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Transmitted Off Crossed Polarized Light Illumination

Halotrichite is a fibrous iron-aluminum sulfate. Its refractive indices are 1.480 and 1.490. It has
oblique extinction of about 35 degrees and a positive sign of elongation. See L. Miles, T. Hopen, and
R. Kuksuk, "Identification of Halotrichite", Microscope Magazine, vol. 38, No. 1, pp. 41-46, and for additional data.