Optical, Compositional, Morphological, and X-Ray Data on Eleven Particles of Amphibole From Libby, Montana, U.S.A.

Amphibole crystals from the former vermiculite mine near Libby, Montana, were examined using three different analytical methods: optical and morphological measurements were made using spindle-stage methods, unit-cell refinements were per-formed using single-crystal X-ray-diffraction techniques, and chemical analyses were performed using an electron microprobe. The eleven samples display a continuum of morphology and conformity to a single crystal, and range from stubby single-crystal cleavage fragments to elongate near-bundles of fibers. Variations in conformity to the properties of a single crystal are reflected in the character of the X-ray reflections on X-ray rotation photographs. Using current nomenclature recommendations, all of the samples examined consist of winchite. Covariance among several properties was observed. The index of refraction and birefringence are dependent on Mg content. There is also a correlation between birefringence and particle morphology. Winchite particles with a high birefringence and high width/thickness aspect ratio tend to be single crystals, whereas samples with a low birefringence and low width/thickness ratio tend to be more polycrystalline.