Titanium, aluminum and interlayer cation substitutions in biotite from high-grade gneisses ' West Greenland
A detailed electron microprobe study has been carried out on biotite in pelitic to mafic metamorphic rocks from two localities in West Greenland. Comparison of analyses from similar rock types indicates higher Ti, K/(K+Na), and possibly (K+Na), similar AlIv, and lower AlvI in granulite us. amphiboli...
| Main Author: | |
|---|---|
| Other Authors: | |
| Format: | Text |
| Language: | English |
| Subjects: | |
| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.553.2191 http://rruff.info/doclib/am/vol68/AM68_880.pdf |
| Summary: | A detailed electron microprobe study has been carried out on biotite in pelitic to mafic metamorphic rocks from two localities in West Greenland. Comparison of analyses from similar rock types indicates higher Ti, K/(K+Na), and possibly (K+Na), similar AlIv, and lower AlvI in granulite us. amphibolite grade samples. Normalization of the analyses on the basis of7 octahedral and tetrahedral cations results in calculated total positive charge that exceeds the theoretical maximum of 22.0 in all cases, assuming an anion framework of l0 oxygen and 2(OH+F+Cl). In granulite grade biotite this charge excess correlates well with 2Ti, indicating that vacancy- or oxy-substitution [Ti4 * + n: 2R2 * or Tia+ 1 2gz-: 3z+ + 2(OH)-l predominate over ones in which octahedral Ti is balanced by tetrahedral Al. The vacancy substitution is preferred as the best explanation ofthese data. Many analyses yield a charge excess above that accounted for by Ti, and contain Alvl not balanced by AlIv. In amphibolite grade biotite, charge excess correlates with 312 AlvI, suggesting additional vacancies associated with dioctohedral substitution [2,{13 * + n: 3R2*]. Vacancy-formation does not appear to violate any crystal-chemical relationships in biotite, and may be necessary to accommodate substitution of the smaller Al and Ti for Fe and Mg in octahedral positions. These results lead to an iterative normalization procedure for microprobe analyses of biotite [Total Cations- (K+Na+Ca+Ba) + Ti + %Aryl:7.0, where Al]"t: (Al+Cr)vI-Alrv + (K+Na+2Ca+2Ba)1, which allocates a vacancy for each Ti and 2 AlYi. This iterative normalization eliminates charge excess, and permits Fe3 * to be estimated (exclusive of oxyannite). A model chemical system for micas is discussed which illustrates how many complex substitutional mechanisms can be reduced to linear combinations of four simple types |
|---|