Physicochemistry and Mineralogy of Storm Dust and Dust Sediment in Northern China

Dust sediments collected from 1995 to 1998 in Beijing, Dunhuang, Inner Mongolia, Kashi, the Kunlun Mountains, Lanzhou, Ningxia, the Taklimakan Desert, and Xi’an, China, were characterized in terms of their physical, chemical, and mineralogical properties. Most aerosols and dust analysed ranged in texture from silty clay to clay loam. Their median particle diameters (Mds) generally ranged between 5 to 63 μm, coinciding with those of loess from central China and the finest sand from northwestern China. The dust sediments were characterized by a predominance of SiO2 and Al2O3, followed by K2O. Their SiO2/Al2O3 and K2O/SiO2 molar ratios ranged from 5.17 to 8.43 and from 0.009 to 0.0368, respectively. The mass concentration spectrum during a dust storm showed a single peak, rather than the triple peak generally observed under clear sky conditions. The dominant minerals were chlorite, illite, calcite, and dolomite. These physical, chemical, and mineralogical properties were consistent with those of aeolian soils and loess in western and central China. The results suggest that aerosols and fine-grained fractions of dust sediments collected in northern China are mainly composed of soil material transported from the arid and semiarid regions of China and Mongolia by prevailing winds. The rate of deposition and properties of dust falling on eastern China were strongly influenced by meteorological conditions, season, latitude, longitude, and altitude of the sampling sites.

Mineralogy and Chemistry of Sulfides from the Longqi and Duanqiao Hydrothermal Fields in the Southwest Indian Ridge

Recent investigations found that hydrothermal activity and sulfide mineralization occurs along the Southwest Indian Ridge（SWIR）. The Longqi and Duanqiao hydrothermal fields between 49° E and 53° E of the SWIR are two prospective mineralization areas discovered by Chinese scientists. With the aim to determine the mineralogical and chemical characteristics of sulfide minerals, we have conducted detailed studies for samples from the two areas using an optical microscope, X-ray diffractometer, scanning electron microscope, and electron microprobe. The mineralization processes in the Longqi area are divided into three main stages：（1） the low-medium-temperature stage： colloform pyrite（Py I） ＋ marcasite → euhedral pyrite（Py II）,（2） the high-temperature stage： isocubanite（±exsolved chalcopyrite） ＋ pyrrhotite → coarse-grained chalcopyrite（Ccp I）, and（3） the medium–low-temperature stage： sphalerite ＋ fine-grained chalcopyrite inclusions（Ccp II） → aggregates of anhedral pyrite（Py III） ± marcasite → Fe-oxide（-hydroxide） ＋ amorphous silica. The mineralization processes in the Duanqiao area are divided into two main stages：（1） the medium–high-temperature stage： subhedral and euhedral pyrite（Py I′） → coarse-grained chalcopyrite（Ccp I′） and（2） the medium–low-temperature stage： sphalerite → fine-grained chalcopyrite（Ccp II′） ＋ chalcopyrite inclusions（Ccp II′） → silica-cemented pyrite（Py II′） ＋ marcasite → Fe-oxide ＋ amorphous silica. We suggest that the fine-grained chalcopyrite inclusions in sphalerite from Longqi and Duanqiao were formed by co-precipitation and replacement mechanisms, respectively. Primary sphalerites from both fields are enriched in Fe（avg. 5.84 wt% for the Longqi field vs. avg. 3.69 wt% for the Duanqiao field）, Co（avg. 185.56 ppm for the Longqi field vs. 160.53 ppm for the Duanqiao field）, and Cd（avg. 1950 ppm for the Longqi field vs. avg. 525.26 ppm for the Duanqiao field）. Cu contents in pyrite from the Duanqiao field（Py I′： avg. 849.23 ppm and Py II′： avg. 1191.11 ppm） tend to be higher than those from the Longqi field（Py I： avg. 26.67 ppm, Py II： avg. 445 ppm, and Py III： avg. 179.29 ppm）. Chalcopyrite from both fields is enriched in Zn（Ccp I： avg. 3226.67 ppm, Ccp II： avg. 9280 ppm, Ccp I′： avg. 848 ppm, Ccp II′（inclusions）： avg. 1098 ppm, and Ccp II′（fine-grained）： avg. 1795 ppm）. The varying contents of Zn in the different pyrite and chalcopyrite generations may result from the zone refining process. An integrated study of the mineralogy and mineralogical chemistry suggests that the hydrothermal fluids of the Longqi area are likely conditioned with higher temperatures and relatively lower fO2 and fS2 than those of the Duanqiao area, but in contrast to the former, the latter is much affected by the compositions of the surrounding rocks.