Dorrite is a silicate mineral that is isostructural to the aenigmatite group. It is most chemically similar to the mineral rhönite [Ca<sub>2</sub>Mg<sub>5</sub>Ti(Al<sub>2</sub>Si<sub>4</sub>)O<sub>20</sub>], made distinct by a lack of titanium (Ti) and the presence of Fe<sup>3+</sup>. Dorrite is named for Dr. John (Jack) A. Dorr, a late professor at the University of Michigan that researched in outcrops where dorrite was found in 1982. This mineral is sub-metallic resembling colors of brownish-black, dark brown, to reddish brown.
Dorrite was first reported in 1982 by A. Havette in a basalt-limestone contact on Réunion Island off of the coast of Africa. The second report of dorrite was made by Franklin Foit and his associates while examining a paralava from the Powder River Basin, Wyoming in 1987. Analyses determined that this newly found mineral was surprisingly similar to the mineral rhönite, lacking Ti but presenting dominant Fe<sup>3+</sup> in its octahedral sites. Other minerals that coexist with this phase are plagioclase, gehlenite-akermanite, magnetite-magnesioferrite-spinel solid solutions, esseneite, nepheline, wollastonite, Ba-rich feldspar, apatite, ulvöspinel, ferroan sahamalite, and secondary barite, and calcite.
Dorrite can be found in mineral reactions that relate dorrite + magnetite + clinopyroxene, rhönite + magnetite + olivine + clinopyroxene, and aenigmatite + pyroxene + olivine assemblages in nature. These assemblages favor low pressures and high temperatures. Dorrite is stable in strongly oxidizing, high-temperature, low-pressure environments. It occurs in paralava, pyrometamorphic melt rock, formed from the burning of coal beds.
Researchers conclusively determined that dorrite is triclinic-pseudomonoclinic and twinned by a twofold rotation about the pseudomonoclinic b axis. The parameters for dorrite are a=10.505, b=10.897, c=9.019 à, ñ=106.26ð, ò=95.16ð, ó=124.75ð.
Calcium 8.97%<br>Magnesium 5.44%<br>Aluminum 6.04%<br>Iron 37.48%<br>Silicon 6.28%<br>Oxygen 35.79%
CaO 12.55%<br>MgO 9.02%<br>Al<sub>2</sub>O<sub>3</sub> 11.41%<br>Fe<sub>2</sub>O<sub>3</sub> 53.59%<br>SiO<sub>2</sub> 13.44%