Linxia basin is a late Cenozoic fault-block basin along the border between the Tibet plateau and the Loess plateau in Northwestern China. Its formation and sediments’ record may be directly affected by the structural dynamics and the uplift of the Tibet Plateau. For this reason, we studied the mineral characteristics of the Maogou formation of Miocene sediments using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD results showed that quartz, chlorite, illite, calcite, plagioclase, orthoclase, kaolinite, and trace palygorskite are present throughout the sequence of the cross-section in Linxia basin. In contrast, gypsum and ankerite occur occasionally in the upper portion of the Miocene deposits. The content of calcite varied with layers in the profile and was relatively concentrated in mudstone. Chlorite and illite were observed transforming into palygorskite under SEM observation. Based on the mineral characteristics and the change of mineral composition, especially the clay minerals, it could be inferred that an arid environment prevailed in the region with warm and cold intervals during Miocene time.

The provenance of clays in shaley intervals across the Permian-Triassic boundary (PTB) in the Xiakou section was investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM), and the results suggest that the layers have three different provenances. The layer P267-b has a loose texture with an oriented arrangement of detrital clay particles, consisting mainly of illite and minor chlorite with irregular outlines or ragged edges. The dehydroxylation reaction of the clays in this layer is characterized by an intense overlapping endothermic effect at ∼600°C, produced by mixed-layer illite-smectite (I-S) consisting of a mixture of cis -vacant ( cv )and trans -vacant ( tv ) octahedral sheets derived from weathering of detrital illite. Layer P259-b shows a more condensed texture with a dark color, and is composed mainly of I-S and minor illite and chlorite. Evidence for alteration of detrital materials to clay mineral aggregates was observed under SEM. Similar to layer P267-b, an intense dehydroxylation reaction occurs at ∼600°C, indicating clays consisting of a mixture of tv and cv sheets and, therefore, that the sediments were derived from a mixture of terrigenous and volcanic sources, combining the texture and the clay-mineral composition of those sediments. However, the undisturbed lamination and relatively small grain size in this bed indicate a low-energy depositional environment. The clay-mineral compositions of the other layers are mainly of I-S with minor amounts of illite and chlorite. Their endothermic dehydroxylation reaction, however, occurs mainly at ∼660°C, indicating that cv sheets are dominant in the clays, and thus, are derived from smectites of volcanic origin. Observations by SEM show that clay minerals grow at the expense of detrital materials, confirming the diagenetic alteration of volcanic ashes in marine sediments. Illite and chlorite are the detrital clay minerals in the clay layers across the PTB in the Xiakou section. The presence of detrital illite and chlorite in the sediments means that an arid climate prevailed in the region during the end-Permian and early Triassic period.

Clay mineralogy of the Late Pleistocene loess sediments in Tawan,Linxia,northwestern China,was investigated using X-ray diffraction(XRD)and scanning electron microscopy(SEM).The results show that mineral compositions of both the loess sediment and the palaeosol are illite,chlorite,mixed-layer illite-smectite,kaolinite,and minor palygorskite.The mineral assemblage of illite,chlorite,mixed-layer illite-smectite,and kaolinite suggests that the palaeoclimate of the source area changed from warm and humid to cold and arid.The Kubler index of illite crystallinity(IC),full width half maximum of the(001)reflection,ranges from 0.23° to 0.86°.It shows significant difference between the lithological layers.However,the IC curve presents two peaks,suggesting that the palaeoclimate of Late Pleistocene in Linxia experienced two cycles of warm and humid to cold and arid,and the late cycle was more humid and more arid comparative to the former one.Both authigenic and detrital palygorskite occured in the Late Pleistocene loess sediments,the stability of palygorskite indicates that the annual rainfall was around 300 mm during the Late Pleistocene.

The gemmological characteristics, microstructure and mineralogical characteristics of a new kind of chlorite jade——“seraphinite”—— from Russia, which is recently presented in the gem markets, are studied by using refractometer, ultraviolet fluorescence lamp, X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show that the chlorite jade is in dark green, semi-translucent to opaque, and lepidoblastic texture, with hardness of 2.5, refractive index 1.58 and density 2.60 g/cm~ 3 . It has homogeneous texture and less fissures, and no fluorescence under ultraviolet fluorescence lamp. XRD analysis shows that the mineral composition of the chlorite jade is mainly chlorite. SEM results show that the chlorite exhibits well-developed crystal. It also can be seen that the chlorite aggregates in different directions show a crossed distribution. The chemical analysis suggests that the chlorite contains w(MgO) 35.4%, w(Al_ 2 O_ 3 ) 14.8%, w(SiO_ 2 ) 32.6 %, w(FeO) 4.2% and w(H_ 2 O) 13.0%. The jade is composed of almost clinochlore, with a chemical formula Mg_ 6 (Fe_ 0.7 Mg_ 3.8 Al_ 1.5 )(Si_ 6.2 Al_ 1.8 )O_ 20 (OH)_ 16 . The chlorite crystal is in oriented distribution. In addition, the large particles of the schistose crystals exhibit wave-shaped bending. Both the microstructure and mineralogical characteristics are probably responsible for silky luster of the chlorite jade.