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Crystal chemistry of trioctahedral micas in alkaline and subalkaline volcanic rocks: A case study from Mt. Sassetto (Tolfa district, Latium, central Italy)
-
Angela Laurora
,
Annibale Mottana
Veröffentlicht/Copyright:
1. April 2015
Abstract This work provides a crystal-chemical description of trioctahedral micas from volcanic rocks (lavas, tuffs, ignimbrites, and xenoliths) outcropping at Mt. Sassetto (Tolfa district, Tuscan Province, central Italy). Mica crystals vary in composition from ferroan phlogopite to magnesian annite. Heterovalent octahedral substitutions are mainly related to Al3+, Ti4+, and, only in a few samples, to Fe3+. The two main mechanisms regulating Ti inlet into the mica structure are the Ti-oxy [VITi4+VI(Mg,Fe)2+-1(OH)--2O2-2] and Ti-vacancy [VITi4+VI □VI(Mg,Fe)2+-2] substitutions. In these micas, Ti content is the predominant crystal-chemical parameter and significantly affects octahedral and interlayer topology as well. Micas with the highest Ti contents deviate from the expected fractional crystallization trend in the Ti vs. Mg/(Mg + Fetot) diagram, possibly as a consequence of a variation in intensive parameters (T, P, fH₂, fO₂, fH₂O) during crystallization in the magmatic chamber.
In micas with significant Fe3+ contents, the layer charge balance is accomplished by the following mechanisms: VIFe2+-3VIFe23+VI□, VIFe2+-1VIFe3+(OH)--1O2-, and VIFe2+-1VIFe3+IVSi-14+IVAl3+. These micas show mean electron counts greater for the octahedral M2 site than for M1, and unit-cell parameters signifcantly smaller than usual for other micas. Atmospheric weathering is unlikely to produce the observed Fe oxidation. The Fe3+-vacancy and Fe-oxy substitutions could represent secondary, re-equilibration effects related to post-eruptive water-rock hydrothermal processes (acid-sulfate alteration).
Keywords: Volcanic micas; central Italy; mica oxidation; hydrothermalism; crystal chemistry; Ti-substitutions
Received: 2006-6-6
Accepted: 2006-11-16
Published Online: 2015-4-1
Published in Print: 2007-4-1
© 2015 by Walter de Gruyter Berlin/Boston
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Schlagwörter für diesen Artikel
Volcanic micas;
central Italy;
mica oxidation;
hydrothermalism;
crystal chemistry;
Ti-substitutions
Artikel in diesem Heft
- New constraints on metamorphic history of Adirondack diopsides (New York, U.S.A.): Al and δ18O profiles
- Hammadah al Hamra 193: The first amphibole-bearing winonaite
- Crystal chemistry of trioctahedral micas in alkaline and subalkaline volcanic rocks: A case study from Mt. Sassetto (Tolfa district, Latium, central Italy)
- Magnetic excitations and heat capacity of fayalite, Fe2SiO4
- The barometer tremolite + tschermakite + 2 albite = 2 pargasite + 8 quartz: Constraints from experimental data at unit silica activity, with application to garnet-free natural assemblages
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- Dissolution of the (001) surface of galena: An in situ assessment of surface speciation by fluid-cell micro-Raman spectroscopy
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- Co2+-Cu2+ substitution in bieberite solid-solution series, (Co1-xCux)SO4·7H2O, 0.00 ≤ x ≤ 0.46: Synthesis, single-crystal structure analysis, and optical spectroscopy
- Clinoenstatite exsolution in diopsidic augite of Dabieshan: Garnet peridotite from depth of 300 km
- Heat-pulse calorimetry measurements on natural chlorite-group minerals
- Low-temperature behavior of NaGaSi2O6
- Single-crystal elasticity of zoisite Ca2Al3Si3O12(OH) by Brillouin scattering
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