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The high-pressure structural configurations of Ca_0.2Sr_0.8Al₂Si₂O₈ feldspar: The I-I2/c and I2/c-P2₁/c phase transitions

Piera Benna^1,2,*, Fabrizio Nestola³, Tiziana Boffa Ballaran⁴, Toni Bali-uni⁵, Lars Fahl Lundegaard⁶ and Emiliano Bruno¹

¹ Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, I-10125 Torino, Italy
² Istituto di Geoscienze e Georisorse, CNR, Sezione di Torino, Italy
³ Dipartimento di Mineralogia e Petrologia, Università di Padova, C.so Garibaldi 37, I-35137 Padova, Italy
⁴ Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany
⁵ Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark
⁶ Centre for Science at Extreme Conditions, University of Edinburgh, Mayfield Road, Edinburgh EH93JZ, U.K.

Correspondence: ^* E-mail: piera.benna{at}unito.it

Single-crystal in situ high-P X-ray diffraction was performed at P = 0.0001, 3.2, 4.4, 6.2, and 7.4 GPa on synthetic Ca_0.2Sr_0.8Al₂Si₂O₈ feldspar (An₂₀SrF₈₀). Data collections confirmed the displacive first-order triclinic I-monoclinic I2/c phase transition at P ~4.3 GPa found in a previous high-P investigation and defined the first-order monoclinic-monoclinic transformation at P ~7.3 GPa as an I2/c-P2₁/c symmetry change. The structural modifications induced by the increase of pressure inside the stability fields of I and I2/c configurations as well as the structural behavior of the two phase transitions were detailed. The I-I2/c transition is similar to the displacive ferroelastic I-I2/c phase transition observed for the same composition with increasing temperature and is related to the increase of the M-site coordination number, with the consequent regularization of Ca/Sr polyhedra and framework. Variations of the T-O-T bond angles are observed, whereas the O-T-O angles do not change significantly.

The I2/c-P2₁/c transition involves a significant modification of the M-polyhedra. In the M(0)-polyhedron, the two O_C atoms, that were not coordinated in I and I2/c space groups, are now bonded more strongly than either O_B or O_D atoms [the M(0)-O_C(0i) and M(0)-O_C(zi) distances are 2.54 and 2.57 Å, respectively], whereas in the M(i)-polyhedron, it is the second OA(20) oxygen that becomes coordinated and, at the same time, one of the bonds to O_D atoms is broken. Moreover, a significant deformation of the framework is obtained due to the decrease of the symmetry from the loss of the two-fold axis and of half of the centers of symmetry. The transition induces, besides a significant distortion in the T-O-T bond angles, a deformation of the internal O-T-O angles within the tetrahedra. The T₂(00) tetrahedron, with the OA(20)-T₂(00)-O_C(0i) angle of 91° and the O_C(0i)-T₂(00)-O_D(0i) angle of 140°, is the most deformed.

Key Words: Crystal structure • high-pressure studies • phase transition • XRD data

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