Mineralogical Characteristics Study of Natural Cryptomelane in the Xialei Manganese Deposit, Guangxi

In the supergene zones of Xialei manganese deposit in Guangxi, south China, there is abundant resource of natural cryptomelane, which is well known as a manganese oxide octahedral molecular sieve having a well-defined 2×2 tunnel structure (OMS-2) with K~+ situated in tunnel positions. This study was focused on the occurrence characteristics of the cryptomelane-bearing ores and the mineralogical characteristics of natural cryptomelane. The morphology, chemical and structure features of natural cryptomelane were characterized by means of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalyzer(EPMA), energy dispersive spectrometer (EDS) and X-ray fluorescence(XRF). The morphology of the natural cryptomelane crystals via SEM showed short prismatic, needle-like and fibrous shapes with the diameters varying from few nanometers to hundreds nanometres. The MnO_2 content was by and large over 90% and K_2O ranged from 2.52 to 4.37wt%. However, the composition resulted from EDS revealed a higher content of K, 3.37～4.17wt%. The XRD of the samples agreed with a monoclinic cryptomelane and the cell parameters were given as a=9.956?nm,b=2.870?nm,c=9.706?nm and β=90.95°. The phase transformation temperature of the natural cryptomelane was about 635.5?℃ examined by TGA and DTA and the cryptomelane was changed to bixbyite at a higher temperature. These studies reveal the significant features of cryptomelane for the potential application in the environmental materials science.