MINERAL INTERGROWTHS REPLACED BY ELBOW-TWINNED RUTILE IN ALTERED ROCKS

Some aggregates of rutile, classically considered to be ''elbow'' twin ned, instead are topotactic replacements of ilmenite or other hexagona l titaniferous precursors. Twinned ruble can be differentiated from th e reticulated rutile of topotactic replacements by the angle of prism intersections, junction morphology, and the overall form of the aggreg ate. In a special case of topotactic replacement of ilmenite, rutile f orms pseudomorphs of ''trellis''-textured ilmenite lamellae in {111} o f precursor magnetite. We trace the progress of ruble formation throug h the alteration of fine-grained magnetite-bearing host rocks. The seq uential two-step topotaxy from magnetite through ilmenite to ruble req uires ruble prisms to parallel the intersections of {111} planes in pr ecursor magnetite. Some coarse reticulated ruble may result from the s ame paragenetic sequence.