Compression of calcite to 40 KB

Abstract

The compressibility of calcite to 40 kbar has been remeasured by using a piston-cylinder apparatus. Calcite 1 is found to transform to calcite 2 at 14.5 kbar with a volume change of 0.00483 cmS/g, and calcite 2 is found to change to calcite 3 at 17.4 kbar with a volume change of0.012% cmS/g. The volume compression data for the three phases are described by the following quadratic relations: Calcite 1 --(AV/Vo) = -14.07 x lO-.P + 5.107 x lO-6P , Calcite 2 --(AV/Vo) = 0.where P is pressure in kilobars. The compression data for calcite 1 and calcite 3 are in good agreement with those available in the literature. The data exhibiting an abnormally high compression of calcite 2 have been reported for the first time. The compression data for calcite 2 have been used to explain quantitatively the abnormal drop near 15 kbar observed in the ultrasonic sound velocity in calcite. Bridgman [1939] observed that the atmospheric pressure phase of calcite (calcite l) transforms to calcite 2 near 15 kbar, and calcite 2 transforms to calcite 3 near 18 kbar. The compressibility of calcite 3 is abnormal in that it is higher than that of calcite I in spite of the fact that calcite 3 is the denser phase. Adaduroo et al. [1961] studied the compressibility of marble to 30 kbar by using a piston displacement piezometer and to 500 kbar by using shock compression and reported another transition at 11 kbar in addition to the two transitions reported by Bridgman [1939]. In a recent study, Vaidya et al. [1973] obtained compressibility data very similar to those of Bridgman [1939]; the two transitions, however, were not resolved in their experiments. The compressibility of calcite 2 could not be measured in many earlier studies because it exists over a small pressure range [Bridgman, 1939], and often the two transitions are not resolved [Bridgman, 1948; Vaidya et aL, 1973]. Besides a very rough estimate by Wang [1968], the only work reporting the compression of calcite 2 is by ,,ldaduroo et al. [1961]. The interest in the compressibility of calcite 2 stems from the fact that the ultrasonic sound velocity in calcite, limestones, and marbles exhibits a large decrease near 15 kbar [Wang, 1966]. In a further study, Wang [1968] showed that this drop is associated with the calcite l-calcite 2 transition and suggested a mechanism based on the oscillations of the domain boundaries between the two phases caused by ultrasonic waves to explain this drop. Using similar concepts, Walsh [1973] recently discussed the propagation of the ultrasonic waves in rocks undergoing polymorphic transformations. As was pointed out by Walsh [1973], this suggestion has much wider implications in the interpretations of the changes observed in the ultrasonic wave velocities. To understand the nature of the anomaly in calcite, a careful measurement of the compressibilities of calcite I and calcite 2 near the calcite l-calcite

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

“…Davis (1964), Fiquet et al (1994), and Suito et al (2001) carried out high pressure X ñ ray powder diffraction measurements of calcite III at 300 K, and proposed that calcite III is the orthorhombic KNO3 IV type structure with Z = 10, and with densities of 3.11 g/cm 3 at 1.9 GPa, 3.21 g/cm 3 at 2.3 GPa, and 3.28 g/cm 3 at 2.9 GPa, respectively, as shown in Table 1, which are all much higher than the densities of aragonite at these pressures, for instance 3.01 g/cm 3 at 2.0 GPa and 3.05 g/cm crystal X ray analyses of calcite III, Merrill and Bassett (1972) reported that the crystal is C centered monoclinic with Z = 8, and with the density at 300 K and 2 GPa to be 2.95 g/cm 3 (see Table 1), being smaller than that of aragonite (3.01 g/cm 3 ) at this pressure, indicating calcite III is metastable with respect to aragonite at high pressures. This conclusion of the metasatbility of calcite III is in general agreement with the volume compression data measured from piston cylinder experiments by Singh and Kennedy (1974). Smyth and Ahrens (1997) re indexed the powder X ray data of calcite III at 300 K and 4.1 GPa published by Fiquet et al (1994), and suggested a different monoclinic structural model for calcite III, having space group C2 with Z = 6, and the density of 2.95 g/cm 3 , less than aragonite (3.08 g/cm 3 ) at this pressure.…”

“…Then Z = 10 with this volume gives the density of calcite III to be 3.62 g/cm 3 , which is too high (29 % density increase) compared with the density of calcite II of 2.81 g/cm 3 at 1.5 GPa obtained here, and is again highly implausible, considering the fact that the calcite II to III transition proceeds sharply, and that calcite III reverts to calcite immediately on release of pressure, as reported by Singh and Kennedy (1974) and Vo Thanh and Lacam (1984). It is to be noted that aragonite, 8 % more dense than calcite, persists metastably at ambient conditions for thousands of years.…”

“…Figure 1 shows the measured volume compression data of calcite, calcite II, and calcite III, obtained in this study, at pressures up to 4.1 GPa. The volume compression data from piston cylinder experiments up to 4.0 GPa by Singh and Kennedy (1974), and the relative volume of calcite II at 1.5 GPa obtained using powder X ray diffraction by Merrill and Bassett (1975) are also given in Figure 1 for comparison. A Birch Murnaghan fitting of the present data for calcite listed in Table 1, with a fixed K0 = 4, yields the bulk modulus at ambient conditions K0 = 75 GPa, which compares well with the reported value of 71 GPa by Singh and Kennedy (1974), and the values, 73 76 GPa, obtained from the measured elastic constants of calcite (Chen et al, 2001, and references therein).…”

“…The volume compression data from piston cylinder experiments up to 4.0 GPa by Singh and Kennedy (1974), and the relative volume of calcite II at 1.5 GPa obtained using powder X ray diffraction by Merrill and Bassett (1975) are also given in Figure 1 for comparison. A Birch Murnaghan fitting of the present data for calcite listed in Table 1, with a fixed K0 = 4, yields the bulk modulus at ambient conditions K0 = 75 GPa, which compares well with the reported value of 71 GPa by Singh and Kennedy (1974), and the values, 73 76 GPa, obtained from the measured elastic constants of calcite (Chen et al, 2001, and references therein). Combining the present Birch Murnaghan equation for calcite with the measured volume of calcite II at 1.5 GPa (236.9 A 3 ; see Table 1), we find the volume difference between calcite and calcite II to be 0.54 cm Merrill and Bassett (1975) overestimates considerably the volume of calcite II at 1.5 GPa, compared with our data, as shown in Table 1 and Figure 1.…”

“…Calcite is the stable phase at the ambient conditions, aragonite is the high pressure phase, and vaterite is known to be metastable with respect to calcite and aragonite under all known conditions. At room temperature, calcite transforms to calcite II at about 1.4 GPa, and further to calcite III at about 1.8 GPa (Bridgman 1939;Singh and Kennedy, 1974). Both calcite II and III back transform to calcite at the ambient conditions.…”