A Survey of White Mica Crystallinity and Polytypes in Pelitic Rocks of Snowdonia and Llŷn, North Wales

R. J. Merriman and B. Roberts
British Geological Survey, Keyworth, Nottingham NG12 5GG
Department of Geology, Birkbeck College, University of London, 7-15 Gresse Street, London W1P 1PA

Abstract: Pelitic rocks in North Wales, ranging in age from late Precambrian to Silurian were sampled. XRD analysis was used to determine the mineralogy and white mica crystallinity of separated < 2 µm fractions. Results show that three stages of metapelite recrystallization can be distinguished. Stage I metapelites are uncleaved or feebly cleaved with crystallinities > 0.43Δ°2θ; the 1Md polytype dominates < 2 µm fractions, occurring as K-and Na-micas. Stage II metapelites show variable cleavage development and crystallinities in the range 0.26-0.43Δ°2θ; 1Md and 2M1 polytypes occur and K- and Na-micas are commonly regularly interstratified. Stage III metapelites are strongly cleaved with crystallinities < 0.26Δ°2θ; the 2M1 polytype is dominant, occurring as K-mica and paragonite. Pelites bearing pyrophyllite, rectorite, and corrensite are found close to plug-like intrusions and were contact altered prior to regional metamorphism. 1M mica is common in deeply buried but relatively undeformed volcaniclastic rocks of the Arfon Group.

Contours of equal crystallinity (isocrysts) are plotted with metabasite zones on a metamorphic map. This shows that stage I metapelites are equivalent to the subpumpellyite zone (≡?laumontite zone). Stage II metapelites are equivalent to the prehnite-pumpellyite facies and stage III metapelites to the clinozoisite and biotite zones of the greenschist facies.

Keywords: pelitic rocks • mica • crystallinity • Snowdonia • Llŷn • Wales

Mineralogical Magazine; June 1985 v. 49; no. 352; p. 305-319; DOI: 10.1180/minmag.1985.049.352.02
© 1985, The Mineralogical Society
Mineralogical Society (www.minersoc.org)