Growth oscillatory zoning in erythrite: Structural variations in vivianite-group minerals

database, nuove specie, discrediti,
ridefinizioni, classificazioni, ecc.
Avatar utente
Marco E. Ciriotti
Messaggi: 23474
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy

Growth oscillatory zoning in erythrite: Structural variations in vivianite-group minerals

Messaggio da Marco E. Ciriotti » mar 08 ago, 2017 9:57

Prossima pubblicazione.

▪ Antao, S.M. & Dhaliwal, I. (2017): Growth Oscillatory Zoning in Erythrite, Ideally Co3(AsO4)2·8H2O: Structural Variations in Vivianite-Group Minerals. Minerals, 7, 136; doi:10.3390/min7080136.

The crystal structure of an oscillatory zoned erythrite sample from Aghbar mine, Bou Azzer, Morocco, was refined using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data, Rietveld refinement, space group C2/m, and Z = 2. The crystal contains two sets of oscillatory zones that appear to have developed during epitaxial growth. The unit-cell parameters obtained are a = 10.24799(3) Å, b = 13.42490(7) Å, c = 4.755885(8) Å, β = 105.1116(3)°, and V = 631.680(4) Å3. The empirical formula for erythrite, obtained with electron-probe micro-analysis (EPMA), is [Co2.78Zn0.11Ni0.07Fe0.04]∑3.00(AsO4)2·8H2O. Erythrite belongs to the vivianite-type structure that contains M1O2(H2O)4 octahedra and M22O6(H2O)4 octahedral dimers that are linked by TO4 (T5+ = As or P) tetrahedra to form complex layers parallel to the (010) plane. These layers are connected by hydrogen bonds. The average <M1–O>[6] = 2.122(1) Å and average <M2–O>[6] = 2.088(1) Å. With space group C2/m, there are two solid solutions: M3(AsO4)2·8H2O and M3(PO4)2·8H2O where M2+ = Mg, Fe, Co, Ni, or Zn. In these As- and P-series, using data from this study and from the literature, we find that their structural parameters evolve linearly with V and in a nearly parallel manner despite of the large difference in size between P5+ (0.170 Å) and As5+ (0.355 Å) cations. Average <T–O>[4], <M1–O>[6], and <M2–O>[6] distances increase linearly with V. The average <As–O> distance is affected by M atoms, whereas the average <P–O> distance is unaffected because it contains shorter and stronger P–O bonds. Although As- and P-series occur naturally, there is no structural reason why similar V-series vivianite-group minerals do not occur naturally or cannot be synthesized.
Marco E. Ciriotti

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


Chi c’è in linea

Visitano il forum: Bing [Bot] e 2 ospiti