High-pressure single-crystal X-ray diffraction study of jadeite and kosmochlor

High-pressure crystallography and compression behavior of the alkali-scandium-germanate end-members LiScGe2O6 and NaScGe2O6

“…Figure 11 shows the results of the corresponding plot, which clearly reveals four trend lines having a very similar slope, each differentiating the subsets of series with M2= Li, Na, Ca and T= Si, Ge. The slopes of the dashed lines drawn in Figure 11 [38]; NaCrSi 2 O 6 [43]; NaVSi 2 O 6 [12]; NaInSi 2 O 6 [36]; NaGaSi 2 O 6 [44]) HP structures, one P-1 HP structure [12] and even the one unique P2 1 /n phase (LiAlGe 2 O 6 [9,10]). For lowpressure (LP) and HP LiFeGe 2 O 6 second-order EoS were fitted using the data given in [41].…”

Isosymmetric pressure‐induced bonding increase changes compression behavior of clinopyroxenes across jadeite‐aegirine solid solution in subduction zones

“…McCarthy et al classified both aegirine and jadeite in the same category of pyroxenes with antipathetic bonds [ McCarthy et al ., ] and predict both end‐members to display the same Na M2 site coordination number increase at about 20 GPa. The highest‐pressure jadeite study [ Posner et al ., ] did not quite reach high‐enough pressure to assess the discontinuous effects of the bonding change; thus, the current aegirine results provide the most accurate to‐date assessment of what to expect across the solid solution. Because of this compressional behavior duality, in order to better understand the properties of metastable pyroxene‐containing rocks in the deeper parts of the cold subduction zones we need more systematic studies of clinopyroxene minerals at pressures well exceeding 20 GPa.…”

“…Tetrahedral chain rotation makes the c direction more compressible but requires high deformability of the M2 site. Figures and show that the O3‐O3‐O3 angle has a discontinuity at 24.0(1) GPa, but the SiO 4 ‐tetrahedral compression does not exhibit discontinuity like in kosmochlor [ Posner et al ., ]. The high compressibility of the b direction can be attributed to the projection of long and soft Na‐O bonds along this direction.…”

“…The initially unbounded Na-O3(C2, D2) distance is shortened to a comparable length of 2.433(6) Å at 24.0(1) GPa (Table 4) [2008b]. This bonding transition has several discontinuous consequences: (i) pressure dependence of the β angle ( Figure 5) decreases and shows an increase in scatter, (ii) pressure dependence of the tetrahedral chain kinking angle ( Figure 13) decreases, (iii) pressure dependence of polyhedral distortion of M2 decreases (Figure 9), (1) Si-O1(C1) 1.632(4) 1.625 (7) 1.620 (8) 1.606 (8) 1.608 (9) 1.602 (6) 1.593 (7) 1.585 (9) 1.589 (7) 1.576(9) Si-O2(C1) 1.596(4) 1.593 (8) 1.588 (9) 1.591 (9) 1.597 (12) 1.571 (7) 1.566 (6) 1.545 (8) 1.567(9) 1.562(11) Si-O3(C1) 1.642 (3) 1.637 (6) 1.636 (6) 1.629 (7) 1.615(9) 1.613(5) 1.610 (5) 1.613 (6) 1.601 (6) 1.588(9) Si-O3(C2) 1.652(4) 1.656 (7) 1.652 (8) Figures 12 and 13 show that the O3-O3-O3 angle has a discontinuity at 24.0(1) GPa, but the SiO 4 -tetrahedral compression does not exhibit discontinuity like in kosmochlor [Posner et al, 2014]. The high compressibility of the b direction can be attributed to the projection of long and soft Na-O bonds along this direction.…”

High‐pressure Raman spectroscopy of Ca(Mg,Co)Si2O6 and Ca(Mg,Co)Ge2O6 clinopyroxenes

“…Pyroxenes, being major phases in Earth mantle, have been the object of several in situ high‐pressure investigations: Vibrational and elastic properties were measured often on the basis of crystal structure refinements . The phase transitions discovered in the high‐pressure experiments gave hints to the geophysical modelling of the velocity of the seismic waves at the mantle boundaries and proved to be crucial to interpret the evolution of the structural deformations in silicates …”