Study On The Genesis Of Andalusite Black Cross Inclusions In Xixia, Henan Province

Andalusite,as a high aluminum refractory,is widely used and has important economic significance.The andalusite in Yangnaigou,Xixia,Henan Province,is characterized by black cross inclusions,which can also be used in jewelry making in addition to industrial uses.In this paper,the modern testing methods,such as microscopic observation,electron probe analysis,X-ray powder diffraction analysis,UV-vis spectrophotometer analysis,infrared spectrum analysis,Raman spectrum analysis are used.The mineralogical characteristics and black cross inclusion characteristics of andalusite in Yangnaigou,Xixia,Henan Province,are studied systematically,and its formation mechanism and forming conditions are deeply analyzed and discussed.It provides a theoretical basis for the evaluation and utilization of andalusite resources.The andalusite crystals in Xixia,Henan Province are fleshy red,brownish red to yellowish brown.Most of the crystals are nearly quadrilateral columns with black cross inclusions.The results show that the main chemical components of andalusite in Xixia are Al₂O₃ and SiO₂,Fe,Na,K,Ca,Mn,Cr,Ti are less in andalusite,and the content of Fe is not evenly distributed in andalusite.Compared with the standard value,the cell parameters of andalusite are slightly reduced.In the infrared spectrum,the stretching vibration of Al-O shifts slightly to the low frequency direction,and the stretching vibration of Si-O shifts slightly to the high frequency direction compared with the standard value,which is slightly offset from the Al-O bond.It is important that a small amount of impurity ions are mixed into the lattice,which may be caused by the substitution of Al ions by Fe,Ti ions.The absorption regions of andalusite to ultraviolet and visible light are mainly concentrated in two absorption zones with 400and 550 nm centers.The charge transfer of Fe²⁺?Ti⁴⁺is the main reason for the brown-red color of andalusite from Xixia goat in Henan Province.In the center of andalusite,there is a core with the same composition.The black cross-shaped inclusions are distributed along the a and b axes of the crystal,and the overall appearance is fishbone-like.The black cruciform inclusions mainly consist of quartz,biotite,carbonaceous,iron-alumina garnet and other minerals,and the surrounding rock is the main source of their material.In the early stage of regional metamorphism,the core of andalusite was formed by layer growth mechanism in a relatively stable environment.According to Bravi's law,the direction of crystal a and b axes was the fastest growing direction.Carbonaceous,quartz,biotite and other minerals in surrounding rocks wrapped in andalusite would accumulate at axes a and b,forming inclusions of black cross-shaped mixed phase,and continuously grow outward.The crystal planes along the axis a and b are gradually submerged by the denser adjacent surface network.Inclusions at the corner of the cylindrical surface are also the most abundant,and the interface becomes round,showing a fan-shaped appearance.The growth of andalusite crystals is completed.