Cation order in the crystal structure of 'minasgeraisite-(Y)'

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Marco E. Ciriotti
Messaggi: 24838
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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Cation order in the crystal structure of 'minasgeraisite-(Y)'

Messaggio da Marco E. Ciriotti » mar 11 lug, 2017 9:50

Prossima pubblicazione.

Referenza:
▪ Cooper, M.A. & Hawthorne, F.C. (2017): Cation order in the crystal structure of minasgeraisite-(Y). Mineralogical Magazine, 81, (in press).

Abstract:
The crystal structure of minasgeraisite-(Y), triclinic P1, a 9.994(4), b 7.705(3), c 4.764(2) Å, α 90.042(9), β 90.218(14), γ 90.034(9) (°), V 366.8(5) Å3, Z 1, has been refined to an R1 index of 2.86% for 4170 observed (|Fo| > 4σF) reflections. Significant observed (|Fo| > 40–60 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-datolite group. Averaging of the X-ray data in Laue groups 2/m and ⎯1 gives the following agreement indices: 2/m (9.68 %) and ⎯1 (5.68 %). The internal agreement index from averaging of identical reflections collected at multiple positions along the diffraction vector is significantly lower than that for the Laue group ⎯1: Rpsi = 2.40%, where 13,109 reflections were collected, 4,288 are unique for P1 symmetry, and Rpsi is based on a mean data redundancy factor of > 3. Both the data merging and an |E2–1| value of 0.773 indicate that P1 is the correct space group. The general formula for the gadolinite-datolite group is W2XZ2T2O8V2 (Z = 2) which we have expanded to 20 anions (Z = 1) to show the W-site cation ordering present in minasgeraisite-(Y). Bi, Ca and REE are ordered over four W sites, with Bi dominant at W1, Ca dominant at W2, and Y dominant at W3 and W4. The dominant constituent at the X sites is vacancy, and Ca does not occur at the X sites. Significant B and Si are assigned to the Be-dominant Z sites, and the T sites are occupied by Si. The simplified minasgeraisite-(Y) formula (Z = 1) is BiCa(Y,Ln)2(􀂆,Mn)2(Be,B,Si)4Si4O16 [(OH),O]4. Minasgeraisite-(Y) should be assigned to a triclinic subgroup of the gadolinite-datolite group, and its lower symmetry suggests that Ca-substituted gadolinites and hingganites should be examined for evidence of triclinic symmetry associated with cation order at the W sites.
Marco E. Ciriotti

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

Avatar utente
Marco E. Ciriotti
Messaggi: 24838
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: Cation order in the crystal structure of 'minasgeraisite-(Y)'

Messaggio da Marco E. Ciriotti » mer 04 lug, 2018 9:23

Pubblicazione effettuata.

Referenza:
▪ Cooper, M.A. & Hawthorne, F.C. (2017): Cation order in the crystal structure of ‘minasgeraisite-(Y)’: Dedicated to our friend the late Luis Menezes, who brought this mineral to our attention and graciously provided a superior specimen with crystals suitable for single-crystal structure work. Mineralogical Magazine, 81, 301-312.

Abstract:
The crystal structure of ‘minasgeraisite-(Y)’, triclinic P1, a 9.994(4), b 7.705(3), c 4.764(2) Å, α 90.042(9), β 90.218(14), γ 90.034(9) (°), V 366.8(5) Å3, Z 1, has been refined to an R1 index of 2.86% for 4170 observed (|Fo| > 4σF) reflections. Significant observed (|Fo| > 40–60 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-datolite group. Averaging of the X-ray data in Laue groups 2/m and ⎯1 gives the following agreement indices: 2/m (9.68 %) and ⎯1 (5.68 %). The internal agreement index from averaging of identical reflections collected at multiple positions along the diffraction vector is significantly lower than that for the Laue group ⎯1: Rpsi = 2.40%, where 13,109 reflections were collected, 4,288 are unique for P1 symmetry, and Rpsi is based on a mean data redundancy factor of > 3. Both the data merging and an |E2–1| value of 0.773 indicate that P1 is the correct space group. The general formula for the gadolinite-datolite group is W2XZ2T2O8V2 (Z = 2) which we have expanded to 20 anions (Z = 1) to show the W-site cation ordering present in ‘minasgeraisite-(Y)’. Bi, Ca and REE are ordered over four W sites, with Bi dominant at W1, Ca dominant at W2, and Y dominant at W3 and W4. The dominant constituent at the X sites is vacancy, and Ca does not occur at the X sites. Significant B and Si are assigned to the Be-dominant Z sites, and the T sites are occupied by Si. The simplified ‘minasgeraisite-(Y)’ formula (Z = 1) is BiCa(Y,Ln)2(􀂆,Mn)2(Be,B,Si)4Si4O16 [(OH),O]4. ‘Minasgeraisite-(Y)’ should be assigned to a triclinic subgroup of the gadolinite-datolite group, and its lower symmetry suggests that Ca-substituted gadolinites and hingganites should be examined for evidence of triclinic symmetry associated with cation order at the W sites.
Marco E. Ciriotti

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

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