Well-oriented, needlelike inclusions occurring in sagenitic biotite in an orthogneiss from the Tananao complex, northeast Taiwan, have been studied by optical microscopy, transmission and analytical electron microscopy, and electron microprobe analysis. Titanite needles in three orientations combine to form equilateral triangles, and rutile needles in three other orientations generally intersect to form asterisk-shaped units. Titanite needles are elongated parallel to 〈011〉. They are oriented within {001} ofbiotite, and one ofthe three sets, which intersect at angles of 60°, is parallel to a of biotite. The {111} or {433} planes of titanite are approximately parallel to {001} of biotite. Rutile needles are elongated parallel to c, and rutile {100} is parallel to {001} of biotite. Of the three sets of rutile needles intersecting at angles of 60°, one is parallel to b of biotite. The preferred orientation of rutile inclusions in biotite is in accord with their mutually parallel planes of closest-packed anions and chains ofedge-sharing octahedra. Titanite inclusions contain approximately 0.10 (Al + Fe) per formula unit, have space group A2/a, and give electron diffraction patterns that display streaking along b* and c*. Rutile inclusions contain 0.2–0.3 wt% Fe2O3 (total Fe as Fe2O3), exhibit streaking and splitting of reflections in electron diffraction patterns, and display a planar microstructure parallel to {100}, which appears to consist of precipitated platelets that have the hematite structure and a probable composition (Fe,Ti)2O3. The titanite and rutile inclusions are inferred to have topotaxially precipitated through reactions occurring during retrogressive metamorphism when excess Ti and Ca were derived from biotite of igneous origin.

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