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Authigenic palygorskite in Miocene sediments in Linxia basin, Gansu, northwestern China

¹ Faculty of Earth Sciences; ² Postgraduate School, China University of Geosciences, ³ State Key Laboratory of Geological Process and Mineral Resources, Wuhan, Hubei, 430074, P R China

^* E-mail: honghl8311{at}yahoo.com.cn

(Received 24 June 2006; revised 6 November 2006)

The mineralogical characteristics of authigenic palygorskite occurring with chlorite and illite in Miocene sediments in Linxia basin were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The XRD results suggest that the mineral composition of the sediments includes mainly quartz, chlorite, illite, calcite, plagioclase, orthoclase, minor palygorskite and small amounts of gypsum and ankerite. Two kinds of palygorskite were observed in the sediments, relatively straight fibrous particles found in matted, felted masses associated with platy chlorite and silky aggregates found in the void spaces. The former probably replaces chlorite grains, growing from the edges or the fissures of chlorite particles. Chlorite grains exhibit bay-shaped or rounded edges, with ambiguous felted boundaries between chlorite particles, indicative of an intensive dissolution process and the growth of palygorskite at the expense of chlorite. Palygorskite is also observed as inclusions within calcite, sprouting from or coating calcite surfaces, suggesting that palygorskite crystallized from solution. The textural relations of palygorskite and the occurrence of ankerite and the characteristically Fe-bearing palygorskite in the sediments suggest the destruction and hydrolysis of chlorite. The ankerite seems to be preferentially present in the void spaces, closely associated with chlorite and illite; fibrous palygorskite crystallizes at the edges of these clay mineral particles and the platy clay mineral particles are gradually replaced by fibrous palygorskite crystals, suggesting that alteration of chlorite to palygorskite involves an interaction with water during the diagenetic process.

KEYWORDS: palygorskite, chlorite, illite, calcite, Linxia basin, China