New data on hemihedrite from Arizona: formula revised

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Marco E. Ciriotti
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Iscritto il: ven 25 giu, 2004 11:31
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New data on hemihedrite from Arizona: formula revised

Messaggio da Marco E. Ciriotti » mar 27 set, 2016 9:36

Prossima pubblicazione.

Referenza:
▪ Lafuente, B., Downs, R.T., Origlieri, M.T., Domanik, K.J., Gibbs, R.B., Rumsey, M.S. (2017): New data on hemihedrite from Arizona. Mineralogical Magazine, 81, (in press).

Abstract:
Hemihedrite from the Florence Lead-Silver mine in Pinal County, Arizona, USA was first described and assigned the ideal chemical formula Pb10Zn(CrO4)6(SiO4)2F2, based upon a variety of chemical and crystal structure analyses. The primary methods used to determine the fluorine content for hemihedrite were colorimetry, which resulted in values of F that were too high and inconsistent with the structural data, and IR spectroscopic analysis that failed to detect OH or H2O. Our reinvestigation using electron microprobe analysis of the type material, and additional samples from the type locality, the Rat Tail claim, Arizona, and Nevada, reveals the absence of fluorine, while the presence of OH is confirmed by Raman spectroscopy. These findings suggest that the colorimetric determination of fluorine in the original description of hemihedrite probably misidentified F due to the interferences from PO4 and SO4, both found in our chemical analyses. As a consequence of these results, the study presented here proposes a redefinition of the chemical composition of hemihedrite to the ideal chemical formula Pb10Zn(CrO4)6(SiO4)2(OH)2. Hemihedrite is isotypic with iranite with substitution of Zn for Cu, and raygrantite with substitution of Cr for S. Structural data from a sample from the Rat Tail claim, Arizona, indicate that hemihedrite is triclinic in space group P-1, a = 9.4891 (7), b = 11.4242 (8), c = 10.8155 (7) Å, α = 120.368 (2)°, β = 92.017 (3)°, γ = 55.857 (2)°, V =784.88 (9) Å3, Z = 1, consistent with previous investigations. The structure was refined from single-crystal X-ray diffraction data to R1 = 0.022 for 5705 unique observed reflections, and ideal chemical formula Pb10Zn(CrO4)6(SiO4)2(OH)2 was assumed during the refinement. Electron microprobe analyses of this sample yielded the empirical chemical formula Pb10.05(Zn0.91Mg0.02)Σ=0.93(Cr5.98S0.01P0.01)Σ=6.00Si1.97O34H2.16 based on 34 O atoms and six (Cr+S+P) per unit cell.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 23479
Iscritto il: ven 25 giu, 2004 11:31
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Re: New data on hemihedrite from Arizona: formula revised

Messaggio da Marco E. Ciriotti » mar 27 set, 2016 14:07

La revisione della formula, Pb10Zn(CrO4)6(SiO4)2(OH)2, non è passata attraverso la CNMNC dell'IMA, ma è indubbiamente corretta.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

Avatar utente
Marco E. Ciriotti
Messaggi: 23479
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: New data on hemihedrite from Arizona: formula revised

Messaggio da Marco E. Ciriotti » gio 26 ott, 2017 22:00

Pubblicazione effettuata.

Referenza:
▪ Lafuente, B., Downs, R.T., Origlieri, M.T., Domanik, K.J., Gibbs, R.B., Rumsey, M.S. (2017): New data on hemihedrite from Arizona. Mineralogical Magazine, 81, 1021-1030.

Abstract:
Hemihedrite from the Florence Lead-Silver mine in Pinal County, Arizona, USA was first described and assigned the ideal chemical formula Pb10Zn(CrO4)6(SiO4)2F2, based upon a variety of chemical and crystal structure analyses. The primary methods used to determine the fluorine content for hemihedrite were colorimetry, which resulted in values of F that were too high and inconsistent with the structural data, and IR spectroscopic analysis that failed to detect OH or H2O. Our reinvestigation using electron microprobe analysis of the type material, and additional samples from the type locality, the Rat Tail claim, Arizona, and Nevada, reveals the absence of fluorine, while the presence of OH is confirmed by Raman spectroscopy. These findings suggest that the colorimetric determination of fluorine in the original description of hemihedrite probably misidentified F due to the interferences from PO4 and SO4, both found in our chemical analyses. As a consequence of these results, the study presented here proposes a redefinition of the chemical composition of hemihedrite to the ideal chemical formula Pb10Zn(CrO4)6(SiO4)2(OH)2. Hemihedrite is isotypic with iranite with substitution of Zn for Cu, and raygrantite with substitution of Cr for S. Structural data from a sample from the Rat Tail claim, Arizona, indicate that hemihedrite is triclinic in space group P-1, a = 9.4891 (7), b = 11.4242 (8), c = 10.8155 (7) Å, α = 120.368 (2)°, β = 92.017 (3)°, γ = 55.857 (2)°, V =784.88 (9) Å3, Z = 1, consistent with previous investigations. The structure was refined from single-crystal X-ray diffraction data to R1 = 0.022 for 5705 unique observed reflections, and ideal chemical formula Pb10Zn(CrO4)6(SiO4)2(OH)2 was assumed during the refinement. Electron microprobe analyses of this sample yielded the empirical chemical formula Pb10.05(Zn0.91Mg0.02)Σ=0.93(Cr5.98S0.01P0.01)Σ=6.00Si1.97O34H2.16 based on 34 O atoms and six (Cr+S+P) per unit cell.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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