| Dorrite | |
| Boxbgcolor: | purple |
| Boxtextcolor: |
|
| Category: | Inosilicate Sapphirine supergroup |
| Formula: | Ca2Mg2Fe43+(Al4Si2)O22 |
| Imasymbol: | Dor[1] |
| Molweight: | 893.97 g/mol |
| Strunz: | 9.DH.40 |
| Dana: | 69.2.1a.2 |
| System: | Triclinic Unknown space group |
| Unit Cell: | a = 9.98, b = 5.08 c = 5.24 [Å]; β = 99.9° |
| Color: | Dark red-brown to dark brown |
| Habit: | Anhedral; Small prismatic crystals; Pseudomonoclinic |
| Twinning: | Common, producing a pseudomonoclinic symmetry |
| Cleavage: | Good cleavage assumed to be parallel to and |
| Fracture: | Irregular |
| Mohs: | 5 |
| Luster: | Submetallic |
| Streak: | Grey |
| Diaphaneity: | Subopaque |
| Density: | 3.959 g/cm3 |
| Refractive: | 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: | 90° |
| Absorption: | Very strong |
| References: | [2] [3] |
Dorrite is a silicate mineral that is isostructural to the aenigmatite group.[2] It is most chemically similar to the mineral rhönite [Ca<sub>2</sub>Mg<sub>5</sub>Ti(Al<sub>2</sub>Si<sub>4</sub>)O<sub>20</sub>], 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.
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]
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]
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]
Calcium 8.97%
Magnesium 5.44%
Aluminum 6.04%
Iron 37.48%
Silicon 6.28%
Oxygen 35.79%[3]
CaO 12.55%
MgO 9.02%
Al2O3 11.41%
Fe2O3 53.59%
SiO2 13.44%[3]