IMA 2016-083 = potassic-magnesio-arfvedsonite

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Marco E. Ciriotti
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IMA 2016-083 = potassic-magnesio-arfvedsonite

Messaggio da Marco E. Ciriotti » mer 04 lug, 2018 9:51

Prossima pubblicazione.

Referenza:
▪ Dyulgerov, M., Oberti, R., Platevoet, B., Kadiyski, M., Rusanov, V. (2018): Potassic-magnesio-arfvedsonite – KNa2(MgFe2+Fe3+)5Si8O22(OH)2: mineral description and crystal chemistry. Mineralogical Magazine, 82, (in press).

Abstract:
The complete mineral description of potassic-magnesio-arfvedsonite, a recently approved (IMA-CNMNC 2016-083) new species of the amphibole supergroup is provided using electron microprobe and LA-ICP-MS analysis, single-crystal structure refinement, Mössbauer and Raman spectroscopy as well as measurement of optical and physical properties. The holotype material was found in syenitic and granitic dyke rocks in association with quartz, potassium feldspar and aegirine-augite from the Buhovo-Seslavtsi pluton (Bulgaria). Potassic-magnesio-arfvedsonite is monoclinic C2/m, with unit-cell parameters: a = 9.9804(11), b = 18.0127(19), c = 5.2971(6) Å, β = 104.341(2)°, V = 922.61 Å3. In transmitted plane-polarized light (λ = 590 cm-1), potassic-magnesio-arfvedsonite is pleochroic (X yellow pale-green, Y green, Z dark-violet brown). It is biaxial (-), α = 1.645(2), β = 1.655(2), γ = 1.660(2) and 2V (meas): 70°. The empirical unit formula obtained from EMP analysis and structure refinement is A(K0.86Na0.0.08)0.94 B(Na1.74Ca0.25 Mn2+0.01)2.00 C(Mg2.67Fe2+1.42Fe3+0.76Ti0.12Mn2+0.03)5.00 TSi8 O22 W(OH1.58F0.22O0.20)2.00. The Fe3+/Fetot ratio (0.35) is coherent with both the Mössbauer spectra and the single-crystal structure refinement. The 10 strongest X-ray powder reflections [d values (in A˚), I, (hkl)] are: 8.519, 80.5, (110); 3.402, 67.3, (131); 3.295, 41.0, (240); 3.173, 65.0, (310); 2.752, 35.6, ( ̅31); 2.715, 100.0 (151); 2.591, 44.1, (061); 2.542, 73.2, ( ̅02); 2.348, 38.5, ( ̅51); 2.174, 42.0, (261). Potassic-magnesio-arfvedsonite is the product of strongly peralkaline and potassic (perpotassic) magma compositions. Trace-elements analysis shows that this amphibole did not exert significant control on trace elements distribution in the crystallizing peralkaline magma.
Marco E. Ciriotti

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