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Fluorlamprophyllite, Na 3 (SrNa)Ti 3 (Si 2 O 7 ) 2 O 2 F 2 , a new mineral from Poços de Caldas alkaline massif, Morro do Serrote, Minas Gerais, Brazil
2017; Cambridge University Press; Volume: 82; Issue: 1 Linguagem: Inglês
10.1180/minmag.2017.081.027
ISSN1471-8022
AutoresMarcelo B. Andrade, Hexiong Yang, Robert T. Downs, G. Färber, R.Ruiz Filho, S. H. Evans, C. W. Loehn, Benjamin N. Schumer,
Tópico(s)Chemical Synthesis and Characterization
ResumoABSTRACT A new mineral species, fluorlamprophyllite (IMA2013-102), ideally Na 3 (SrNa)Ti 3 (Si 2 O 7 ) 2 O 2 F 2 , 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)[(Na 3 Ti)F 2 ][Ti 2 (Si 2 O 7 ) 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 ~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 α = 1.735(7), β = 1.749(7) and γ = 1.775(9) and 2V meas = 72(3)°. An electron microprobe analysis produced an average composition (wt.%) (9 points) of Na 2 O 10.63(30), K 2 O 0.47(3), SiO 2 30.51(13), SrO 18.30(24), MgO 0.81(17), Al 2 O 3 0.23(2), CaO 1.11(7), MnO 5.03(38), TiO 2 27.41(87), Fe 2 O 3 2.45(37), F 2.86(23), plus H 2 O 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.% H 2 O brought [F+(OH)] = 2 pfu, yielding (Na 2.63 Sr 1.35 Mn 0.54 Ca 0.15 Mg 0.15 K 0.08 ) Σ4.90 (Ti 2.63 Fe 0.24 Al 0.04 ) Σ2.91 Si 3.89 O 16 [F 1.15 (OH) 0.85 ] Σ2.00 . The mineral is monoclinic, with space group C 2 /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) Å 3 . The 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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