攀枝花钒钛磁铁矿中纳-微米尖晶石的 出溶序次及其成因机制
Sequence and Genetic Mechanism of Nano-Micron Spinel- Exsolution from Panzhihua V-Ti Magnetite Deposit

钛磁铁矿内部尖晶石出溶体的成分组成和形成机制对估算磁铁矿固溶体的成分以计算铁钛氧化物的氧逸度-温度具有重要意义.为了探究攀枝花钒钛磁铁矿中尖晶石的成因及形成机制，运用岩相观察和各种微区原位观测手段，系统研究了各类尖晶石的矿物学特征.尖晶石的粒度在纳微米之间，有3种类型：第1种为钛磁铁矿晶界处不规则的大颗粒尖晶石，Mg#为60～70；第2种为钛磁铁矿颗粒内部的粒状尖晶石，Mg#为71～77；第3种为沿钛磁铁矿(100)方向定向分布的尖晶石片晶，Mg#为75～77.3类尖晶石分别与磁铁矿主晶具有相同的取向关系：｛111｝Mag//｛111｝Spl，｛110｝Mag//｛110｝Spl和｛100｝Mag//｛100｝Spl.3类尖晶石均是磁铁矿主晶的出溶体，钛磁铁矿晶界处及其内部的粒状尖晶石的形成与某些晶体缺陷关系密切，是在降温过程中较早出溶的产物，尖晶石片晶在两者之后以旋节分解出溶形成.
Detailed characterization and formation mechanism of the spinel exsolution in titanomagnetite is crucial for reconstructing the composition of magnetite solid solution precursor and for application to the FeTi oxide oxythermometer. The chemical composition, topographic characteristics and crystallographic relationships of spinel exsolution were studied systematically to probe into its sequence and genetic mechanism through petrographic observation and the integrated use of insitu microanalysis methods. The results show that there are three output forms of the spinel exsolution. One spinel exsolution is on the edge of the titanomagnetite with Mg# range from 60-70. Another is granular spinel exsolution distributing dispersedly with big particlesize, with Mg# range from 71-77. The third spinel exsolution is present as lamellae parallelling to the ｛100｝ of the titanomagnetite with Mg# range from 75-77. Three different types of spinel exsolution are all the magnesiaalumina spinel and have closepacked oxygen planes and directions parallel to those in the host magnetite with ｛111｝Mag//｛111｝Spl, ｛110｝Mag//｛110｝Spl and ｛100｝Mag//｛100｝Spl respectively. Analysis suggests that three different types of spinel exsolution are all the product of the magnetite solid solution in the process of slow cooling with different sequence and genetic significance. The granular spinel exsolution in the titanomagnetite and the spinel exsolution on the edge of the titanomagnetite are related to some crystallographic defects in titanomagnetite and exsolved at the early stages of exsolution. The spinel lamellae exsolved in the way of spinodal decomposition parallelling to ｛l00｝