Am. Hofmeister et al., Prevalence and origin of birefringence in 48 garnets from the pyrope-almandine-grossularite-spessartine quaternary, AM MINERAL, 83(11-12), 1998, pp. 1293-1301
Forty garnets are anisotropic among 48 in a suite spanning the quaternary s
ystem Mg3Al2Si3O12-Fe3Al2Si3O12-Ca3Al2Si3O12-Mn3Al2Si3O12 (Py-Al-Gr-Sp). Th
e eight isotropic specimens are too thin (<0.4 mm) for detection of weak an
isotropy. Birefringence (delta) in the remaining 40 garnets is low (delta =
0.0001 to 0.0006) and undulatory in appearance, suggesting that most optic
al anomalies in quaternary garnets, including the pyrope-almandine-spessart
ine ternary, originate through residual strain. Multiple or alternate origi
ns are not precluded for the few samples with nearly uniform retardation or
unusual sector twinning. An inverse correlation exists between degree of b
irefringence and stresses encountered during tectonic deformation. That is,
mantle garnets from kimberlites (e.g., Py(37)Al(36)Gr(27)) have the highes
t delta values. As geologic setting is difficult to separate from compositi
on, the inverse trend suggests that birefringence arises partially from int
ernal factors. We propose that the mismatch in size between Ca2+ and Mg2+ e
xacerbates retention of residual strain. Short-range ordering is not entire
ly ruled out, but clustering has only been inferred for synthetics in the m
iddle 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 studi
es of samples from similar localities.