The Canadian Mineralogist
Volume 34, pages 605-614 (1996)

MINERAL INTERGROWTHS REPLACED BY "ELBOW-TWINNED" RUTILE IN ALTERED ROCKS

ERIC R. FORCE
U.S. Geological Survey, Gould-Simpson Building, University of Arizona, Tucson, Arizona 85721, U.S.A.

R. PETER RICHARDS
Morphogenesis, Inc., 154 Morgan Street, Oberlin, Ohio 44074, U.S.A.

KEITH M. SCOTT
Division of Exploration and Mining, CSIRO, P.O. Box 136, North Ryde NSW 2113, Australia

PAGE C. VALENTINE
U.S. Geological Survey, Gosnold Building, Quissett Campus, Woods Hole, Massachusetts 02543, U.S.A.

NEIL S. FISHMAN
U.S. Geological Survey, Mail Stop 939, Denver Federal Center, Denver, Colorado 80225, U.S.A.

Abstract

Some aggregates of rutile, classically considered to be "elbow-twinned", instead are topotactic replacements of ilmenite or other hexagonal titaniferous precursors. Twinned rutile can be differentiated from the reticulated rutile of topotactic replacements by the angle of prism intersections, junction morphology, and the overall form of the aggregate. In a special case of topotactic replacement of ilmenite, rutile forms pseudomorphs of "trellis"-textured ilmenite lamellae in {111} of precursor magnetite. We trace the progress of rutile formation through the alteration of fine-grained magnetite-bearing host rocks. The sequential two-step topotaxy from magnetite through ilmenite to rutile requires rutile prisms to parallel the intersections of {111} planes in precursor magnetite. Some coarse reticulated rutile may result from the same paragenetic sequence.