RUTILE

TiO₂ 
Tetragonal

Rutile was first reported from the Franklin Marble by Fowler (1825) as “red oxide of titanium.” It was later described by Palache (1935), and reported from the Buckwheat Dolomite by Peters et al. (1983) and Germine (1985). It has not been found in the zinc orebodies at either Franklin or Sterling Hill.         

Description

	Figure 22-74. Rutile from the corundum-margarite occurrence at Sterling Hill. Field of view is 0.5 mm in maximum dimension.

Local rutile is predominantly fibrous, acicular, or prismatic in habit; among such habits are prismatic crystals, stout equant crystals, and aggregates of filamentary crystals similar to the “mountain-leather” variety of tremolite.

The color of most material is exceedingly dark red or dark brown, appearing black except in cases where red-orange internal reflections are evident under intense illumination. Some acicular crystals may have a brassy or bronzy color. A significant number of these have been mislabeled “millerite,” as reported by Thomas (1956). Millerite is not known to occur locally, and specimens so-labeled have been verified as rutile using XRD methods. The luster of freshly broken surfaces is adamantine. Stout crystals can be confused with brookite.

Composition

Rutile is a titanium dioxide mineral. Peters et al. (1983) have reported less than 1.0 wt. % Cr as a substituent for Ti, and Germine (1985) reported small amounts of iron; no other analytical data are known.

Occurrence and paragenesis

Local rutile is found in the Buckwheat Dolomite, the Franklin Marble, and assemblages derived in part from the marble. Rutile from the Buckwheat Dolomite occurs both in cavities and within the dolomite itself, suggesting an early formation relative to the other vug-occupying minerals in this rock unit (Peters et al., 1983) (Figure 22-56). Some of this material is extremely acicular and may have surficial coatings giving anomalous colorations.

Rutile is found in large (up to 5-7 mm) equant crystals associated with zircon, corundum, anorthite, and other species in the margarite occurrence in the Franklin Marble described herein from near the Sterling Hill orebody and accessed through its adits. Some of these rutile crystals have slightly to substantially rounded edges (Figure 22-74); they may be resorbed, as is common for some marble minerals (Rutstein, 1982), or may be abraded if the rutile crystals are pre-metamorphic detrital relicts. The crystals are homogeneous and do not have compositional zoning or thortveitite inclusions, as does some of the associated gahnite (Dunn and Frondel, 1990).

Palache noted the close association of rutile and corundum, and it is supported here. Rutile has been found in acid-insoluble residues during studies of the Franklin Marble and is probably a locally common mineral in the marble where Ti concentrations are high. Palache reported its occurrence in a number of the local marble quarries. Puffer et al. (1987) reported the occurrence of rutile fibers in Franklin Pond.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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