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Strontian-loparite and strontio-chevkinite: Two new minerals in rheomorphic fenites from the Paraná Basin carbonatites, South America

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Abstract

Strontian-loparite is recognized at Sarambi and Chiriguelo, Paraguay and at Salitre I, Brazil as an accessory mineral in rheomorphic sanidine-aegirine-nepheline dikes associated with carbonatite plugs. Strontio-chevkinite is recognized only at Sarambi in the same rheomorphic fenites hosting Strontian-loparite, with lamprophyllite as an additional strontium-bearing mineral.

Electron microbeam analyses of strontian-loparite yield fairly constant TiO2 contents (−40 wt.%), with REE (25–30 wt.%, and Ce>La), SrO (8–24 wt.%), Na2O (2.5–7.8 wt.%), CaO (1.9–2.8 wt.%), Nb2O5 (2.7–8.8 wt.%), FeO (≃1 wt.%), and Zr, Al, Cr, Y, Mn, Mg, Ba, and K≃0.5 wt.% (oxides). Representative formulae for samples from Sarambi are (Sr0.42 REE0.27 Na0.20 Ca0.09 K0.01 Ba0.005)0.995 (Ti0.94 Zr0.002 Fe0.025 Nb0.036)1.003 O3, which is similar to Chiriguelo but contrasts with Salitre I (Sr0.14 REE0.32 Ca0.09 Ba0.004 Na0.44 K0.006)1.00 (Ti0.88 Nb0.12 Fe0.02 Zr0.005)1.025 O3, which has lower Sr but higher Na+Nb. Approximately 50% of the solid solution series between loparite and tausonite (SrTiO3) is present, establishing this as a new mineral series of cubic perovskite-type structures. The symmetry is Pm3m, a=3.886(1) Å, V=58.687(6) Å3,Z=1, with the strongest reflections at 2.746(100)(110), 1.587(50)(211), 1.942(40)(200), 1.374(30)(220), and 1.0398 (25)(321), Å. Calculated density is 5.26 gm/cm3. The mineral is idiomorphic in cubes, exhibits fluorite-type interpenetrating twins and has a metallic luster. It is opaque in tones of gray-white, with deep ruby red internal reflections. Spectral reflectances in air and oil, respectively are: 470 nm 17.9–18.5%, 5.7–5.8%; 546 nm 16.6–17.2%, 5.0–5.1%, 589 nm 16.3–16.8%, 4.8–5.0%; 650 nm 16.1–16.5%, 4.7–4.8%. Vickers microhardness values range from 1,206–1,150 kg/mm2.

Strontio-chevkinite compositions average TiO2=23.16, SiO2=20.45, ZrO2=10.3, FeO=6.0, CaO=2.0, SrO=19.6, La2O3=9.18, Ce2O3=9.35 (all wt.%), with minor contents of Nb2O5, Al2O3, Cr2O3, MnO, MgO, BaO, Na2O, K2O, PbO and Y2O3. A modified formula for the mineral is (Sr2[La, Ce]1.5Ca0.5)4 Fe 2+0.5 Fe 3+0.5 (Ti, Zr)2 Ti2 Si4 O22. The symmetry is P21/a and the cell parameters area=13.56 Å,b=5.70 Å,c=11.10A,β=100.32°,V=844.86 Å3, with Z=2. The strongest reflections are 3.01(100)(401), 1.97(75)(024), 2.19(70)(42¯1) 2.51(40)(022), 2.74(30)(004), and 2.85(25)(020) Å. Calculated density is 5.44 gm/cm3. The mineral is opaque with a submetallic luster and thick parallel or interpenetrating twins are apparent in polished sections. It has a weak to medium anisotropy in air and oil immersion, with estimated reflectances of ≃10% and ≃2%, respectively. The mineral is dark gray in reflected light and has a characteristic fleshpink color.

Temperature estimates from alkali feldspar-nepheline and inferredT °C from strontian-loparite and strontiochevkinite, coupled with model temperatures for fenitization and rheomorphism, along with high Sr ∶ Ba and Na ∶ K ratios, imply that the new minerals and their host rocks were generated at moderate depths (≃5 km) at high oxygen pressures (much greater than MH) and crystallized atT≃500–550° C. The very high oxidation states may have stabilizedtetravalent iron, and it is proposed that the semi-conducting properties of strontium titanates could potentially serve to determine theT andfO2 of naturally occurring dielectric minerals.

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  1. Department of Geology, University of Massachusetts, 01003, Amherst, Massachusetts, USA

    Stephen E. Haggerty

  2. 48 Page Brook Road, 01741, Carlisle, Massachusetts, USA

    Anthony N. Mariano

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Haggerty, S.E., Mariano, A.N. Strontian-loparite and strontio-chevkinite: Two new minerals in rheomorphic fenites from the Paraná Basin carbonatites, South America. Contr. Mineral. and Petrol. 84, 365–381 (1983). https://doi.org/10.1007/BF01160288

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