Dorrite is a silicate mineral that is isostructural to the aenigmatite group. Although it is most chemically similar to the mineral rhönite a₂Mg₅Ti(Al₂Si₄)O₂₀ the lack of titanium (Ti) and presence of Fe³⁺ influenced dorrite's independence. 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.

Discovery

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 Fe³⁺ in its octahedral sites. Other minerals that coexist with this phase are plagioclase, gehlenite- akermanite, magnetite-magnesioferrite-spinel solid solutions,

esseneite Esseneite is a relative rare mineral of the pyroxene group, with formula CaFeAlSiO6. It is the ferric-iron-dominant member. Esseneite is an iron-analogue of other pyroxene-group members, davisite, grossmanite, and kushiroite. It is a metamorphic ...

, nepheline, wollastonite, Ba-rich feldspar,

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...

, ulvöspinel, ferroan sahamalite, and secondary

barite Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate ( Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...

, and

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

Occurrence

Dorrite can be found in mineral reactions that relate dorrite + magnetite + clinopyroxene, rhönite + magnetite + olivine + clinopyroxene, and aenigmatite +

pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...

+ 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.

Crystallography

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 Å, α=106.26°, β=95.16°, γ=124.75°.

Chemical Composition

Calcium 8.97%
Magnesium 5.44%
Aluminum 6.04%
Iron 37.48%
Silicon 6.28%
Oxygen 35.79%

Oxides

CaO 12.55%
MgO 9.02%
Al₂O₃ 11.41%
Fe₂O₃ 53.59%
SiO₂ 13.44%

References

{{reflist Natural materials Inosilicates Triclinic minerals