Study On The Chromogenic Factors Of Purple Orthoclase In Yaojiazhuang Pyroxene Syenite In Hebei Province

Feldspar,as a kind of common rock forming mineral composed of calcium,sodium and potassium aluminosilicates,is the main component mineral of almost all magmatic rocks except a small amount of ultrabasic rocks.There are many kinds of feldspar with various colors,such as white,pink,yellow,light green,etc.but the feldspar itself should be colorless and transparent.On the one hand,the color may be due to the change of composition caused by weathering;On the other hand,the structure of feldspar has changed,including crystal structure and internal structure of feldspar;The more important reason may be that the feldspar contains other components and some special inclusion minerals.Relatively speaking,alkaline rocks,as a special kind of magmatic rocks,are strongly alkali rich,and most of the main minerals are alkaline feldspar,showing different colors,often reaching the gem level.There is a kind of biotite pyroxene syenite in Triassic alkaline complex in yaojiazhuang,Hebei Province.The overall color ratio is high.Orthoclase is gray white and purplish red.The purplish red feldspar is large in size and very bright in color.It is distributed between bright green biotite and green black pyroxene.It is very beautiful,rare and of great ornamental value.It can reach gem level,but there are too many fissures,It can’t be cut,but the study of its chromogenic factor will provide an important basis for the study of this kind of feldspar gem grade minerals.Based on the petrology and geochemistry of syenite and the mineral structure and geochemistry of feldspar,this paper studies the composition and structural characteristics of feldspar by means of infrared,ultraviolet,Raman,electron microprobe and TIMA,and then finds out the chromogenic factors of purplish red feldspar,so as to provide theoretical basis for the development of this kind of ornamental stone.The conclusions are as follows:1.Microscopic and TIMA scanning show that the orthoclase grains of the augite syenite with grayish white orthoclase are small,but the content is low and high.The orthoclase grains of the augite syenite with purplish red orthoclase are large,but the content is slightly low.Geochemical analysis shows that the two types of augite syenite have the same composition.Relatively speaking,the content of Al₂O₃ in augite syenite with purplish red orthoclase is lower than that of augite syenite with grayish white orthoclase,which indicates that weathering is not the cause of the coloration of purplish red orthoclase,on the contrary,it may lead to the gradual change of purplish red to grayish white.The contents of Mn O and Fe O in the augite syenite with purplish red orthoclase are lower than those in the augite syenite with grayish white orthoclase,and the contents of Fe O and Mn O in the augite and mica are higher.Therefore,more Fe²⁺and Mn²⁺ions enter the orthoclase of the augite syenite with purplish red orthoclase,resulting in the occurrence of purplish red syenite,Therefore,the high content of Fe O and Mn O is one of the reasons why orthoclase appears purplish red.2.EBSD analysis shows that the crystal structure of two kinds of orthoclase with different colors is similar,and there is no obvious difference,which can be basically excluded.The results of TIMA analysis show that the internal structure of orthoclase is obviously different,the gray orthoclase is relatively broken,the fine minerals are distributed along the fracture filling,the filling minerals are rich,there are quartz and amphibole,the fine mineral inclusions are relatively less,while the purple orthoclase is relatively complete,the fracture is less,there is no other minerals except amphibole,The distribution of fine mineral inclusions is dispersive,which is the reason why orthoclase is purple.3.Raman analysis shows that the purplish red orthoclase contains anatase and sphene,while the gray white orthoclase has no such chromogenic minerals.Anatase,sphene and rutile can be transformed into each other and present red.The mineral inclusions in purplish red orthoclase should be one of the reasons for its purplish red.