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Drift Composition and Surficial Geology of the Trutch Map Area (94G),
Northeastern British Columbia
Geological Survey of Canada Open File D3815 |
1J.B. Percival, Mineral Resources Division, Geological Survey of Canada, 601 booth Street, Ottawa, Ontario, K1A 0E8 (Email: Jeanne.Percival@nrcan.gc.ca)
The mineralogy of bulk materials and clay-size separates is determined by X-ray powder diffraction analysis (XRD). Suspensions (in water) of the samples are pipetted onto glass slides and air-dried overnight to produce oriented mounts. X-ray patterns of the air-dried samples are recorded on a Philips PW1710 automated powder diffractometer equipped with a graphite monochromator, Co Kα radiation at 40 kV and 30 mA. The samples are also X-rayed following saturation with ethylene glycol and heat treatment (550° C).
Treatments: | |
air-dried: | 2-72 °2θ |
glycolation: | 2-72 °2θ |
heat (550° C for 2 hours): | 2-35 °2θ |
JADE® (Materials Data, Inc.) is a PC-based program that allows X-ray powder-diffraction pattern processing. This program enables manipulation of the X-ray pattern for optimization (e.g., correction for background, instrument error) in identification of mineral species. JADE can be teamed with the Powder Diffraction File (PDF) of the International Centre for Diffraction Data (ICDD) and other databases. Semi-quantitative analyses are possible through comparison with a set of reference standards using a pre-determined reference intensity ratio (RIR). The RIR's available from the ICDD database are based on a weighted factor of a mineral relative to corundum.
The weighted factors or RIR's used at the GSC laboratory have been, in many cases, recalculated using quartz as the internal standard. Selected minerals were prepared as pressed powder mounts in various proportions with quartz (e.g., 25%/75%, 50%/50% etc.). JADE was then used to do a pattern simulation for quantitative analysis. Based on the varying proportions, an average RIR was calculated. A user file was created containing minerals from the ICDD database with the new calculated RIR values as well as selected reference minerals from the National Mineral Collection (e.g., clay minerals) from the GSC.
Semi-quantitative analyses appear reasonable when minerals in the samples can be matched to the standards. Difficulty arises when clay minerals of varying composition (e.g., expandable layers, mixed-layers) are encountered, or when minerals species overlap (e.g., kaolinite and chlorite). Also, there are a limited number of standards in the user file; these may not be an exact match to the mineral being analyzed (e.g., actinolite rather than ferro-hornblende). Some of these problems have been overcome by using Co radiation (which enables splitting of the kaolinite and chlorite peaks) but others have not yet been resolved (expandable minerals). No estimate of error is possible at this time, although it could be as high as ± 20-25%.
Table 1: Qualitative clay mineralogy by X-ray diffraction analyses. Minerals arranged in order of decreasing abundance, based on peak intensities (A = abundant; m = minor; tr = trace). ML refers to mixed-layer clay mineral.
Sample No. | Mineralogy | Comments |
98BJB0009 | Quartz, Illite, Calcite (A) Dolomite, Chlorite (m) Plagioclase (m-tr) Hematite or Goethite, ML, K-Feldspar? (tr) |
Either hematite or goethite are present in trace amounts. ML is an illite-smectite. |
98BJB0041 | Illite, Calcite (A) Chlorite, Quartz (m) Plagioclase, Dolomite (m-tr) ML, K-Feldspar? (tr) |
Table 2: Semi-quantitative clay mineralogy by X-ray diffraction analyses, using JADE® software.
Sample No. | Qtz | Pl | Kfs | Chl | Ill | ML | Cal | Dol | Hem/Goe | Comments |
98BJB0009 Air-dried | 25 | 4 | tr? | 7 | 27 | tr | 28 | 9 | tr | Mixed-layer is probably an illite-smectite clay mineral. |
-glycol | 22 | 3 | tr? | 8 | 30 | tr | 28 | 7 | tr | |
98BJB0041 Air-dried | 14 | 7 | tr? | 18 | 30 | tr | 26 | 5 | Mixed-layer is probably an illite-smectite clay mineral. | |
-glycol | 21 | 4 | tr? | 8 | 37 | tr | 24 | 6 |
Qtz = quartz; Pl = Plagioclase Feldspar; Kfs = K-Feldspar; Chl = Chlorite; Ill = Illite; ML = Mixed-layer Clay Mineral; Cal = Calcite; Dol = Dolomite; Hem = Hematite; Goe = Goethite.
Both sample contain similar minerals but in varying proportions. The glycol sample is used to confirm presence of smectite and mixed-layer clay minerals containing an expandable component. Heat-treatment is used to confirm the presence of smectite and chlorite and to distinguish chlorite from kaolinite and vermiculite. Use the air-dried data keeping in mind that no estimate for ML is given.
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