IMA 2018-129 = davidbrownite-(NH4)

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Marco E. Ciriotti
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IMA 2018-129 = davidbrownite-(NH4)

Messaggio da Marco E. Ciriotti » mar 03 set, 2019 17:54

Prossima pubblicazione.

Referenza:
▪ Kampf, A.R., Cooper, M.A., Rossman, G.R., Nash, B.P., Hawthorne, F.C., Joe Marty, J. (2019): Davidbrownite-(NH4), (NH4,K)5(V4+O)2(C2O4)[PO2.75(OH)1.25]4·3H2O, a new phosphate-oxalate mineral from the Rowley mine, Arizona, USA. Mineralogical Magazine, 83, (in press).

Abstract:
Davidbrownite-(NH4), (NH4,K)5(V4+O)2(C2O4)[PO2.75(OH)1.25]4·3H2O, is a new mineral species from the Rowley mine, Maricopa County, Arizona, U.S.A. It occurs in an unusual bat-guano-related, post-mining assemblage of phases that include a variety of vanadates, phosphates, oxalates and chlorides, some containing NH4+. Other secondary minerals found in association with davidbrownite-(NH4) are antipinite, fluorite, mimetite, mottramite, quartz, rowleyite, salammoniac, struvite, vanadinite, willemite and wulfenite. Crystals of davidbrownite-(NH4) are light green-blue needles or narrow blades up to about 0.2 mm in length. The streak is white, luster is vitreous, Mohs hardness is ca. 2, tenacity is brittle and fracture is splintery. There are two good cleavages in the [010] zone, probably {100} and {001}. The measured density is 2.12(2) g·cm-3. Davidbrownite-(NH4) is optically biaxial (+) with α = 1.540(2), β = 1.550(5), γ = 1.582(2) (white light); 2V = 58.5(5)°; moderate r > v dispersion; orientation Z = b, Y ≈ a. Pleochroism: X pale blue, Y = nearly colorless, Z = light blue; Y < X < Z. Electron microprobe analysis gave the empirical formula [(NH4)3.11K1.73Na0.09]Σ4.93[(V4+1.92Mg0.01Al0.02)Σ1.95O2](C2O4) [(P3.94As0.12)Σ4.06O10.94(OH)5.06]·3H2O, with the C and H content provided by the crystal structure. Raman and infrared spectroscopy confirmed the presence of NH4 and C2O4. Davidbrownite-(NH4) is monoclinic, P21/c, with a = 10.356(6), b = 8.923(5), c = 13.486(7) Å, β = 92.618(9)°, V = 1244.9(12) Å3, and Z = 2. The crystal structure of davidbrownite-(NH4) (R1 = 0.0524 for 2062 Io > 2I reflections) consists of a chain structural unit with the formula {(V4+O)2(C2O4)[PO2.75(OH)1.25]4}5–, and a disordered interstitial complex containing five large monovalent cations (NH4+ and K+) and three H2O groups pfu. Strong hydrogen bonds form links within and between the chains.

NdR: La specie è dedicata al cristallografo Prof. emerito I. David Brown (1932-) che sviluppò la famosa e utilissima Bond-Valence Theory.
- Brown, I.D. (2002): The Chemical Bond in Inorganic Chemistry. The Bond Valence Model. Oxford University Press, Oxford.
- Brown, I.D. (2016): The Chemical Bond in Inorganic Chemistry: The Bond Valence Model. 2nd Edition. Oxford University Press, Oxford.

Si tratta del secondo minerale (il primo è la phoxite) contenente sia il gruppo fosfato che quello ossalato. Una gradevolissima nuova specie minerale in piacevoli spray di cristalli lamellari allungati, quasi sempre associati ad antipinite.
Marco E. Ciriotti

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