| Dorrite | |
|---|---|
 Reddish-brown dorrite crystals from Chelyabinsk, Russia | |
| General | |
| Category | Inosilicate Sapphirine supergroup |
| Formula (repeating unit) | Ca2Mg2Fe4(Al4Si2)O22 |
| IMA symbol | Dor |
| Strunz classification | 9.DH.40 |
| Dana classification | 69.2.1a.2 |
| Crystal system | Triclinic Unknown space group |
| Unit cell | a = 9.98, b = 5.08 c = 5.24 ; β = 99.9° |
| Identification | |
| Formula mass | 893.97 g/mol |
| Color | Dark red-brown to dark brown |
| Crystal habit | Anhedral; Small prismatic crystals; Pseudomonoclinic |
| Twinning | Common, producing a pseudomonoclinic symmetry |
| Cleavage | Good cleavage assumed to be parallel to {010} and {001} |
| Fracture | Irregular |
| Mohs scale hardness | 5 |
| Luster | Submetallic |
| Streak | Grey |
| Diaphaneity | Subopaque |
| Density | 3.959 g/cm |
| Refractive index | α=1.82 β=1.84 γ=1.86 |
| Birefringence | δ = 0.040 |
| Pleochroism | X=red-orange to brown Y=yellowish brown Z=greenish brown |
| 2V angle | 90° |
| Absorption spectra | Very strong |
| References | |
Dorrite is a silicate mineral that is isostructural to the aenigmatite group. It is most chemically similar to the mineral rhönite , made distinct by a lack of titanium (Ti) and the presence of Fe. 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 Fe 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.
Occurrence
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.
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
- Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
- ^ Cosca, Michael A.; Rouse, Roland R.; Essene, Eric J. (December 1988). "Dorrite , a new member of the aenigmatite group from a pyrometamorphic melt-rock". American Mineralogist. 73 (11–12): 1440–1448.
- ^ Barthelmy, Dave. "Dorrite Mineral Data". webmineral.com. Retrieved 11 December 2015.