Crystallographic relationship of troilite (pyrrhotite) in apatite host from the southern Sulu area, China

• Main Authors: Tsung-Han Tsai, 蔡宗翰
• Other Authors: Shu-Cheng Yu
• Format: Others
• Language: zh-TW
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/75814276704545988423

碩士 === 國立成功大學 === 地球科學系碩博士班 === 93 === 　Precipitate textures are not uncommon in ultrahigh-pressure (UHP) minerals. Precipitate in UHP minerals may have taken place during nearly isothermal decompression processes, in contrast to exsloution occurred with falling temperature in primary igneous minerals. Consequently, not only the mineral identity between the precipitated phase and the host matrix can be drastically different, their crystal structures may also be completely unrelated. 　The present research samples, hornblende-schists, were collected from Jiaocho(shihu), a small town in the southern Sulu UHP metamorphic belt . The major mineral composition of the schist involves hornblende, epidote, plagioclase, with minor amount of biotite, titanite and rutile. Occurred as an accessory mineral component in this hornblende-schist, apatite crystals are easily recognizable. Many distinct and well-oriented troilite (cannt exclude pyrrhotite with troilite superstructure) rods within the apatite host were identified in this study. The rods of troilite were crystallized in parallel arrangement and in two different orientations, one parallel with and the other perpendicular to the c-direction of the apatite host. These troilite rods distribute rather evenly in the apatite crystal. Based on optical examination and electron diffraction analysis, the rods of troilite have two kinds of different form in the apatite host : one is long rods, 10 ~ 50 μm in length and 1 ~ 3 μm in diameter ; the other is short rods, 1 ~ 4 μm in length and 0.5 ~ 1 μm in diameter. The growth direction of these two kinds of troilite rods are about 90° with long rods running in the c-axis of the apatite, but these two forms do not cross with each other. The approximately relative crystallographic orientation between the two kinds of troilite and the host phase, as determined from the electron diffraction patterns are long troilite rods(deviation : 1 ~ 2° ) : [ 2 -1 -2 ]Ap // [ 1 -1 0 ]Tr, ( 1 2 0 )Ap // ( 0 0 1 ) Tr, and short troilite rods : [ 2 -1 -2 ]Ap // [ 2 -2 1 ] Tr, ( 1 2 0 )Ap // ( 1 1 0 ) Tr. 　In view of the more compressible structural channel occupied by the larger F-ions running in the c-axis of the apatite host, the orientation of the troilite rods crystallized in the c-direction of the apatite matrix is consistent with the structural consideration. Since the structure of troilite is of NiAs type, and two successive FeS6 octahedra are linked together in c-axis by sharing octahedral face, and this structural feature results into a comparatively unstable and consequently energetic growth direction. This structural argument can be equally applied to the exsolution mechanism induced either by a temperature change or by a pressure effect.