In:
Physics and Chemistry of Minerals, Springer Science and Business Media LLC, Vol. 48, No. 9 ( 2021-09)
Abstract:
The thermal behaviour of a natural allanite-(Ce) has been investigated up to 1073 K (at room pressure) by means of in situ synchrotron powder X-ray diffraction and single-crystal neutron diffraction. Allanite preserves its crystallinity up to 1073 K. However, up to 700 K, the thermal behaviour along the three principal crystallographic axes, of the monoclinic β angle and of the unit-cell volume follow monotonically increasing trends, which are almost linear. At T 〉 700–800 K, a drastic change takes place: an inversion of the trend is observed along the a and b axes (more pronounced along b ) and for the monoclinic β angle; in contrast, an anomalous increase of the expansion is observed along the c axis, which controls the positive trend experienced by the unit-cell volume at T 〉 700–800 K. Data collected back to room T , after the H T experiments, show unit-cell parameters significantly different with respect to those previously measured at 293 K: allanite responds with an ideal elastic behaviour up to 700 K, and at T 〉 700–800 K its behaviour deviates from the elasticity field. The thermo-elastic behaviour up to 700 K was modelled with a modified Holland–Powell EoS; for the unit-cell volume, we obtained the following parameters: V T0 = 467.33(6) Å 3 and α T0 ( V ) = 2.8(3) × 10 –5 K −1 . The thermal anisotropy, derived on the basis of the axial expansion along the three main crystallographic directions, is the following: α T0 ( a ):α T0 ( b ):α T0 ( c ) = 1.08:1:1.36. The T -induced mechanisms, at the atomic scale, are described on the basis of the neutron structure refinements at different temperatures. Evidence of dehydroxylation effect at T ≥ 848 K are reported. A comparison between the thermal behaviour of allanite, epidote and clinozoisite is carried out.
Type of Medium:
Online Resource
ISSN:
0342-1791
,
1432-2021
DOI:
10.1007/s00269-021-01154-6
Language:
English
Publisher:
Springer Science and Business Media LLC
Publication Date:
2021
detail.hit.zdb_id:
1463021-7
detail.hit.zdb_id:
131393-9
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