Description and crystal structure of Manganlotharmeyerite, Ca(Mn3+,□,Mg)2{AsO4,[AsO2 (OH 2]}2(OH,H2O)2, from the Starlera Mn deposit, Swiss Alps, and a redefinition of lotharmeyerite

Joël Brugger, Sergey V. Krivovichev, Uwe Kolitsch, Nicolas Meisser, Michael Andrut, Stefan Ansermet, Peter C. Burns

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Abstract

Manganlotharmeyerite, Ca(Mn3+,□,Mg 2{AsO4,[AsO2(OH)2]}2 (OH,H2O)2, is a new member of the tsumcorite group of minerals, of general formula Me(1)Me(2)2(XO4 2(OH,H2O)2, in which Me(1) represents Pb2+, Ca2+, Na+ and Bi3+, Me(2) represents Fe3+, Mn3+, Cu2+, Zn2+, Co2+, Ni2+, Al3+ and Mg2+, and X can be P5+, As5+, V5+ or S6+. Manganlotharmeyerite occurs with sailaufite, (Ca,Na,□ 2Mn3O2(AsO4)2(CO 3 •,3H2O, tilasite and calcite in discordant veinlets within massive braunite ores at the Starlera Mn deposit in the Eastern Alps of Switzerland. Manganlotharmeyerite and the associated arsenates are the product of the latest stage of As remobilization during retrograde Alpine metamorphism of Triassic carbonate-hosted, syngenetic exhalative Mn-ores. Manganlotharmeyerite forms elongate platy crystals and aggregates up to 1 mm in length, but optically homogeneous crystals do not exceed 100 μm in length. The mineral has a Mohs hardness of about 3, with a Dcalc of 3.75(2) and a Dmeas of 3.77(2) g/cm3; it is brittle, with an irregular fracture and a distinct cleavage parallel to {001}. Manganlotharmeyerite is transparent to translucent with an adamantine luster. It is brown-red to dark reddish orange in color, and has a light brown streak. The new mineral species is biaxial positive, with α 1.785(5), β 1.814(5) and γ 1.854(5), 2V(meas.) about 85°, 2V(calc.) 82°, dispersion r < v weak. The average composition of the mineral, measured with an electron microprobe, is [wt%]: V2O5 0.67, As2O5 53.41, MgO 3.95, Mn2O3 16.70, CaO 12.42. Fe2O3, 0.91, NiO 1.07, minor quantities (< 1 wt%) of Al2O3, SrO, Na2O, CoO, and ZnO, H2Ocalc 8.84, sum 99.3, resulting in the empirical chemical formula (Ca0.94Sr0.01Na0.01 (Mn3+ 0.450.26Mg0.20Ni 0. 03 Fe0.02Zn0.02Al0.01Co0.01 2(As0.98V0.02)2O 10H4.16. The structural chemical formula of the mineral derived on the basis of the crystal- structure refinement is Ca(Mn3+ 0.590 .24Mg2+ 0.17)2{(AsO4 0.76[AsO2(OH)2]0.24} 2[(OH 0. 59(H2O)0.41]2. Manganlotharmeyerite is monoclinic, C2/m, a 9.043(1), b 6.2314(7), c 7.3889(9) Å, β 116.392(2)°, V 372.99(9) Å3 and Z = 2. Single-crystal structure refinement (R1 = 0.025) confirmed isotypy with the tsumcorite structure. The strongest ten lines in the X-ray powder-diffraction pattern [d in Å (I)(hkl)] are: 4.93(80) (1̄10), 4.61(50)(111), 3.426(60 (2̄02), 3.182(100)(1̄12), 3.122 (50)(020), 2.927(70)(201), 2.822(70 (02̄1), 2.718(80)(3̄11), 2.555(100)(2̄21,3̄12), 2.134(70 (202,221). Unit-cell parameters from the powder-diffraction data are: a 9.074(5), b 6.239(3), c 7.406(4), β 116.60(3)°. The nomenclature of members of the tsumcorite group with Me(1) = Ca and X = As (lotharmeyerite subgroup) is based on the occupant of the Me(2) site. Manganlotharmeyerite represents the Mn3+- dominant member of the group, and lotharmeyerite is redefined to represent the Zn-dominant member. The lotharmeyerite subgroup now contains six phases: cabalzarite with Me(2) = Mg, ferrilotharmeyerite (Fe3+), cobaltlotharmeyerite (Co), nickellotharmeyerite (Ni), lotharmeyerite (Zn) and manganlotharmeyerite (Mn3+).

Original languageEnglish
Pages (from-to)1597-1608
Number of pages12
JournalCanadian Mineralogist
Volume40
Issue number6
DOIs
Publication statusPublished - 1 Jan 2002
Externally publishedYes

Keywords

  • Lotharmeyerite
  • Manganlotharmeyerite
  • New mineral species
  • Starlea Mn deposit
  • Structure refinement
  • Swiss Alps
  • Tsumcorite group

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