DICKITE FROM DOWNCAST PILLAR IN THE SHAFT OF YUNGANG COAL FIELD, DATONG

Dickite found in the shaft of Yungang Coal Field occurs in downcast pillar close to coal seams at the top of Datong Formation of Middle Jurassic. It fills in pores and cracks of rocks, being in spotted block and network of vein. The main characteristics of this mineral are as follows : it is white-powdery aggregate with well lubricating,in most cases, its crystal takes the shape of hexa- gonal tabular;refractive index : Ng = l.567-1.568, Nm =1.563-1.564, Np = 1.561-1.562 Ng-Np=0.006 Ng//b, ApΛ(010)CΛNp=11°αΛNm=15°, (+) 2V=65°, parallel extinction, length slow. Under the scanning electromicroscope, its shapes are hexagonal prism or pagoda-like. Its chemical compositions are : SiO₂ = 46.54%, Al₂O₃ = 38.99%, H₂O⁺ 13.93%, SiO₃: Al₂O₃ = 202 : 100. Its thermography curve has a intense endothermic peak at 670°, the curve is gently slope in the side of lower temperature and steeply in the side of higher temperature. In the figure of X-ray diffraction curves, peaks at d (021) =4.2442, d (012) = 3.9549,d (022) =3.7855 appear in the range of 2θ = 20°-24°;and between 2θ = 34°and 40°,four continnuous diffraction peaks appear.In the infrared spectrogram absorption, peaks at 3708,3640,3614cm^(-1) occur in high frequency range, the frequency is from higher to lower, while the intensity of absorption peaks is from weak to strong. All of these data fully prove that the dickite from this district is similar to that from other districts. Owing to the fact that this mineral is found at the depth of about 400m under the earth's surface and intergrows with pyrite, the dickite may be formed in the condition of the reducing environment with less than 15℃ temperature.