STRUCTURAL CHARACTERISTICS OF FELDSPAR FROM HOLOCENE VOLCANIC ERUPTIVE ROCK ON THE TIANFENG PROFILE IN TIANCHI AND THEIR GEOLOGICAL IMPLICATIONS

We have studied the surface features and structural state of the feldspar from the Holocene volcanic eruptive rock on the Tianfeng profile using data from observations under scanning electron microscope and analysis of x ray diffraction.On this profile black pumice appears at the top and darkgrey pumice is present at the bottom.The result shows that the feldspar in these pumice rocks has fresh surface,with slightly weathering for individual cases,and occasionally seen as non-crystalline floc of varied thickness at concaves on its grain surface.This feldspar belongs to the high sandidine-high albite series of the alkali feldspar.The intensity of its characteristic diffraction peak and its plane mesh d value are roughly consistent with the PDF data of anorthoclase.And its constitution((Or37～41,Ab63～59)also implies that it is anorthoclase.It has the following parameters:ordering degree 0.045～0.089,h value-1.010～-1.361,and the lowest crystalline temperature 800～990℃which is up to 990℃for the black floc at the top.Our study suggests that the younger volcanic eruption at Tianchi has a lower ordering degree of feldspar and a higher temperature reflected by its structural state.Because the structure of the present feldspar at Tainchi is the result of the cooling process of the hot volcanic rock,the temperature when the feldspar was formed might be very high.The difference of structural state in this feldspar is probably associated with ages and temperatures of volcanic eruptions. The study of its features has some theoretical and applied significance.