Dorite [Ca ₂ (Mg ₂ Fe (super 3+) ₄ )(Al ₄ Si ₂ )O ₂₀ ], a new member of the aenigmatite group from a pyrometamorphic melt-rock Available to Purchase

Dorrite, a new mineral of the aenigmatite group, has been found in a paralava from the Powder River Basin, Wyoming. The type specimen used for X-ray analysis has been quantitatively analyzed, yielding the formula (Ca_1.92Na_0.07K_0.01)(Ca_0.05Mg_1.05Mn_0.03$Fe0.442+$Ti_0.05-$Fe4.383+$⁠)(⁠$Fe2.003+$Al_2.40Si_1.60)O₂₀; the end-member formula is inferred to be Ca₂(Mg₂$Fe43+$⁠)(Al₄Si₂)-O₂₀. The chemical compositions of individual dorrite grains within the same thin section display wide variations in tetrahedral Al/Fe³⁺, but contain dominant Fe³⁺ in the octahedral sites and little Ti⁴⁺. As is typical of aenigmatite-group minerals, dorrite is triclinic and twinned, producing a pseudomonoclinic symmetry. The space group is P1 or P1 with refined parameters a = 10.505(3), b = 10.897(3), c = 9.019(1) Å, α = 106.26(2)°, β = 95.16(2)°, and γ = 124.75(2)°; V = 772.5(4) ų (Z = 2) and ρ_calc = 3.959 g/cm³. Dorrite has very strong absorption and high relief with approximate refractive indices of α = 1.82, β = 1.84, and γ = 1.86 (all ±0.01). Dorrite is nearly opaque in thin section, whereas in ultrathinned areas it displays the following pleochroic formula: X = red-orange to brown, Y = yellowish brown, and Z = greenish brown. It occurs as anhedral to prismatic grains up to 0.1 mm in length and in close association with esseneite or titanian andradite. Other minerals coexisting with dorrite include plagioclase, gehlenite-akermanite, and magnetite-magnesioferrite-spinel solid solutions, with lesser Ba-rich feldspar, wollastonite, ulvospi-nel, nepheline, apatite, ferroan sahamalite, and secondary barite and calcite. The mineral is named for J. A. Dorr, Jr., late professor at the University of Michigan.

A series of mineral reactions relate dorrite + magnetite + clinopyroxene, rhonite + magnetite + olivine + clinopyroxene, and aenigmatite + pyroxene + olivine assemblages that are widespread in nature. The conditions necessary for crystallizing these assemblages appear restricted to low pressures and high temperatures. Dorrite is stable in strongly oxidizing, high-temperature, low-pressure environments.