Weathering of phlogopite to beidellite in a karstic environment

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 discov- ered in the Grotta del Cervo cave, Pietrasecca, central Italy. Phlogopite crystals are a mixture of 1M and 2M1 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 (K0.92Na0.03Ba0.03Ca0.02)(Mg2.05Al0.01Fe0.19 Fe0.40 Mn0.01Ti0.150.19)(Si2.63Al1.37)O10.16(OH1.57F0.27). 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 pseudomor- phic 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 anal- cime, 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.