Chromite

Spectrum Details

Modality

FTIR

Category

mineral

Material Type

Oxide

Sample ID

HS281.1B, HS281.2B, HS281.3B, HS281.4B

Collection Locality

Sierra Leone

Spectral Purity

1c2_3_4_ # Chromite HS281.1B # 1= 0.2-3, 2= 1.5-6, 3= 6-25, 4= 20-150 microns SPECTRAL_PURITY: 1c2_3_4_ # Chromite HS281.2B # 1= 0.2-3, 2= 1.5-6, 3= 6-25, 4= 20-150 microns SPECTRAL_PURITY: 1c2b3c4b # Chromite HS281.3B # 1= 0.2-3, 2= 1.5-6, 3= 6-25, 4= 20-150 microns SPECTRAL_PURITY: 1c2_3_4_ # Chromite HS281.4B # 1= 0.2-3, 2= 1.5-6, 3= 6-25, 4= 20-150 microns

Composition / XRD

None # XRF, EPMA, ICP(Trace), WChem

Sample Description

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. Grain size fractions are indicated by the extension after the sample number: .1B = IMAGE_OF_SAMPLE:

XRD Analysis

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.

X Units

cm⁻¹

Y Units

Absorbance

Data Points

1,801