Fluorlamprophyllite, Na3(SrNa)Ti3(Si2O7)2O2F2, a new mineral from Poços de Caldas alkaline massif, Morro do Serrote, Minas Gerais, Brazil

Abstract

ABSTRACTA new mineral species, fluorlamprophyllite (IMA2013-102), 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 lustre. 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) and 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 (Na2.63Sr1.35Mn0.54Ca0.15Mg0.15K0.08)Σ4.90(Ti2.63Fe0.24Al0.04)Σ2.91Si3.89O16[F1.15(OH)0.85]Σ2.00. The mineral 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. The structure is a layered silicate inasmuch as the O atoms are arranged in well-defined, though not necessarily close-packed layers.