Growth structure and crystal symmetry of grossular garnets from the Jeffrey Mine, Asbestos, Quebec, Canada

Birefringent, orange grossular garnets from the Jeffrey mine, Asbestos, Quebec, Canada, have noncubic ordered arrangements of the Al³⁺, Fe³⁺, and/or OH⁻ ions and rhombic growth hillock with four vicinal faces on the (110) face. The surface features correspond to internal growth patterns observed in thin section between crossed polarizers. The growth steps are symmetrically inclined to the (001) and (110) planes on the (110) face, and the ordered arrangement of Al³⁺, Fe³⁺, and/or OH⁻ ions is symmetrical with respect to these planes, which produces sectoral twins on the crystal face. The twofold axis (b axis) is normal to the (110) growth face; therefore the crystal symmetry is monoclinic. Colorless and green grossular garnets show fine growth steps parallel to [001] on the (110) face; therefore their crystal symmetry is orthorhombic, although some parts are strained.