析出物的現象普遍存在於超高壓變質礦物中，然而超高壓礦物中析出的成因可能不同於在火成岩中之出溶現象。超高壓礦物的析出現象主要可能是肇因於在超高壓變質環境下，受壓力影響將不同離子充填進入主晶結構，之後超高壓岩體快速上升即相當於等溫減壓的過程而析出。

　　本研究樣本取自江蘇省石湖(南蘇魯超高壓變質帶)之角閃石片岩。主要組成礦物為角閃石、綠簾石、斜長石和少量黑雲母、榍石及金紅石。在其附屬礦物氟-磷灰石中發現不透光析出物，由穿透式電子顯微鏡半定量和選區電子繞射分析結果得知，該硫化鐵為隕硫鐵，但不排除為磁黃鐵礦具有隕硫鐵超結構，成分約在Fe0.93S ~ Fe0.95S範圍。產狀有兩種，分別為(1)長桿狀長10 ~ 50 µm，寬0.5 ~ 1 µm；(2)短桿狀長1 ~ 4 µm，寬約0.5 ~ 1 µm。長短桿狀隕硫鐵的生長方向約成90度交角且沒有相交的現象，皆有優勢排列。長桿狀隕硫鐵之長邊平行磷灰石c軸生長，廣泛均勻分佈沒有集中於磷灰石中央或邊緣，相對於長桿狀隕硫鐵，短桿狀隕硫鐵的數量則較為稀少。經電子繞射分析長短桿狀兩種形式的隕硫鐵與磷灰石各大致有以下結晶順構關係(偏差約在1 ~ 2度)：
(1)長桿狀：[ 2 -1 -2 ]Ap // [ 1 -1 0 ]Tr；( 1 2 0 )Ap // ( 0 0 1 )Tr。
(2)短桿狀：[ 2 -1 -2 ]Ap // [ 2 -2 1 ]Tr；( 1 2 0 )Ap // ( 1 1 0 )Tr。

　　兩種結晶的生長方式主要是受主晶氟-磷灰石與隕硫鐵之晶體結構影響。磷灰石是由CaO5X (X=F,OH,Cl)多面體沿c軸方向呈六次對稱分佈。這些多面體沿c軸方向重複出現(Hughes et al, 1989)。而隕硫鐵結構是NiAs型的結構，在c軸方向因相鄰兩個FeS6正八面體是以共面關係相連，增高了其排斥力的作用，較為不穩定。因此推測當隕硫鐵的c軸方向與氟-磷灰石c軸方向垂直時，能生長為長桿狀；反之當隕硫鐵的c軸方向與氟-磷灰石c軸方向平行時，則生長為短桿狀。實驗結果顯示，則是分別在垂直面與平面產生角度上的旋轉偏差。但仍然證實隕硫鐵能夠生成為長桿狀，主要是氟-磷灰石結構沿著c軸方向是比較容易壓縮，而隕硫鐵在平面方向上結構較為穩定所影響；另一方面短桿狀隕硫鐵則是在結構可以延伸的平面方向，但卻受制於磷灰石同為平面方向的網狀相連結構，而形成短桿狀。隕硫鐵的不同產狀，最主要可能是由於結晶階段不同或變質過程中受到溫壓與應力變化所造成的，則需要更進一步的相關研究。

　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.

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