Crystal chemistry of barian enrichment in micas from metasomatized inclusions in serpentinite, Motagua Fault Zone, Guatemala
Harlow, George E.
European Journal of Mineralogy Volume 7 Number 4 (1995), p. 775 - 790
published: Aug 1, 1995
manuscript accepted: Jan 23, 1995
manuscript received: Sep 12, 1994
ArtNo. ESP147050704006, Price: 29.00 €
Abstract One to three coexisting micas with relatively high barium content, particularly in grain rims where coexisting celsian or hyalophane are present, have been found in jadeitites, albitites, and micaceous rocks from a serpentinite-matrix melange along the Motagua Valley of Guatemala. Barian phengitic muscovite and/or phlogopite, with 0.4 to over 4 wt. % BaO, (± paragonite) are found in the jadeitites. Barian phengite, from 1.2 to 8.9 wt. % BaO (0.25 cats per interlayer site), dominates in mica-rich rocks where celsian, cymrite, hyalophane and barite are minor but common. Compared to other reports of barian micas coexisting with Ba phases, the BaO values here are lower, probably due to low contents of Cr, Ti and Mn, lower formation temperatures and higher formation pressures. Barium content in phengite always exceeds that in coexisting phlogopite (Ph/PhlDBa = 1.2 to 9.5) which is likely due to the steric effects of high Mg and Si content in phengite and low Fe, Mn, and Ti in phlogopite. Coexisting paragonite and late stage preiswerkite have low but not always negligible barium content, as high as 0.3 wt. % BaO. The exchange BaAlK-1Si-1 is most commonly observed for zoning of Ba in micas. However, correlated phengite exchange, MgSiAl-1AL-1, in certain rocks leads to an effective Ba(Mg,Fe2+)K-1AL-1 exchange which indicates a positive P/T growth trajectory and a pressure-volume limitation for increased tetrahedral aluminum in the mica structure.