Compositions of trioctahedral micas in the Cornubian batholith

Abstract

New major and trace element chemical analyses of trioctahedral micas of the Cornubian batholith emphasize their enrichment in a 'trace-alkali element' association (Li, Rb, Cs, F, Nb, Mn) and their deficiency in a 'femic element' association (Zr, Ce, Th as well as Mg and Ti) compared with micas from many other granite suites, although there are similarities with some Hercynian granites and with rare metal pegmatites. The new data demonstrate a continuous series from siderophyllite through zinnwaldite to lepidolite, principally as a result of Li-R 2 § substitution as indicated by Foster (1960), although a more complete replacement is (Li, AI) = (Fe z+, Fe 3 § Ti). It is suggested that the ranges of these micas are based upon the Li content in the unit cell formula and the ratio of Li to R 2+, in effect, a compromise between the ranges proposed by Foster (1960) and Rieder (1970). Microprobe analyses lack Li20, but can be plotted on FeO-SiO 2 and FeO-AlzO3-SiO 2 diagrams (wt. or atom %) in order to locate compositions within the trioctahedral LiFe micas and to distinguish between lepidolite and muscovite.An examination of 55 new mica analyses shows that hornfels biotites are richer in Mg and that the Cornubian Type A granite (as classifed by Exley and Stone, 1982) micas are richer in Ti and Fe compared with those of Type B granites. Micas from microgranite dykes appear to be poorer in femic elements and richer in trace alkalis and F compared with their Type B granite hosts, consistent with their differentiation from the latter. Mica chemistry is consistent with the magmatic evolution of the A-B-microgranite sequence in the biotite granites, the transformation of Types B to D upon emplacement of E, the derivation of Types E from B by extreme differentiation or metasomatic transformation and mobilization, and the in situ differentiation of Types G from E.

The petrogenesis of the Early Permian Variscan granites of the Cornubian Batholith: Lower plate post-collisional peraluminous magmatism in the Rhenohercynian Zone of SW England

“…The mica formulas have been calculated to 22 oxygen atoms and the Li 2 O content calculated according to Tischendorf et al (1997). All biotites are trioctahedral, consistent with the previous studies of Stone et al (1988), as the octahedral Y-site always contains more than 5 atoms per 11 oxygen atoms and the interlayer cation site atoms have a total of approximately 2. Mica names are in accordance with the IUGS classification of Rieder et al (1998).…”

“…Stone et al, 1988;Henderson and Martin, 1989). The mica formulas have been calculated to 22 oxygen atoms and the Li 2 O content calculated according to Tischendorf et al (1997).…”

Fluorite Deposits at Voznesenka in the Khanka Massif, Russia: Geology and Age of Mineralization

“…3, 2003 Fluorite Deposits at Voznesenka, Russia 201 Foster (1960). Protolithionite is an old term corresponding to ferroan zinnwaldite and lithian siderophyllite (Stone et al, 1988;Rieder et al, 1998).…”

Química mineral de micas do maciço granítico Serra Branca, Goiás: processos de alteração hidrotermal como modificadores do reequilíbrio cristaloquímico

“…Uma vez que esse elemento não é analisado em microssonda eletrônica, uma série de formulações empíricas foi proposta na literatura com vistas à sua obtenção indireta (Foster 1960, Monier & Robert 1986, Fonteilles 1987, Stone et al 1988, Henderson & Martin 1989, Tindle & Webb 1990. Entretanto, estas fórmulas aplicam-se às micas fortemente litiníferas (lepidolitas s.l.…”

“…A boa correlação positiva (0,963) entre SiO 2 e Li 2 O em mica branca mostrada na figura 12 explica-se pelo fato de que em micas tri-octaédricas ocorre aumento de Si, diminuição de Al IV e Fe 2+ com o aumento de Li (Stone et al 1988). Não obstante os baixos valores em Li obtidos por via úmida e o número pouco representativo de amostras analisadas (Tab.…”