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Cordierites from the Lepontine Alps: Na + Be → Al substitution, gas content, cell parameters, and optics

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Abstract

Na, Be cordierites (Alpe Sponda) and normal Na-, Be-poor cordierites (Miregn) from the same tectonic unit are compared. The Alpe Sponda samples (Na2O: 1.1–1.5 wt.%, BeO: 0.6–0.8 wt.%), which occur in paragonite mica schists, reveal low optic angles (2V x : 45°–55°) and low distortion indices (Δ: 0.139–0.172). Chemical analyses suggest a substitution of the type Na + Be → Al. Heating in a reducing atmosphere expels the volatile channel occupants and increases 2 V x to 72.4°–73.9° and Δ to 0.194–0.223.

The normal cordierites from Miregn replace kyanite and occur in biotite mica schists interbedded with leucocratic layers. These specimens exhibit optic angles between 64° and 79°; Δ ranges between 0.232 and 0.250.

The gas contents (2.0 and 2.3 wt.%) of cordierites from Alpe Sponda are significantly higher than in the Miregn samples (1.5 and 1.8 wt.%). IR spectra show that H2O is the major component in both localities and CO2 does not exceed 0.4 wt.%. Degassed Na, Be cordierites have higher refractive indices than degassed normal cordierites with the same F-value (F[mol] = (Fe + Mn)/(Fe + Mn + Mg)). This behavior is mainly caused by smaller cell dimensions and increased density due to the Na + Be → Al substitution.

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Authors and Affiliations

  1. Laboratorium für chemische und mineralogische Kristallographie, Universität Bern, Freiestr. 3, CH-3012, Bern, Schweiz

    Th. Armbruster & A. Irouschek

  2. Mineralogisch-Petrographisches Institut, Universität Basel, Bernoullistr. 30, CH-4056, Basel, Schweiz

    Th. Armbruster & A. Irouschek

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  1. Th. Armbruster
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  2. A. Irouschek
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Dedicated to Prof. E. Wenk on his 75th birthday

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Armbruster, T., Irouschek, A. Cordierites from the Lepontine Alps: Na + Be → Al substitution, gas content, cell parameters, and optics. Contrib Mineral Petrol 82, 389–396 (1983). https://doi.org/10.1007/BF00399715

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  • DOI: https://doi.org/10.1007/BF00399715

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