河北平泉光头山碱性花岗岩中的钠铁非石

AENIGMATITE FROM GUANGTOUSHAN ALKALI CRANITE,PINGQUAN, HEBEI PROVINCE, CHINA

碱性花岗岩一般以含钠质镁铁硅酸盐矿物为特征，但光头山碱性花岗岩中除钠铁闪石和霓辉石外，还有钢铁非石。钠铁非石是一种三斜晶系的富钛、铁、铀的单链非辉石类硅酸盐矿物，通常和钠质辉石共生。和钠质角门石、钠质辉石一样，钠铁非石也是一种碱性指示矿物，只是比较少见。过碱性岩浆岩中是否出现钢铁非石，主要受岩浆的温度、氧逸度和成分的控制。钠铁非石形成于过碱性岩浆演化的晚期阶段。

Alkali granites are mineralogically characterized by sodium amphiboles and sodium pyroxenes such as riebeckite, arfvedsonite, aegirine and aegirine-augite. Guangtoushan alkali granite is located in the northern part of Pingquan County, Hehei Province, China. The granite contains aenigmatite, a kind of single chain silicate mineral rich in TiO2, FeO and Na2O and being triclinic as revealed by X-ray powder and microprobe analyses. Aenigmatite, like sodium amphiboles and sodium pyroxenes, is also a kind of alkali-indicating mineral. The fact that in Guangtoushan alkali granite aenigmatite coexists with aegirine-augite, boih are the earliest mafic silicates in the rocks and were crystallized from the magmas earlier than arfvedsonite but later than alkali feldspar and quartz shows that aenigmatite was formed in the late stage of magma evolution. But the fact that there is no aenigmatite in the late pegmatite pods intruded into the granite indicates that the temperature, the fugacity of oxygen and chemical composition of the magma for the crystallization of aenigmatite bad been changed when these pegmatite pods were intruding into the granite.