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Allanite at high temperature: effect of REE on the thermal behaviour of epidote-group minerals
Physics and Chemistry of Minerals ( IF 1.2 ) Pub Date : 2021-08-22 , DOI: 10.1007/s00269-021-01154-6
G. Diego Gatta 1 , Francesco Pagliaro 1 , Paolo Lotti 1 , Alessandro Guastoni 2 , Laura Cañadillas-Delgado 3 , Oscar Fabelo 3 , Lara Gigli 4
Affiliation  

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 HT 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: VT0 = 467.33(6) Å3 and αT0(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: αT0(a):αT0(b):αT0(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.



中文翻译:

高温下的红榴石:稀土元素对绿帘石族矿物热行为的影响

已通过原位同步加速器粉末 X 射线衍射和单晶中子衍射研究了天然阿兰石 (Ce) 在高达 1073 K(室温下)的热行为。Allanite 保持其结晶度高达 1073 K。然而,高达 700 K,沿三个主晶轴、单斜β角和晶胞体积的热行为遵循单调增加的趋势,几乎是线性的。在Ť  > 700-800 K,急剧变化发生:沿观察到的趋势的反转一个b轴(沿更加明显b)和单斜晶系β角度;相反,沿着所述观察到的膨胀的异常增加Ç轴,其控制通过在单元电池容积经历的积极趋势Ť  > 700-800 K.收集的数据返回到室温Ť时,H后Ť实验中,显示晶胞参数与之前在 293 K 下测量的参数显着不同:allanite 以理想的弹性行为响应,最高可达 700 K,而在T  > 700–800 K 时,其行为偏离弹性场。高达 700 K 的热弹性行为是用改进的 Holland-Powell EoS 建模的;对于晶胞体积,我们获得了以下参数:V T0  = 467.33(6) Å 3和 αT0 ( V ) = 2.8(3) × 10 –5  K -1。基于沿三个主要结晶方向的轴向膨胀导出的热各向异性如下:α T0 ( a ):α T0 ( b ):α T0 ( c ) = 1.08:1:1.36。原子尺度上的T诱导机制是根据不同温度下的中子结构细化来描述的。 报告了在T ≥ 848 K时脱羟基作用的证据。进行了褐铝石、绿帘石和斜纹石的热行为之间的比较。

更新日期:2021-08-23
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