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Research Article| March 01, 1976 Biotite-induced grussification of the Boulder Creek Granodiorite, Boulder County, Colorado DANA ISHERWOOD; DANA ISHERWOOD 1Department of Geological Sciences, University of Colorado, Boulder, Colorado 80302 Search for other works by this author on: GSW Google Scholar ALAYNE STREET ALAYNE STREET 2Geography Department, Cambridge University, Cambridge, England Search for other works by this author on: GSW Google Scholar Author and Article Information DANA ISHERWOOD 1Department of Geological Sciences, University of Colorado, Boulder, Colorado 80302 ALAYNE STREET 2Geography Department, Cambridge University, Cambridge, England Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1976) 87 (3): 366–370. https://doi.org/10.1130/0016-7606(1976)87<366:BGOTBC>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation DANA ISHERWOOD, ALAYNE STREET; Biotite-induced grussification of the Boulder Creek Granodiorite, Boulder County, Colorado. GSA Bulletin 1976;; 87 (3): 366–370. doi: https://doi.org/10.1130/0016-7606(1976)87<366:BGOTBC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Grus formation in the Precambrian Boulder Creek Granodiorite of Colorado is the result of biotite expansion along basal cleavages. A petrographic study of the grus, impregnated in plastic resin to retain its original fabric, reveals expanded biotite fragments separated by void space with numerous microfractures through quartz and feldspar grains. Mineral alteration during grussification includes hornblende solution and argillation of feldspar. X-ray analysis of the weathered biotite indicates the formation of hydrobiotite and biotite-hydrobiotite interlayer combinations. Chemical changes in the composition of the biotite are not excessive and consist mainly of oxidation of Fe+2 to Fe+3 with replacement of K+1 by water molecules. These changes have decreased the biotite density from 3.08 g/cm3 to 2.50 g/cm3. Modal analyses of fresh and weathered samples suggest that grussification is a function of biotite content: the greater the biotite percentage, the greater the susceptibility for granular disintegration to occur. The formation of microfractures and the expansion of the biotite have reduced the bulk density from 2.67 g/cm3 for the original rock to 1.98 g/cm3 for the grus. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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