2V is the angle between the two optic axes in a biaxial crystal. Measured values of 2V for this mineral vary widely from about 60° to 90°. It is also possible to calculate a theoretical value of 2V from the measured values of the refractive indices. The calculated value varies from 68° to 88°. The direction perpendicular to the plane containing the two optic axes is called the optical direction Y. In magnesiohastingsite Y is parallel to the b crystal axis. The optical direction Z lies in the plane containing the two optic axes and bisects the angle between them. In magnesiohastingsite the angle between Z and the c crystal axis is 15° to 19°. If the color of the incident light is changed, then the refractive indices are modified, and the value of 2V changes. This is known as dispersion of the optic axes. For magnesiohastingsite the effect is weak, with 2V larger for red light than for violet light (r>v). Pleochroism is variable in green, yellow-green, bluish-green and brown. Physical properties Cleavage is good with cleavage surfaces intersecting at about 56° and 124°, as is common with all the amphiboles. The mineral displays simple or multiple twinning parallel to a prism face. It is a brittle mineral, with Mohs hardness 5 to 6 and specific gravity 3.2, with increasing magnesium content causing a decrease in specific gravity. It is not radioactive and it is insoluble in hydrochloric acid. Occurrence and associations The type locality is in the Canadian National Railway tunnel, Montreal, Quebec, Canada. It is common in amphibolite, schist and pegmatitic gabbro. It is also found in welded tuffs, granodiorite, granite and tonalite. Associated minerals include quartz, orthoclase, plagioclase, biotite, magnetite and apatite. Barkevikite and the magnesium-rich members of the hastingsite group are found in diorite, essexite and related calcium-rich rocks. Localities include: The granitic batholiths of the Scottish Highlands The Swiss and Italian Alps The Harz Mountains, Germany Finland and Sweden Japan, where it is widespread The Southern California and Sierra Nevada batholiths, California, US Langban, Sweden, in skarn In a xenolith in diorite porphyry in the Mount Hilliers complex, Henry Mountains, Garfield County, Utah References External links Jmol: http://rruff.geo.arizona.edu/AMS/viewJmol.php?id=11242 Amphibole group Monoclinic minerals Minerals in space group 12