Is there tetrahedral Fe/sup 3 +/ in biotite
Tetrahedral Fe/sup 3 +/ has been observed in Moessbauer and optical studies of Al-deficient micas, including synthetic ferri-annites, annites from banded iron formations and phlogopites from deep-seated rocks. In such samples Si + Al < 4 (per 11 0), and 0.10-0.67 formula units of Fe/sup 3 +/ fill the tetrahedral sites in the structure. The authors also discovered several Al-rich biotites which contain small amounts of Fe/sub tet//sup 3 +/ based on their spectroscopic data. Fe/sup 3 +/ appears to be displacing some of the Al/sup 3 +/ into the octahedral site. Examination of the literature shows nine other cases of Fe/sub tet//sup 3 +/ in trioctahedral 1M micas where Si + Al > 4. Traditionally, the effects of cation substitutions on the physical properties have been considered to be dependent on the size difference between the octahedral and tetrahedral layers of the structure. Much attention has focused on the substitution of the larger Fe/sup 2 +/ for Mg/sup 2 +/ and other cations in the octahedra of relatively simple synthetic compositions. However, in the natural micas studied here high fO/sub 2/ and high proportions of Al/sup 3 +/, Fe/sup 3 +/, and Ti/sup 4 +/ in the compositions raise the issue of structural readjustments based on substitution of small cations into the structure. In our samples, the average octahedral cation size is 0.67 A. If many small octahedral cations are incorporated into the structure during biotite formation, considerable octahedral flattening and (in response) tetrahedral rotation must occur to stabilize the mica. Perhaps the high degree of tetrahedral rotation allows accommodation of the larger Fe/sub tet//sup 3 +/ instead of Al/sub tet//sup 3 +/.
- Research Organization:
- Massachusetts Institute of Technology, Cambridge, (USA)
- OSTI ID:
- 6610685
- Report Number(s):
- CONF-8510489-
- Journal Information:
- Geol. Soc. Am., Abstr. Programs; (United States), Vol. 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28 Oct 1985
- Country of Publication:
- United States
- Language:
- English
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