Abstract: 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.

Key words: Tianchi volcano, x ray diffraction, structural state of feldspar, temperature

摘要： 利用扫描电镜和x射线衍射分析数据研究天池火山天文峰剖面全新世喷发物中的长石表面特征和结构状态,结果显示:天文峰剖面从顶部黑色浮岩向下到暗灰色浮岩中,长石表面比较新鲜,个别长石有极轻微风化,在长石颗粒表面凹处偶有沉积厚度不同的非晶质絮状物。长石为碱性长石中的高透长石-高钠长石系列,特征衍射峰的强度、面网d值与歪长石PDF数据基本相符,物相组成为(Or37～41,Ab63～59),表明天池火山喷发物中的长石属歪长石;长石有序度介于0.045～0.089之间;η参数介于-1.010～-1.361之间;最低结晶温度为800～990℃,且顶部黑色浮岩中长石的最低结晶温度高达990℃。研究认为,有天池火山喷发的年代越新,长石有序度越低,其结构状态所记录的温度越高的趋势。目前测得的天池火山长石结构是由高温结构随温度下降转变而来,因此天池火山长石最初形成的温度可能更高。天池火山全新世喷发物中长石结构状态特征的差异可能与火山喷发的年代、喷发温度有关。研究天池火山全新世喷发物中的长石结构状态特征具有一定的理论和实际应用价值。

关键词: 天池火山, x射线衍射, 长石结构状态, 温度