Mineral accumulations induced by biological activity on granitic rocks in Qinghai Plateau, China

Abstract

Fragments of weathered granitic rocks from the Kunlun Shan. Qinghai Plateau (China) were investigated to elucidate the influence of biotic crusts on the breakdown of granitic rocks in an alpine environment. Scanning electron microscopy with energy dispersive system and X-ray diffractometry were used to describe the nature and properties of mineral accumulations on the rock surface. Results showed that organic salts such as calcium oxalate and calcium formate are associated with Aspicilia caesiocinera (Nyl.ex Malbr.) Arnold, Caloplaca sp., Xanthoria elegans (Link) Th.Fr., and Lecidea plana (Lahm) Nyl. Secondary accumulations of 2 : 1 clays minerals are found in A. caesiocinera while oxides of manganese are associated with X. elegans. Coatings of goethite (iron oxides) are believed to form from biological activity associated with the presence of hyphae and rodlet structures on the flakes. Calcium oxalate crystallizes into several morphologies such as druse, hexagonal plates, and lenticular containing between 20 and 48 per cent calcium by weight. Calcium formate and iron oxide (goethite) occur together in the form of 'red' desert varnish. Observed 'black' coatings contain as much as 37 per cent manganese and 22 per cent iron. Clay accumulations have plate-like morphology and contain c. 2 : 1 silicon to aluminium contents. We argue that organic acids from the activities of biotic crusts contribute to the breakdown of granitic rocks. Fungi accelerate the breakdown of granitic rocks through the growth of fungal hyphae along the 001 cleavage planes in primary chloritic minerals. © 2003 John Wiley and Sons, Ltd.