Abstract: 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 MnO₂ content was by and large over 90% and K₂O 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 = 9.956nm, b = 2.870nm, c = 9.706nm 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.

Key words: natural cryptomelane, manganese oxide octahedral molecular sieve, occurence, Xialei manganese deposit in Guangxi

摘要： 锰钾矿是一种有孔道结构的锰氧化物八面体分子筛（OMS-2），具有良好的催化氧化性和离子交换性。通过对广西下雷锰矿床的氧化锰矿体中锰钾矿的赋存特征和矿物学特征的研究，表明天然锰钾矿主要充填矿石裂隙和孔洞产出，具有后期多次成矿的特点。利用X-射线粉晶衍射、扫描电镜、电子探针、电子能谱和X荧光光谱对天然锰钾矿的形貌特征、化学成分、结构特征进行研究，结果表明天然锰钾矿晶体形态主要为针状、纤维状，沿 b 轴延伸，直径几 nm 至几百 nm；集合体规则排列呈片状、板状或放射状；晶体结构属于单斜晶系，具有沿 b 轴方向的孔道结构，K 位于孔道位置；化学成分 w(MnO₂) 含量一般大于90％，w(K₂O) 为2.52％～4.37％，单晶成分 w(Mn) = 56.72%～65.32%，w(O) = 30.51%～33.90%，w(K) = 3.37%～4.17%；差热－热重分析表明天然锰钾矿的结构热稳定性较好，635.5℃时相转变为方铁锰矿。

关键词: 天然锰钾矿, 八面体分子筛, 产出特征, 广西下雷锰矿