The crystal structure of dissakisite-(La) and structural variations after annealing of radiation damage
-
Barbara Lavina
,
Susanna Carbonin
Abstract
The crystal chemistry of the new mineral dissakisite-(La), the Mg-analogue of allanite-(La) with ideal formula CaLaAl2MgSi3O12(OH), was studied by single-crystal X-ray diffraction and Mössbauer spectroscopy. Diffraction data indicated that it has a partially metamict state with expanded cell parameters a = 8.959, b = 5.7226, and c = 10.232 Å, β = 115.19°, and V = 474.7 Å3. The crystal structure was refined to R(I/σ(I) > 4) of 3.45% and wR2 of 8.52% (S.G. P21/m). The displacement parameters are unusually large as consistent with positional disorder. Ca and REE are partitioned between the A1 and the A2 sites, Fe2+ between the M3 and the A1 sites, and Fe3+ between the M3 and the M2 sites. This confined to A1. The Mössbauer spectrum implies the presence of five different iron sites, three of which are attributed to Fe2+ at the M3 site, one to Fe2+ at the A1 site, and one to Fe3+ at a rather distorted octahedral site.
Annealing at 700 °C caused contraction of cell edges (b and c shrank by 0.5%, a by 0.3%, β only slightly decreased; ~1% in V) and of bond distances, with a consequent overall increase in bond valences. Displacement parameters decreased by about 30%, leading to a significant increase from 1495 to 1627 in the number of reflections with I/σ(I) > 4. Structure rearrangement was achieved by means of flexible points such as the two-coordinated oxygen atoms O8 and O9, and by greater volume decreases in M3, A1, A2, and Si3 polyhedra. Si tetrahedra behaved non-rigidly, at variance with what was observed for isomorphous substitution and thermal expansion.
Despite the high Th content and the age of the mineral, the crystal structure is well preserved. The above paradox could be explained by a thermal event after formation of the mineral that reversed previous damage, or by a prolonged period under relatively high-temperature conditions.
© 2015 by Walter de Gruyter Berlin/Boston
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- The rate of ferrihydrite transformation to goethite via the Fe(II) pathway
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Articles in the same Issue
- Dana Lecture. Rates of Fe, Mg, Mn, and Ca diffusion in garnet
- An FTIR study of hydrogen in anorthoclase and associated melt inclusions
- First occurrence of iodine in natural sulfosalts: The case of mutnovskite, Pb2AsS3(I,Cl,Br), a new mineral from the Mutnovsky volcano, Kamchatka Peninsula, Russian Federation
- The real topological conÞ guration of the extra-framework content in alkali-poor beryl: A multi-methodological study
- A low-temperature heat-capacity study of synthetic anhydrous Mg-cordierite (Mg2Al4Si5O18)
- Dissolution kinetics of pyrochlore ceramics for the disposition of plutonium
- Oxy-amphibole equilibria in Ti-bearing calcic amphiboles: Experimental investigation and petrologic implications for mantle-derived amphiboles
- Fe2+/Fe3+ charge ordering in contact layers of lamellar magnetism: Bond valence arguments
- Calcic amphibole growth and compositions in calc-alkaline magmas: Evidence from the Motru Dike Swarm (Southern Carpathians, Romania)
- Anomalously high Fe contents in rehomogenized olivine-hosted melt inclusions from oxidized magmas
- The rate of ferrihydrite transformation to goethite via the Fe(II) pathway
- Phase transitions and volumetric properties of cryolite, Na3AlF6: Differential thermal analysis to 100 MPa
- The crystal structure of dissakisite-(La) and structural variations after annealing of radiation damage
- Effects of metal protection coils on thermocouple EMF in multi-anvil high-pressure experiments
- First-principles study of the OH-stretching modes of gibbsite
- In situ X-ray diffraction study of phase transitions of FeTiO3 at high pressures and temperatures using a large-volume press and synchrotron radiation
- High-pressure proton disorder in brucite
- Microhardness, toughness, and modulus of Mohs scale minerals
- Effects of pH, temperature, and aqueous organic material on the dissolution kinetics of meta-autunite minerals, (Na, Ca)2-1[(UO2)(PO4)]2⋅3H2O
- InterstratiÞ ed kaolinite-smectite: Nature of the layers and mechanism of smectite kaolinization
- The kinetics of the α → β transition in synthetic nickel monosulÞ de
- An electron microprobe analysis, secondary ion mass spectrometry, and single-crystal X-ray diffraction study of phlogopites from Mt. Vulture, Potenza, Italy: Consideration of cation partitioning
- Unique W-rich alloy of Os and Ir and associated Fe-rich alloy of Os, Ru, and Ir from California
- Schreyerite, V2Ti3O9: New occurrence and crystal structure
- A cold-sealing capsule design for synthesis of fluid inclusions and other hydrothermal experiments in a piston-cylinder apparatus
- Complete solid-solution between Na3Al2(PO4)3 and Mg3Al2(SiO4)3 garnets at high pressure
