Fluorlamprophyllite, Na-3(SrNa)Ti-3(Si2O7)(2)O2F2, a new mineral from Pocos de Caldas alkaline massif, Morro do Serrote, Minas Gerais, Brazil

Author(s)

Andrade, Marcelo B. ^{[1, 2]} ; Yang, Hexiong ^[2] ; Downs, Robert T. ^[2] ; Farber, Gunnar ^[3] ; Contreira Filho, Reynaldo R. ^[4] ; Evans, Stanley H. ^[2] ; Loehn, Clayton W. ^[5] ; Schumer, Benjamin N. ^[2]

Affiliation

^[1] Univ Sao Paulo, Sao Carlos Inst Phys, Dept Phys & Interdisciplinary Sci, POB 369, BR-13560970 Sao Carlos, SP - Brazil

^[2] Univ Arizona, Dept Geosci, 1040 East 4th St, Tucson, AZ 85721 - USA

^[3] Bornsche Str 9, D-39326 Samswegen - Germany

^[4] Rua Murici 62, BR-13098315 Campinas, SP - Brazil

^[5] Louisiana State Univ, Shared Instrumentat Facil, Baton Rouge, LA 70803 - USA

• Document type: Journal article
• Source: MINERALOGICAL MAGAZINE; v. 82, n. 1, p. 121-131, FEB 2018.
• Web of Science Citations: 5

Abstract

A new mineral species, fluorlamprophyllite (IMA2013-102), ideally Na3(SrNa)Ti-3(Si2O7)(2)O2F2, has been found in the Pocos de Caldas alkaline massif, Morro do Serrote, Minas Gerais, Brazil. Alternatively, the idealized chemical formula could be written as (SrNa){[}(Na3Ti)F-2]{[}Ti-2(Si2O7)(2)O-2], 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 similar to 3; cleavage is perfect on [100] and no parting was observed. The calculated density is 3.484 g/cm(3). Optically, fluorlamprophyllite is biaxial (+), with alpha= 1.735(7), beta = 1.749(7) and gamma = 1.775(9) and 2V(meas) = 72(3)degrees. 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)(Sigma 4.90)(Ti2.63Fe0.24Al0.04)(Sigma 2.91)Si3.89O16{[}F-1.15(OH)(0.85)](Sigma 2.00). 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) A, beta = 96.794(2)degrees and V = 727.5(1) A(3). The structure is a layered silicate inasmuch as the O atoms are arranged in well-defined, though not necessarily close-packed layers. (AU)

• FAPESP's process: 13/03487-8 - Center of characterization of new mineral species: Raman spectroscopy, electron microprobe and X-ray and neutron diffraction
• Grantee: Marcelo Barbosa de Andrade
• Support Opportunities: Research Grants - Young Investigators Grants