Weathering of phlogopite to beidellite in a karstic environment

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ID: 298799
2009
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Abstract
The extent of natural weathering of phlogopite crystals in a karstic environment and the composition of its weathering products have been examined through a multi-method investigation carried out on a volcanoclastic deposit discovered in the Grotta del Cervo cave, Pietrasecca, central Italy. Phlogopite crystals are a mixture of 1*M* and 2*M*~1~ polytypes and show mixed-layering with a low amount (1-4%) of smectite in the glass groundmass. Phlogopite crystals vary in composition from phlogopite to ferroan phlogopite. Their average structural formula is \\\[(\\mathrm{K}\_{0.92}\\mathrm{Na}\_{0.03}\\mathrm{Ba}\_{0.03}\\mathrm{Ca}\_{0.02})(\\mathrm{Mg}\_{2.05}\\mathrm{Al}\_{0.01}\\mathrm{Fe}\_{0.19}\^{2{+}}\\mathrm{Fe}\_{0.40}\^{3{+}}\\mathrm{Mn}\_{0.01}\\mathrm{Ti}\_{0.15}{\\square}\_{0.19})(\\mathrm{Si}\_{2.63}\\mathrm{Al}\_{1.37})\\mathrm{O}\_{10.16}(\\mathrm{OH}\_{1.57}\\mathrm{F}\_{0.27}).\\\] Micromorphological, X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), scanning electron microscopy (SEM), and microprobe analyses (EMPA) showed that the mineralogical evolution of phlogopite crystals is consistent with *in-situ* pseudomorphic weathering of mica to Al-rich dioctahedral (rather than Mg-rich) clay minerals (beidellite-like) via phlogopite-smectite mixed-layer phases. The main mechanism of transformation is a layer-by-layer modification in which water composition, enriched in Si and Al from glass dissolution, is a major control. Mg loss, Ca and Na exchange for K, enrichment in Si and Al are the main processes required for beidellitization of phlogopite. Glass alteration produced sequentially smectite, chabazite, phillipsite and analcime, as leaching progressed and the interstitial fluids became increasingly alkaline. The type of zeolite found in the several samples (in the chabazite, phillipsite, analcime series) indicates that the alteration extent increases from wall to the ceiling and to the floor of the cave.
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Authors Luca Aldega, Javier Cuadros, Angela Laurora, André L. Rossi
Journal american journal of science
Year 2009
DOI
10.2475/08.2009.03
URL
https://ajsonline.org/article/61793.pdf
Keywords Keywords not found

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