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Clay Minerals; September 2000; v. 35; no. 4; p. 693-708
© 2000 Mineralogical Society of Great Britain and Ireland
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Research Paper

Retarded illite crystallinity caused by stress-induced sub-grain boundaries in illite

G. GIORGETTI1,2,*, I. MEMMI1 and D. R. PEACOR2

1 Department of Geological Sciences, The University of Siena, Via Laterina, 8, 53100 Siena, Italy, 2 Department of Geological Sciences, University of Michigan, Ann Arbor MI 48109-1063,USA

* E-mail: giorgettig{at}unisi.it

(Received 2 November 1999; revised 14 January 2000)

An XRD-TEM study was carried out on low-grade metapelites (Transantarctic Mountains) to determine the cause of apparent inconsistences in grade as determined by IC and independent geologic relations. The illite crystallinity (IC) data indicate that the three units investigated (BT, RBT, MS) were affected by very low- (IC = 0.24°{Delta}2{theta} in BT) to low-grade metamorphism (0.19°{Delta}2{theta} in MS). In all three samples, mica crystals are of a size typical of the epizone, but the mean size increases from BT to RBT and MS, due to the increasing strain features from BT to MS. These results indicate that strain-induced reduction in crystal size was retained in BT samples (with anomalously high IC values). Microtextures in RBT and MS (with smaller IC) samples suggest a recovery of sub-grain boundaries. A decrease in crystal size may occur with increasing grade where strain rates are high relative to the rate of recrystallization.

KEYWORDS: illite crystallinity (IC), stress-induced boundaries, TEM, Antarctica




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