TITLE: Chromite HS281 DESCRIPT
MINERAL: Chromite (Spinel group)
COLLECTION_LOCALITY: Sierra Leone
ORIGINAL_DONOR: Hunt and Salisbury Collection
CURRENT_SAMPLE_LOCATION: USGS Denver Spectroscopy Laboratory
ULTIMATE_SAMPLE_LOCATION: USGS Denver Spectroscopy Laboratory
Forms series with Magnesiochromite and with Hercynite. Dimorphous with Donathite.
Some Mg is always present substituting for Fe+2 and some Al and Fe+3 may substitute for chromium. There is extensive solid solution between chromite and magnesiochromite, MgCr2O4. Occurs chiefly in ultramafic igneous rocks and is a common constituent of peridotites and other ultrabasic rocks and of serpentines derived from them.
Sieve interval 74 - 250µm.
40 kV - 30 mA, 6.5-9.5 keV
File: chrom281.out, *.mdi; chrm281b.out, *.mdi (smear mounts on quartz plates)
References: Huebner's reference patterns
Found: Spinel with ao=8.25 angstroms, minor chlorite and spinel with ao=8.30 angstrom; unidentified weak reflections at 2.58. 2.54. 2.44, 2.030, 2.002, and 1.544 angstroms.
Comment: The peaks are sharp and well formed, suggesting good crystallinity and compositional heterogeneity. The spinel compositions are distinct, indicating a bimodal distribution of composition rather than a continuous range of solid solution. Most members of the spinel group, including chromite (ao=8.33-8.38 angstroms), have unit cells larger than those observed. Galaxite has ao=8.28 angstroms, close to the observed values, and spinel, hercynite, and gahnite have smaller unit cells (ao=8.08-8.14 angstroms). If the HS281 contains chromite, it has substantial solid solution, most likely with with spinel (MgAl2O4). The textural relationship between the two grains is not known, but if one forms cores, the other mantles, optical spectra may see only one spinel composition. The chlorite has a weak 14 angstroms reflection.
J.S. Huebner, J. Pickrell, and T. Schaefer, 1994, written communication.
COMPOSITIONAL_ANALYSIS_TYPE: None # XRF, EM(WDS), ICP(Trace), WChem
50 vol% chromite
30 vol% Fe-stained feldspar
20 vol% white mica
Chromite has transparent edges, otherwise opaque. Spectrum albedo so low that any features due to impurities are obscured. G. Swayze.
LIB_SPECTRA_HED: where Wave Range Av_Rs_Pwr Comment
LIB_SPECTRA: splib04a r 1020 0.2-3.0µm 200 g.s.= 225 µm
LIB_SPECTRA: splib05a r ____ 0.2-3.0µm 200 g.s.=
LIB_SPECTRA: splib06a r ____ 1.5-25µm 250 g.s.=