A Mn-bearing oxidized variety of beraunite occurs in a phosphatic nodule in a granite at Mangualde, Portugal. It forms sheaf-like aggregates of subparallel needles. The elongated prisms have a rhombic section. The prevalent forms are (301) and (30-1) (both closely striated) and, in cases, a very thin (001). Mn-bearing beraunite is red, vitreous and transparent. Dmeas 2.99 g/cm3. Optically it is biaxial (+) with (-) elongation: orientation Y=b, X:c = +10°, 2Vz = 78(2)°, with medium dispersion of 2Vz, r > v, indices of refraction alphacalc 1.768, betameas 1.774, gammacalc 1.783. Pleochroism X = Y pale yellow, Z red orange. Chemical analysis by ICP gives: Li2O 0.02, Na2O 0.15, CaO 0.17, MnO 4.23, FeO 0.01, Fe2O3 49.49, P2O5 31.62, H2O 15.38, total 101.07 wt.%, leading to the formula Fe3+5.65Mn0.54Na0.04Ca0.03Li0.01 (OH)5(PO4)4.06(OH)0.95(H2O)4.81. Mn-bearing beraunite is monoclinic, space group C2/c, with a 20.760(3), b 5.154(1), c 19.248(2) Å, beta 93.55(1)°, V 2055.4(3) Å3 and Z = 4. The strongest reflections in an X-ray powder diffraction pattern [d in Å (Iobs) (hkl)] are: 10.39(vS)(200), 9.62(vs)(002), 7.28(S)(-202), 4.82(mS)(111), 3.081(S)(314). Eighteen previously unobserved reflections were recorded. The structure of the Mn-bearing beraunite has been refined to an R of 0.034 and an Rw of 0.029 for 2690 observed [I>3sigma(I)] reflections. Hydrogen bonding is proposed, based on the refined positions of hydrogen atoms. Mn2+ partially substitutes for Fe3+ in the Fe1 and Fe2 sites.
|Autori:||MARZONI FECIA DI COSSATO Y; ORLANDI P; PASERO M|
|Titolo:||Manganese-bearing beraunite from Mangualde, Portugal: mineral data and structure refinement|
|Anno del prodotto:||1989|
|Appare nelle tipologie:||1.1 Articolo in rivista|