Halotrichite
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
www.mindat.org/min-1809.html for additional data.