Crystal structure of the OH-dominant gadolinite-(Y) analogue

database, nuove specie, discrediti,
ridefinizioni, classificazioni, ecc.
Rispondi
Avatar utente
Marco E. Ciriotti
Messaggi: 25626
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Crystal structure of the OH-dominant gadolinite-(Y) analogue

Messaggio da Marco E. Ciriotti » lun 08 mag, 2017 19:40

Referenza:
▪ Chukanov, N.V., Aksenov, S.M., Rastvetaeva, R.M., Kristiansen, R., Pekov, I.V, Belakovskiy, D.I., Van, V.K., Bychkova, Y.V., Bychkova, Y.V., Britvin, S.N. (2017): Crystal structure of the OH-dominant gadolinite-(Y) analogue, (Y,Ca)2(Fe,☐)Be2Si2O8(OH,O)2 from Heftetjern pegmatite, Norway. Acta Crystallographica, B73, (in press).

Abstract:
A hydroxyl-dominant analogue of gadolinite-(Y) (OH-Gad) has been discovered in the Heftetjern granitic pegmatite, Southern Norway, in association with late-stage REE-minerals. The empirical formula based on 10 O apfu is (Y1.285Ca0.55Ce0.07La0.04Nd0.01)tot1.955Fe2+0.57Be2.02Si1.995O8.48(OH)1.52. The mineral is monoclinic, space group P21/c, a = 4.7514(10), b = 7.5719(16), c = 9.9414(2) Å, β = 90.015(4)°, V = 357.663(3) Å3, and Z = 2. Density calculated using the empirical formula is 3.903 g/cm3. The crystal structure was refined to R = 0.0217 for 776 > 2σ(I). OH-Gad is isostructural with gadolinite-(Y) and it is characterized by the predominance of OH– over O2– at the anionic Ø-site. Refined crystal-chemical formula is (Z = 2): A(Y1.25Ca0.55Ce0.2)X(Fe2+0.57☐0.43)ZBe2TSi2O8Ø[(OH)0.86O0.59(OH)*0.55]. Possible orientation and local environment of hydroxyl group were suggested based on bond-valence sum calculations and geometrical analysis of the crystal structure. The IR spectrum confirms disordering of hydrogen atoms. OH-Gad is seems to be a potentially new mineral – the first simultaneously hydroxyl- and iron-dominant member of gadolinite subgroup. It is a OH-analogue of gadolinite-(Y) and Fe2+-analogue of hingganite-(Y).
Marco E. Ciriotti

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

Avatar utente
Marco E. Ciriotti
Messaggi: 25626
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: Crystal structure of the OH-dominant gadolinite-(Y) analogue

Messaggio da Marco E. Ciriotti » mar 03 ott, 2017 14:30

Pubblicazione effettuata.

Referenza:
▪ Chukanov, N.V., Aksenov, S.M., Rastvetaeva, R.M., Kristiansen, R., Pekov, I.V, Belakovskiy, D.I., Van, V.K., Bychkova, Y.V., Bychkova, Y.V., Britvin, S.N. (2017): Crystal structure of the OH-dominant gadolinite-(Y) analogue, (Y,Ca)2(Fe,☐)Be2Si2O8(OH,O)2 from Heftetjern pegmatite, Norway. Acta Crystallographica, B73, 899-906.
Marco E. Ciriotti

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

Avatar utente
Marco E. Ciriotti
Messaggi: 25626
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: Crystal structure of the OH-dominant gadolinite-(Y) analogue

Messaggio da Marco E. Ciriotti » lun 06 mag, 2019 17:00

Prossima pubblicazione.

Referenza:
▪ Cooper, M.A., Hawthorne, F.C., Miyawaki, R., Kristiansen, (2019): Cation Order in the Crystal Structure of ‘Ca-hingganite-(Y)'. Canadian Mineralogist, 57, (in press).

Abstract:
The crystal structure of ‘Ca-hingganite-(Y)', triclinic P1, a 9.863(4), b 7.602(3), c 4.762(2) Å, α 90.002(15), β 90.073(7), γ 90.020(5)°, V 357.1(5) Å3, Z = 1, has been refined to an R1 index of 2.28% for 4186 observed (|Fo| > 4σF) reflections. Significant observed (|Fo| > 20–50 σF) reflections violate the presence of a 21-screw axis and an a-glide plane, negating the space group P21/a previously found for minerals of the gadolinite supergroup. Averaging of the X-ray data in Laue groups 2/m and forumla gives the agreement indices 2/m (2.90%) and formula (2.61%). The internal agreement index from averaging of identical reflections collected at multiple positions along the diffraction vector (Rpsi) is 2.12%, where 13,801 reflections were collected, 4204 are unique for P1 symmetry, and Rpsi is based on a mean data-redundancy factor of >3. The |E2 – 1| value of 0.783 is in agreement with P1 as the correct space group. The general formula for the gadolinite supergroup is A2MQ2T2O8φ2 (Z = 2), which we have expanded to 20 anions (Z = 1) to show the A-site cation ordering present in ‘Ca-hingganite-(Y)'. Ca and Y(Ln) are ordered over four A sites: Y(Ln) is dominant at A1 [= 0.90 Y + 0.07 Er + 0.03 Ca], Ca is dominant at A2 [= 0.92 Ca + 0.08 Y], and there is near-equal Ca and Y(Ln) at A3 [= 0.55 Ca + 0.42 Y + 0.03 Er] and A4 [= 0.50 Ca + 0.47 Y + 0.03 Er]. The resulting site-specific empirical formula is A(Ca2.021Y1.811Ln0.133)Σ3.965M(□1.191Fe0.794Mn0.009Mg0.006)Σ2Q(Be2.24B1.58Si0.224)Σ4.044TSi4O16φ[(OH)2.382O1.618]Σ4. The dominant constituent at the M site is vacancy, and Ca does not occur at the M site. Significant B and Si are assigned to the Be-dominant Q sites, and the T sites are occupied by Si. The simplified formula for ‘Ca-hingganite-(Y)' (Z = 1) is A[(Y,Ln)Ca(Y,Ca,Ln)2]M(□,Fe)2Q(Be,B,Si)4TSi4O16[(OH),O]4.

NdR: dall'abstract non è facile intendere se il gruppo spaziale è P1 oppure P-1.
Marco E. Ciriotti

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

Rispondi

Chi c’è in linea

Visitano il forum: Nessuno e 1 ospite