Magnesiohastingsite is a calcium-containing amphibole and a member of the hornblende group. It is an inosilicate (chain silicate) with the formula NaCa2(Mg4Fe3+)(Si6Al2)O22(OH)2 and molar mass 864.69 g. In synthetic magnesiohastingsite it appears that iron occurs both as ferrous iron Fe2+ and as ferric iron Fe3+, but the ideal formula features only ferric iron. It was named in 1928 by Marland P. Billings. The name is for its relationship to hastingsite and its magnesium content. Hastingsite was named for the locality in Dungannon Township, Hastings County, Ontario, Canada. Calcic (containing calcium) amphiboles include: The tremolite – actinolite – ferro-actinolite series The hornblende group The kaersutite – ferro-kaersutite series Joesmithite The hornblende group includes: The edenite – ferro-edenite series The tschermakite – ferrotschermakite series The pargasite – ferro-pargasite series The magnesiohastingsite – hastingsite series The magnesiosadanagaite – sadanagaite series 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 nα = 1.652 to 1.676, nβ = 1.661 to 1.695, nγ = 1.666 to 1.706. Synthetic material has nα = 1.642 to 1.657, nγ = 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