Dorrite

Dorrite is a silicate mineral that is isostructural to the aenigmatite group.[1] 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.[1] 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.[2] This mineral is sub-metallic resembling colors of brownish-black, dark brown, to reddish brown.

Dorrite
General
CategoryInosilicate
Sapphirine supergroup
Formula
(repeating unit)
Ca2Mg2Fe43+(Al4Si2)O22
Strunz classification9.DH.40
Dana classification69.2.1a.2
Crystal systemTriclinic
Unknown space group
Unit cella = 9.98, b = 5.08
c = 5.24 [Å]; β = 99.9°
Identification
Formula mass893.97 g/mol
ColorDark red-brown to dark brown
Crystal habitAnhedral; Small prismatic crystals; Pseudomonoclinic
TwinningCommon, producing a pseudomonoclinic symmetry
CleavageGood cleavage assumed to be parallel to {010} and {001}
FractureIrregular
Mohs scale hardness5
LusterSubmetallic
StreakGrey
DiaphaneitySubopaque
Density3.959 g/cm3
Refractive indexα=1.82
β=1.84
γ=1.86
Birefringenceδ = 0.040
PleochroismX=red-orange to brown
Y=yellowish brown
Z=greenish brown
2V angle90°
Absorption spectraVery strong
References[1][2]

Discovery

Dorrite was first reported in 1982 by A. Havette in a basalt-limestone contact on Réunion Island off of the coast of Africa.[1] 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.[1] 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.[1]

Occurrence

Dorrite can be found in mineral reactions that relate dorrite + magnetite + clinopyroxene, rhönite + magnetite + olivine + clinopyroxene, and aenigmatite + pyroxene + olivine assemblages in nature.[1] These assemblages favor low pressures and high temperatures.[1] 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.[2]

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°.[2]

Chemical Composition[2]

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

  1. 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.
  2. Barthelmy, Dave. "Dorrite Mineral Data". webmineral.com. Retrieved 11 December 2015.
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