Weathering of Phlogopite to Beidellite in a karstic environment

The extent of natural weathering of phlogopite crystals in a karsticenvironment 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 ItaIy.Phlogopite crystaIs are a mixture of 1M and 2M1 polytypes and show mixed-layering with a low amount (1-4%) of smectite in the glass groundmass. PhlogopitecrystaIs vary in composition from phlogopite to ferroan phlogopite. Their average structural formula is(K0.92Na0.03Ba0.03Ca0.02) (Mg2.05Al0.01Fe2+0.19Fe3+0.40Mn0.01Ti0.150.19) (Si2.63Al1.37)O10.16(OHI.57F0.27) Micromorphological, X-ray diffraction (XRD), Fourier Transform Infrared (FTIR),scanning electron microscopy (SEM), and microprobe analyses (EMPA) showed thatthe mineralogical evolution of phlogopite crystals is consistent with in-situ pseudomorphicweathering of mica to Al-rich dioctahedral (rather than Mg-rich) day mineraIs (beidellite-like) via phlogopite-smectite mixed-Iayer phases. The main mechanism of transformation is a layer-by-Iayer modification in which water composition, enriched inSi and Al from glass dissolution, is a major control. Mg loss, Ca and a 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 a1kaIine.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 thefloor of the cave.