Magnesiohastingsite

Unit cell

Magnesiohastingsite belongs to the monoclinic crystal system, point group 2/m, space group C2/m. It has two formula units per unit cell (Z = 2). The unit cell parameters both for natural and for synthetic material are a = 9.9 Å, b = 18.0 Å, c = 5.3 Å, β = 105°.

Structure

The structure of all amphiboles is based on a double chain of linked SiO₄ tetrahedra, with composition (Si₄O₁₁)_n. The inner tetrahedra are referred to as T1 and the outer ones as T2. The pattern in the double chain repeats after a block of two T1 and two T2 tetrahedra, with a repeat distance of approximately 5.3 Å, and this determines the length of the unit cell along the c crystal axis. The tetrahedra within a chain all point outwards in the same direction, and the chains are linked back-to-back by cations to form I-beams. The I-beams themselves are then linked together to form the complete structure.

Optical properties

The mineral is subopaque and vitreous, green to dark green or black in color and with a pale grey-green to pale brownish-green streak. It is biaxial (-) with refractive indices for natural material in the range n_α = 1.652 to 1.676, n_β = 1.661 to 1.695, n_γ = 1.666 to 1.706. Synthetic material has n_α = 1.642 to 1.657, n_γ = 1.653 to 1.672. An increase in magnesium content causes a decrease in refractive indices. The maximum birefringence (the difference in refractive index between light travelling through the crystal with different polarizations) is δ = 0.012 to 0.033. The optic angle 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 calciium-rich rocks. Localities include: