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

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

34 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)1293-1301
Number of pages9
JournalAmerican Mineralogist
Issue number11-12 PART 1
Publication statusPublished - 1998
Externally publishedYes

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology


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