Modal mineralogy of some granitic rocks from eastern Maricopa and northern Gila Counties, Arizona

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Author
Spencer, J.E.
Skotnicki, S.J.
Richard, S.M.
Issue Date
2003-12-01
Keywords
Arizona Geological Survey Open File Reports
Mesa Quadrangle
Arizona
Gila County
Maricopa County
Geology
classification
potassium feldspar
plagioclase feldspar
quartz

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Citation
Spencer, J.E., Skotnicki, S.J., and Richard, S.M., 2003, Modal mineralogy of some granitic rocks from eastern Maricopa and northern Gila Counties, Arizona. Arizona Geological Survey Open File Report, OFR-03-09, 18 p.
Description
Granitic rocks are classified by their relative proportions of quartz, plagioclase feldspar, and potassium feldspar. Geologic mapping in the Mesa 19 x 29 Quadrangle by the Arizona Geological Survey, done in part with federal funds provided by the STATEMAP program, covered large areas of granitic rocks. In order to properly classify and name these rocks, a set of sample was collected (by SJS) and sent to Quality Thin Sections (QTS) in Tucson, Arizona, where the samples were cut into slabs (typically about 8 x 15 cm) and chemically stained to allow distinction ofK-feldspar and plagioclase. The plagioclase stain was not effective in most cases, for reasons unknown to the responsible commercial technician at QTS, but most of the slabs took the K-feldspar stain which clearly identified the K-feldspar. After spraying the slabs with clear acrylic spray, quartz could generally be distinguished from plagioclase by the lack of secondary mineral development in the quartz. Plagioclase also tended to have a white chalky color, possibly in part due to attack by hydrofluoric acid during the staining process, and this chalky white contrasts with the gray color of quartz. Slab mineral populations were determined by point counting using a binocular microscope and a set of counting grids consisting of clear acetate sheets with a rectangular grid of tiny dots marking the counting points. As the grid was traversed, the mineral underneath each dot was counted. The counting grid was selected to use a grid with the largest spacing possible so that ~ 1000 points could be counted over the entire cut face of the slab. The largest grid size available (#104.5 = 2.8 mm dot spacing) was used for all but one sample. Seven rows of points were counted in each pass across the sample, and the sub totals for each 'pass' were recorded. Variation in mineral content with each pass reflects mineral population heterogeneity. Variations are not presented in the data table, but could be determined from the data presented. ( 18 pages)
URI
http://hdl.handle.net/10150/629911
Additional Links
https://library.azgs.arizona.edu/
Language
en
Series/Report no.
OFR-03-09
Rights
Arizona Geological Survey. All rights reserved.
Collection Information
Documents in the AZGS Document Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu.
North Bounding Coordinate
34.0644
South Bounding Coordinate
32.7715
West Bounding Coordinate
-113.17
East Bounding Coordinate
-109.94
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