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IMA 2013-102 = fluorlamprophyllite

Posted by Marco E. Ciriotti  
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Marco E. Ciriotti April 18, 2018 09:29AM
Referenza:
▪ Andrade, M.B., Yang, H., Downs, R.T., Farber, G., Contreira Filho, R.R., Evans, S.H., Loehn, C.W., Schumer, B.M. (2018): Fluorlamprophyllite, Na3(SrNa)Ti3(Si2O7)2O2F2, a new mineral from Poços de Caldas alkaline massif, Morro do Serrote, Minas Gerais, Brazil. Mineralogical Magazine, 82, 121-131.

Abstract:
A new mineral species, fluorlamprophyllite (IMA 2013-102), ideally Na3(SrNa)Ti3(Si2O7)2O2F2, has been found in the Poços de Caldas alkaline massif, Morro do Serrote, Minas Gerais, Brazil. Alternatively, the idealized chemical formula could be written as (SrNa)[(Na3Ti)F2][Ti2(Si2O7)2O2], setting the large interlayer cations before the cations of the layer. Fluorlamprophyllite is the F-analogue of lamprophyllite. It is associated with aegirine, analcime, natrolite, nepheline and microcline. Fluorlamprophyllite crystals are brownish-orange and bladed. The mineral is transparent with a pale yellow streak and an adamantine luster. It is brittle and has a Mohs hardness of ~3; cleavage is perfect on {100} and no parting was observed. The calculated density is 3.484 g/cm3. Optically, fluorlamprophyllite is biaxial (+), with α = 1.735(7), β = 1.749(7), and γ = 1.775(9), 2Vmeas = 72(3)°. An electron microprobe analysis produced an average composition (wt%) (9 points) of Na2O 10.63(30), K2O 0.47(3), SiO2 30.51(13), SrO 18.30(24), MgO 0.81(17), Al2O3 0.23(2), CaO 1.11(7), MnO 5.03(38), TiO2 27.41(87), Fe2O3 2.45(37), F 2.86(23), plus H2O 1.00 (added to bring the total close to 100 %), -O=F -1.20, with the total = 98.61%. The elements Nb and Ba were sought, but contents were below microprobe detection limits. The resultant chemical formula was calculated on the basis of 18 (O + F) atoms per formula unit. The addition of 1.00 wt% H2O brought [F+(OH)] = 2 pfu, yielding (Na2.63 Sr1.35Mn0.54 Ca0.15Mg0.15 K0.08)Σ=4.90 (Ti2.63Fe0.24Al0.04)Σ=2.91Si3.89O16[F1.15(OH)0.85]Σ=2.00. It is monoclinic, with space group C2/m and unit-cell parameters a = 19.255(2), b = 7.0715(7), c = 5.3807(6) Å, β = 96.794(2)°, and V = 727.5(1) Å3. Its structure is a layered silicate inasmuch as the O atoms are arranged in well-defined, though not necessarily close-packed layers.
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Ronnie Van Dommelen August 10, 2018 12:33PM
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