The crystal data and stability of calcite III at high pressures based on single-crystal X-ray experiments

Abstract

Single crystal X ray experiments have been performed for calcite, calcite II, and calcite III at 300 K and pressures up to 4.1 GPa. The volume decrease from calcite to calcite II is estimated to be 0.54 cm 3 /mol (1.5 %) at 1.5 GPa, which compares well with the value 0.48 cm 3 /mol in the calcite to calcite II transition at 1.4 GPa reported previously based on piston cylinder volume compression experiments. The crystal data of calcite III have been determined using single crystal X ray analyses at 300 K and pressures between 2.3 and 4.1 GPa. The calcite III crystal is found to be monoclinic with the space group either C2, Cm, or C2/m from the systematic absence of reflections, and with the refined cell parameters of a = 8.707(7), b = 9.315(6), c = 6.154(4) A , and = 115.18(2) at 2.3 GPa. These crystal data of calcite III at 2.3 GPa yield a density of 2.94 g/cm 3 for Z = 8, which is much less than the previously estimated densities, ranging from 3.11 to 3.28 g/cm 3 at pressures from 1.9 to 2.9 GPa, using powder X ray analyses based on orthorhombic structural models for calcite III, but is generally consistent with the reported data by the piston cylinder experiments. Thus the present study reveals the density of calcite III is substantially less than that of aragonite (3.02 g/cm 3 at 2.3 GPa), indicating that calcite III is metastable with respect to aragonite at high pressures.

Puzzling calcite-III dimorphism: crystallography, high-pressure behavior, and pathway of single-crystal transitions

“…Indeed, the lattice can be transformed by [−1,0,0/0,−1,1/1,1,1] to a I-centered monoclinic lattice with β ≈ 106.8° with the a-axis corresponding to the former trigonal c-axis direction of calcite-I. In contrast, vector transformation by [0,1,1/0,−1,1/1,0,0] perfectly matches the monoclinic setting according to the C-centered lattice described by Hagiya et al (2005), with β ≈ 106.8° and the [101] direction of this monoclinic cell corresponding to the c-axis direction of calcite-I (Fig. 6).…”

“…5). Merging the experimental data of determined volumes with those reported by Merlini et al (2012) and Hagiya et al (2005), all transitions from calcite-I to calcite-III are marked by pronounced volume discontinuities at the relevant transition pressures, i.e., -1.3 % for the I-II transition at 1.62 gPa, −3.4 % for the II-IIIb at 1.85 gPa, and −1.8 % at 3.37 gPa. From a thermodynamic point of view, all transitions can therefore be considered first order in character.…”

“…In addition to solving the structure of calcite-VI, the authors confirmed and identified the existence of indeed two individual triclinic polymorphs forming on transition from calcite-II on crossing the phase boundary. The so-called phase IIIb, which is apparently pseudomonoclinic with a C-centered lattice, reveals a strong relationship to the lattice geometry reported by Hagiya et al (2005). It was reported to transform on compression out of the calcite-II stability field only on low compression rates and therefore has been considered potentially to be metastable or to represent a transient state, depending on the experimental pressure-time pathway (Merlini et al 2012).…”

“…This includes the changes in its structural state under non-ambient conditions tracing back to the origins of experimental highpressure (high-P) research (Bridgman 1925). It has been known for more than 75 years (Bridgman 1939) that calcite undergoes a series of transitions under pressure, which involve at the current level of knowledge in total ten proven structural polymorphs, i.e., calcite (here referred to as calcite-I), calcite-II, calcite-III, calcite-IIIb, calcite-IV, calcite-V, calcite-VI, aragonite, post-aragonite, and vaterite (Davis 1964;Tyburczy and ahrens 1986;Biellmann et al 1993;Fiquet et al 1994;Smyth and ahrens 1997;Suito et al 2001;Hagiya et al 2005;Catalli and Williams 2005;Ono et al 2005Ono et al , 2007Oganov et al 2006Oganov et al , 2008Merlini et al 2012).…”

“…a first model for the crystal structure was deduced from high-P powder X-ray data with a monoclinic structure apparently matching a large extent of the observed diffraction pattern (Smyth and ahrens 1997). Hagiya et al (2005) reported different monoclinic lattice metrics with systematic absences corresponding to C2, Cm, or C2/m symmetry based on single-crystal diffraction experiments, but did not provide a structure solution. Wang and Zheng (2011) indicated the existence of two types of calcite-III from in situ raman spectroscopic investigations on powders and considered the calcite-III dimorphism to originate from the effects of non-hydrostaticity.…”