Dorrite
Dorrite is a silicate mineral that is isostructural to the aenigmatite group.[2] Although it is most chemically similar to the mineral rhönite [Ca2Mg5Ti(Al2Si4)O20], the lack of titanium (Ti) and presence of Fe3+ influenced dorrite's independence.[2] 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.[3] This mineral is sub-metallic resembling colors of brownish-black, dark brown, to reddish brown.
| Dorrite | |
|---|---|
 Reddish-brown dorrite crystals from Chelyabinsk, Russia | |
| General | |
| Category | Inosilicate Sapphirine supergroup |
| Formula (repeating unit) | Ca2Mg2Fe43+(Al4Si2)O22 |
| IMA symbol | Dor[1] |
| 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 [Å]; 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/cm3 |
| 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 | [2][3] |
Discovery
Dorrite was first reported in 1982 by A. Havette in a basalt-limestone contact on Réunion Island off of the coast of Africa.[2] 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.[2] 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.[2]
Occurrence
Dorrite can be found in mineral reactions that relate dorrite + magnetite + clinopyroxene, rhönite + magnetite + olivine + clinopyroxene, and aenigmatite + pyroxene + olivine assemblages in nature.[2] These assemblages favor low pressures and high temperatures.[2] 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.[3]
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°.[3]
Chemical Composition
Calcium 8.97%
Magnesium 5.44%
Aluminum 6.04%
Iron 37.48%
Silicon 6.28%
Oxygen 35.79%[3]
Oxides
CaO 12.55%
MgO 9.02%
Al2O3 11.41%
Fe2O3 53.59%
SiO2 13.44%[3]
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 [Ca2 (Mg2 Fe3+4)(Al4Si2) O20], 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.