Prevalence and origin of birefringence in 48 garnets from the pyrope-almandine-grossularite-spessartine quaternary

Anne M. Hofmeister, Rand B. Schaal, Karla R. Campbell, Sandra L. Berry, Timothy J. Fagan

研究成果: Article査読

31 被引用数 (Scopus)

抄録

Forty garnets are anisotropic among 48 in a suite spanning the quaternary system Mg3Al2Si3O12-Fe3Al2Si3O12-Ca3Al2Si3O12-Mn3Al2Si3O12 (Py-Al-Gr-Sp). The eight isotropic specimens are too thin (<0.4 mm) for detection of weak anisotropy. Birefringence (δ) in the remaining 40 garnets is low (δ = 0.0001 to 0.0006) and undulatory in appearance, suggesting that most optical anomalies in quaternary garnets, including the pyrope-almandine-spessartine ternary, originate through residual strain. Multiple or alternate origins are not precluded for the few samples with nearly uniform retardation or unusual sector twinning. An inverse correlation exists between degree of birefringence and stresses encountered during tectonic deformation. That is, mantle garnets from kimberlites (e.g., Py37Al36Gr27) have the highest δ values. As geologic setting is difficult to separate from composition, the inverse trend suggests that birefringence arises partially from internal factors. We propose that the mismatch in size between Ca2+ and Mg2+ exacerbates retention of residual strain. Short-range ordering is not entirely ruled out, but clustering has only been inferred for synthetics in the middle of the Py-Gr binary, a composition range unknown for natural samples, and domains are precluded in the natural samples by the recent X-ray studies of samples from similar localities.

本文言語English
ページ(範囲)1293-1301
ページ数9
ジャーナルAmerican Mineralogist
83
11-12 PART 1
DOI
出版ステータスPublished - 1998
外部発表はい

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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