ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract A mica whose structural formula: (K1.76Na0.31)(Fe2.22Mn1.29Mg0.99Ti0.28Al0.24□0.98) ·(Si7.33Al0.67)O20.26(F2.16OH1.58) closely approximates that of tetrasilicic potassium mica K2(M 5 2+ □)Si8O20(OH,F)4 where M2+ represents Mg2+, Fe2+, Mn2+, ..., has been discovered in the matrix of a peralkaline rhyolite (comendite) of the Mont-Dore massif (France). These micas had been obtained previously by synthesis only. In the groundmass of the rock, the micaceous phase is accompanied by a manganoan arfvedsonite, pyrophanite, magnetite, apatite, sphene, zircon and fluorite. The crystallographic properties of the mica are typically that of a tetrasilicic mica, with d 060 = 1.533Å and space group C2/m. There is a regular decrease of d 060 (parameter b) with the ionic radius of the octahedral cation in synthetic micas containing Fe2+, Co2+, Mg2+, Ni2+. The purely Mn2+ end-member could not be synthesised; its instability is discussed on the basis of structural considerations. The conditions of crystallization of the micaceous phase are estimated to be 760 ° C, 800 bars with a f o 2=10−14.7 bar. This mica has crystallized from a residual liquid, with high activity of silica and low activity of alumina, whose origin is discussed. The name MONT-DORITE is proposed for this natural tetrasilicic mica having Fe/Fe+Mg 〉1/2 and Fe/Fe+Mn 〉1/2. This name is from the stratovolcano Mont-Dore.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00371894
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