Dorrite
(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]
Unknown space group
c = 5.24 [Å]; β = 99.9°
β=1.84
γ=1.86
Y=yellowish brown
Z=greenish brown
Dorrite is a silicate mineral that is isostructural to the aenigmatite group.[2] It is most chemically similar to the mineral rhönite [Ca2Mg5Ti(Al2Si4)O20], made distinct by a lack of titanium (Ti) and the presence of Fe3+.[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.
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.
- ^ a b c d e f g h 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.
- ^ a b c d e f Barthelmy, Dave. "Dorrite Mineral Data". webmineral.com. Retrieved 11 December 2015.
