MINERAL: Almandine (Garnet group)
COLLECTION_LOCALITY: Jackson Co., North Carolina
ORIGINAL_DONOR: Wards Science
CURRENT_SAMPLE_LOCATION: USGS Denver Spectroscopy Laboratory
ULTIMATE_SAMPLE_LOCATION: USGS Denver Spectroscopy Laboratory
Forms series with Pyrope and with Spessartine.
Optical examination gives the following mode:
95 vol% almandine
4 vol% biotite
1 vol% magnetite
Rose pink color in hand sample, original sample was a almandine-biotite schist. Sample was hand-picked and sieved so that current sample consists of < 250-µm grain fraction. Sample could benefit from more handpicking under a microscope. Biotite forms almandine coated flakes.
40 kV - 30 mA, 7.3-9.5 keV
File: alma475.out, *_mdi (smear mount on quartz plate)
References: Borg and Smith (1969), JCPDS #9-427, DHZ, v. 1a (1982), Huebner's reference patterns
Found: Garnet, minor mica, trace kaolin mineral, unidentified reflection at 4.59 Angstroms
Comment: All reflections sharp, indicating good crystallinity. Resolved three distinct garnet compositions; the major garnet has ao=11.51 Angstroms, consistent with almandine-rich garnet; the minor garnets have larger ao (to 11.57 Angstroms) indicating solid solution, perhaps with spessartine or grossular. Mica identification was based on two basal reflections, each a doublet (two mica compositions?). The kaolinite mineral identification was also based on two basal reflections; that at 7 Angstroms is also a doublet! This is an unusual assemblage; was the sample contaminated during collection or preparation?
J.S. Huebner, J. Pickrell, and T. Schaefer, 1994, written communication.
COMPOSITIONAL_ANALYSIS_TYPE: EM(CDS) # XRF, EPMA, ICP(Trace), WChem
|COMPOSITION:||O=Cl,F,S||wt%||#correction for Cl, F, S|
Microprobe analyses are average of five spot locations done on
different grains. All Fe is expressed as FeO, and the Fe2O3 content
was not determined.
Optical examination gives the following mineral mode:
95 vol% pyrope
4 vol% biotite
1 vol% magnetite or other opaque
This sample has a bimodal grain size distribution:
1st population: avg grain size = 200 µm
2nd population: avg grain size = 25 µm
avg. grain size = 199 µm
Smaller grains partially coat larger grains. G. Swayze
Fine pyrope grains adhere to biotite contaminant surfaces. Some pyrope grains (~10%) have dark honey color perhaps due to mafic inclusions. Most grain surfaces have conchoidal fractures since garnet has no cleavage; parting surfaces not apparent in this sample.
SPECTRAL_PURITY: 1b2b3b4_ # 1= 0.2-3, 2= 1.5-6, 3= 6-25, 4= 20-150 microns