CORE Metadata, citation and similar papers at core.ac.uk Provided by Institutional Research Information System University of Turin Phys Chem Minerals DOI 10.1007/s00269-016-0847-0 ORIGINAL PAPER High‑pressure structural configuration and phase transition in celsian, BaAl2Si2O8 Nadia Curetti1,2 · Piera Benna1,2 · Emiliano Bruno1,2 Received: 10 July 2016 / Accepted: 22 September 2016 © Springer-Verlag Berlin Heidelberg 2016 Abstract In situ high-pressure X-ray diffraction study was changes with compression in celsian are the deformation in performed on celsian (Cls97Or3) from Jakobsberg, Sweden. the Ba polyhedra and the variation in the T–O–T angles. A single crystal of celsian was loaded in an ETH-type dia- mond anvil cell, and unit-cell parameters were measured at Keywords Crystal structure · High-pressure studies · 20 different pressures up to 6.0 GPa at room T. The evolu- Celsian · Equation of state · Phase transition tion of the unit-cell parameters and volume as a function of pressure shows a discontinuity at P ~ 5.7 GPa indicat- ing a displacive first-order phase transition. The P–V data Introduction were fitted by a second-order Birch–Murnaghan EoS only up to 2.55 GPa, because at higher pressures a slight change Barium feldspar, celsian (formula: BaAl2Si2O8, symbol: in the compressional behavior of the unit-cell volume is Cls, space group: I2/c), belongs to the alkaline-earth feldspar observed, indicating a pre-transition volume softening. with Al:Si ratio equal to 1:1 and contains the heaviest cation The resulting EoS coefficients are V 1461.4(1) 3 and normally found in feldspars. Newnham and Megaw (1960) 0 = Å K 88.1(6) GPa. A second crystal of celsian was loaded determined the structure at room condition of a celsian from T0 = in the DAC cell, and single-crystal in situ HP X-ray dif- Broken Hill, Australia, containing an appreciable amount fraction was performed at P 0.0001, 2.1, 4.2, 5.5, 5.9, of K (Cls Or ), using Fourier difference maps and least = ~84 ~18 6.5 and 7.8 GPa. The data collections between 0 and squares methods. The authors observed that the Al, Si distri- 5.5 GPa show only a- and b-type reflections confirming the bution among the tetrahedral sites is partially ordered (“each I2/c space group. The appearance of c and d-type reflec- predominantly aluminum tetrahedron is surrounded by four tions at 5.9, 6.5 and 7.8 GPa, the analysis of the systematic predominantly silicon tetrahedra, and vice versa”) and that absence and the structural refinements define the HP phase the Ba atom influences the surrounding silicate framework. transition as an I2/c–P21/c transition. The most significant Ba atom is ninefold coordinated, with an irregular configura- tion very similar to that of K in sanidine or microcline, but the Ba–O distances are nearly all shorter, particularly for oxygens OA(2) and OB. Moreover, the Ba atom has a marked aniso- Electronic supplementary material The online version of this tropic temperature factor with the shortest axis of the vibra- article (doi:10.1007/s00269-016-0847-0) contains supplementary tion ellipsoid close to a*. The structure refinement conducted material, which is available to authorized users. by Griffen and Ribbe (1976) in nearly pure celsian (Cls95Or5) * Piera Benna from Jakobsberg, Sweden, confirms the results of Newnham [email protected] and Megaw (1960) “particularly with regard to the tetrahedral O O distances, O–T–O angles, and mean T–O and Ba, K–O 1 Dipartimento di Scienze della Terra, Università degli Studi di ··· Torino, Via Valperga Caluso 35, 10125 Turin, Italy bond lengths,” despite the difference in chemical composition. The aim of the present work is to investigate the high- 2 CrisDi Interdepartmental Center for Crystallography, Università degli Studi di Torino, Via P. Giuria 5, 10125 Turin, pressure behavior of celsian and to detail the possible pres- Italy ence of phase transitions, in order to contribute to the 1 3 Phys Chem Minerals knowledge of structural evolution of feldspars at HP. Recent Experimental methods study performed on natural and synthetic feldspars reveals several complexities at high pressures as “elastic softening The nearly pure celsian (Cls97Or3) (formula: and completely new symmetries and sequences of phase Ba0.97K0.03Al1.95Si2.04O8) studied in this work, comes from transformations” (Nestola et al. 2008). While in the alkali Jakobsberg, Sweden. A single crystal of celsian of size feldspars (AlSi feldspars) investigated at HP no phase tran- 120 80 50 μm3 was selected on the base of its opti- 3 × × sitions were observed (Downs et al. 1994; Allan and Angel cal features and loaded in an ETH-type diamond anvil cell 1997; Nestola et al. 2008; Curetti et al. 2011), in Al2Si2 (DAC, hereafter) (Miletich et al. 2000). A foil of T301 feldspars several pressure-induced phase transitions have steel 250 μm thick was used as a gasket to hold the crys- been found. The Val Pasmeda anorthite (macroscopic order tal, and it was pre-indented to 110 μm before drilling a ¯ ¯ parameter Q 0.92) shows a reversible first-order P 1–I 1 hole by spark erosion (∅ 250 μm). A 16:3:1 mixture of od = phase transition between 2.6 and 2.7 GPa (Angel et al. 1988; methanol:ethanol:water was used as hydrostatic pressure- Angel 1988, 1992). With increasing disorder (Q 0.78), transmitting medium. The internal pressure and the uncer- od = the transition becomes continuous in character and there is tainty on P were calculated by using a crystal of quartz as an increase in the pressure of the transition (P 4.8 GPa) standard internal (using 12 reflections) (Angel et al. 1997). T = (Angel 1992). In the synthetic Ca0.2Sr0.8Al2Si2O8 feldspar The unit-cell parameters were determined at room T in (An20SrFsp80), triclinic at room temperature, Nestola et al. the P range 0.0001–6.0 GPa from the setting angles of 27 (2004) and Benna et al. (2007) observed at P ~ 4.3 GPa a reflections centered in eight positions (King and Finger ¯ first-order I 1–I2/c transformation, and a further transition at 1979; Angel et al. 2000), using a Siemens P4 four-circle P ~ 7.3 GPa, strongly first order, to the P21/c space group, X-ray diffractometer equipped with point detector (MoKα never observed in feldspars until then. Recently, Pandolfo radiation; Department of Earth Sciences, University of et al. (2011) found in synthetic strontium feldspar (SrAl- Torino) and with the SINGLE Control Program software Si O ) the same first-order I2/c to P2 /c transition at P 6.6 (Angel and Finger 2011). The unit-cell parameters were 2 2 8 1 = GPa. Curetti et al. (2015) observe the I2/c–P21/c transition in measured at overall 20 different pressures and are reported synthetic lead feldspar (PbAl2Si2O8) at P ~ 8 GPa. in Table 1. Table 1 Unit-cell parameters of P (GPa) a ( ) b ( ) c ( ) β (°) V ( 3) celsian as a function of pressure Å Å Å Å (esd in brackets) 0.0001a 8.615(6) 13.0312(16) 14.377(2) 115.13(2) 1461(1) 0.0001 8.615(8) 13.031(2) 14.376(4) 115.10(3) 1461(2) 0.26(2) 8.604(4) 13.0199(12) 14.3673(17) 115.124(15) 1457(1) 0.38(2) 8.599(4) 13.0153(12) 14.3614(18) 115.128(16) 1455(1) 1.10(2) 8.569(4) 12.9875(13) 14.3226(19) 115.119(17) 1443(1) 1.36(2) 8.559(4) 12.9807(12) 14.3117(18) 115.114(16) 1440(1) 1.70(3) 8.546(4) 12.9683(11) 14.2946(17) 115.103(15) 1435(1) 2.16(3) 8.526(4) 12.9508(13) 14.2726(19) 115.091(18) 1427(1) 2.55(3) 8.512(3) 12.9403(9) 14.2579(13) 115.101(11) 1422(1) 2.79(3) 8.494(4) 12.9287(14) 14.243(2) 115.106(18) 1416(1) 3.13(3) 8.474(4) 12.9170(11) 14.2220(16) 115.080(15) 1410(1) 3.44(3) 8.463(3) 12.9049(10) 14.2051(15) 115.047(14) 1405(1) 3.80(3) 8.438(4) 12.8851(13) 14.1760(19) 114.989(17) 1397(1) 3.94(5) 8.426(3) 12.8778(10) 14.1647(14) 114.973(13) 1393(1) 4.33(2) 8.413(3) 12.8615(12) 14.1422(15) 114.912(14) 1388(1) 4.67(6) 8.391(4) 12.8463(14) 14.1163(18) 114.857(16) 1381(1) 5.05(5) 8.369(4) 12.8264(15) 14.0858(18) 114.832(17) 1372(1) 5.18(5) 8.364(4) 12.8238(13) 14.0808(17) 114.815(15) 1371(1) 5.59(5) 8.343(5) 12.8023(19) 14.053(2) 114.78(2) 1363(1) 5.90(6)b 8.315(8) 12.707(3) 14.023(4) 114.36(4) 1350(2) 6.01(4)b 8.315(8) 12.705(3) 14.012(4) 114.32(4) 1349(1) b P21/c space group a In air 1 3 Phys Chem Minerals Table 2 Single-crystal data at different pressures P (GPa) 0.0001a 2.1 4.2 5.5 5.9 6.5 7.8 Space group I2/c I2/c I2/c I2/c P21/c P21/c P21/c a (Å) 8.628(9) 8.527(3) 8.422(3) 8.363(3) 8.3414(7) 8.313(3) 8.2791(19) b (Å) 13.0380(10) 12.9449(5) 12.8618(4) 12.8089(4) 12.7458(3) 12.6714(4) 12.5505(3) c (Å) 14.44(4) 14.312(13) 14.169(11) 14.114(13) 14.0574(16) 13.994(12) 13.896(9) β (°) 115.1(2) 114.98(8) 114.89(7) 114.75(7) 114.383(10) 114.13(8) 113.78(5) 3 V (Å ) 1471(4) 1432.0(14) 1392.3(12) 1373.0(14) 1361.2(2) 1345.3(13) 1321.3(9) Refl.