红柱石、蓝晶石和夕线石的相变与变形

Polymorphism and deformation of andalusite,kyanite and sillimanite

红柱石、蓝晶石和夕线石是铝硅酸盐(Al_(2)SiO_(5))的3种同质多像体,其晶体结构、物理性质和温度、压力稳定范围各不相同。研究它们的矿物共生组合、相变和变形,可以解析矿物结晶顺序、变质与变形历史。本文对已有的天然样品和实验岩石学研究成果进行综合分析,发现Al_(2)SiO_(5)的多相转变和相平衡关系受化学组成、流体、温度和压力等环境条件的影响;Al_(2)SiO_(5)的双相或三相共生组合通常是亚稳态的;矿物结晶顺序记录着区域的温度压力轨迹和构造演化历史。变质岩石中,Al_(2)SiO_(5)矿物可以形成不同强度的结晶学优选方位,其中c[001]轴近似平行线理方向,红柱石、蓝晶石和夕线石的主滑移系分别为(110)[001]、(100)[001]和(010)[001]或(100)[001]。目前,红柱石-夕线石转变线位置和三相点的温压条件还存在争议,三相矿物的共生组合是变质过程的亚稳态共存状态,不能代表三相点的温压条件。

The Al_(2) SiO_(5) polymorphs,namely andalusite,kyanite and sillimanite,are characterized by different crystal structures and physical properties stable at different pressure and temperature(p-T)conditions.The intergrown paragenetic assemblage of these three polymorphs as well as their correlated phase transformations and deformations can be used to study mineral crystallization sequence,metamorphism and deformation history of the metamorphic rocks.Studies of natural samples and the experimental petrology indicate that the phase transformation and phase equilibria relationship of Al_(2) SiO_(5) are comprehensively influenced by many factors including the chemical composition of original rocksand metamorphic fluids,and their temperatures and pressures.The associations of two or three Al_(2) SiO_(5) polymorphs of andalusite,kyanite and sillimanite in metamorphic rocks occurred usually as results of metastable phases during metamorphic episodes.The crystallization sequence of those three phases recorded important informations about the p-T paths and the tectonic evolution history of a region.The Al_(2) SiO_(5) minerals in many metamorphic rocks may show weakly to strongly crystallographic preferred orientations(CPO)characterized with c-axes subparallel to mineral lineation,suggesting the importance of the[001]slip system.The fabrics of andalusite and kyanite suggest that their dominant slip systems are of the(110)[001]and(100)[001],respectively.The fabrics of sillimanite aggregates indicate that its main slip system is of the(010)[001]or(100)[001].So far,the position of the andalusite-kyanite transition line in the phase diagram and the temperature and pressure conditions of the triple-point of andalusite,kyanite and sillimanite are still controversial.The intergrown paragenetic assemblage of these three phases of aluminum silicates represents metastable coexistence state in the metamorphic process and cannot be used to accurately represent p-T conditions of the triple point of andalusite,kyanite and sillimanite in the phase diagram.