Do magnesian clays play a role in dolomite formation in alkaline environments? An example from Castañar Cave, Cáceres (Spain)

摘要

The speleothems of Castañar Cave are made of a complex mineralogical association including aragonite, dolomite, huntite, hydromagnesite and a small proportion of Mg-rich clays whose mineralogy is difficult to determine. These clays appear as thin mats composed by fibres that are commonly associated to dolomite. The composition of cave waters and the textural relationship between the minerals indicate that in these alkaline environments there are several possible evaporation/concentration stages (with waters with pH > 8) followed by other stages characterized by freshwater entrances. All this result in complex precipitation sequences in which Ca and Mg are always available, and then it is silica content the main factor driving Mg-clay formation.The high Mg/Ca ratios drive initial aragonite precipitation, which contributes to an additional increase in this ratio enabling precipitation of huntite and Mg-clays (if silica is available). The following stage is dolomite formation either by replacement of metastable huntite or by direct precipitation on the Mg-clays that act as templates.