ZACCARINI, R.W. COOPER, F. PRINCIVALLE |
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Abstract |
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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. |
| Keywords:
X-ray single crystal diffraction. Structural refinement. Cr-spinel. Stillwater. |
| DOI:
10.1344/105.000001706
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Full text (PDF) |
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