钙钛矿族矿物的晶体化学分类和地球化学演化

钙钛矿族矿物是硅不饱和岩石 (如金伯利岩、黄长岩、似长岩、辉闪苦橄岩、单斜辉石岩和碳酸盐岩等 )中普遍存在的副矿物。它们的一般化学式为ABX3 ,其中A代表Na ,Ca ,Sr等 ,B代表Ti,Nb ,Fe3 +,Th等 ,X代表O ,F。钙钛矿结构是理想的等轴钙钛矿结构发生畸变而衍生出的正交和四方结构。自然界中产出的钙钛矿族矿物的化学成分可以用 7端员化合物来表示(即“钙钛矿空间”) ,但大部分落在斜方钠铌矿钠铈钛矿锶钛矿钙钛矿四元体系内 ,并明显受地质或热力学条件的控制。钙钛矿族矿物的一般成分演化趋势为 :狭义的钙钛矿→钠铈钛矿→铈铌钙钛矿→钙铌、铌钙和铌钠铈钛矿→ (等轴或斜方 )钠铌矿。此外 ,钙钛矿族矿物是碱性岩中主要的稀土载体矿物之一 ,它们表现出强烈的轻稀土富集。虽然在许多岩浆组合中钙钛矿是液相线上的稳定相 ,但在亚固相再平衡过程、交代作用和次生蚀变作用中 ,会被其它含钛矿物(如榍石、锐钛矿等 )所交代 ,形成特殊的组合结构

CRYSTAL-CHEMISTRY AND GEOCHEMISTRY OF PEROVSKITE-GROUP MINERALS

Perovskite\|group minerals are ubiquitous accessory minerals in silica\|undersaturated rocks such as kimberlites,foidites,olivinites,clinopyroxenites,and carbonatites.They are expressed by the general structural formula ABX 3,where A stands for Na,Ca,Sr…,B stands for Ti,Nb,Fe 3+ ,Th…,and X stands for O,F.The structure of the perovskite is an orthorhombic and tetragonal one derived from the distortion of the ideal cubic perovskite structure.The major element compositions of natural perovskites may be expressed in terms of only seven end\|member components(“perovskite space”),and are mostly within the quaternary system of lueshite\|loparite\|tausonite\|perovskite,and could be controlled significantly by geological or thermodynamic conditions.The general trend of compositional evolution of perovskite\|group minerals is from perovskite ss . through cerian and cerian niobian perovskite to calcian niobian,niobian calcian and niobian loparite\|(Ce),and then to (iso)lueshite.Perovskite is one of the principal hosts for REE in alkaline rocks,and is typically strongly enriched in LREE.Although perovskite is a stable phase on the liquidus in many magmatic assemblages,it may be replaced by other Ti\|bearing minerals(i.e.titanite,anatase…)during subsolidus reequilibration,metasomatic and secondary alteration.