High pressure behavior of natural fluorapatite and carbonate-fluorapatite

Knowledge of the mineral-physics of apatite helps to assess the geological implications of its actual stability field, but also its mechanical properties - useful for its technological applications. Changes in its chemical and physical properties are observed when carbonate is incorporated into the apatite structure. The entry of CO3 can occur by substitution of both channel anion (type A) and phosphate ion (type B); the first process causes an increase in a and a decrease in c, vice versa the second one a decrease in a and an increase in c. Several in situ HP experiments have been already carried out on synthetic samples of fluor-, hydroxyl-, and chlor-apatite. A study of natural fluorapatite was performed by Matsukage et al. (2004), who found a KT lower than previous works (BM2: KT = 91.6(1.0) GPa vs 97.9(1.9) GPa in Brunet et al., 1999 and 97.8(1.0) GPa in Comodi et al., 2001). Recently Liu et al. (2011) have studied the effects of carbonate substitution on the compressibility of hydroxylapatite: the presence of carbonate (type-A and -B) lowers the value of KT. However, it is difficult to assess if the softening is due to type-A or -B or both. We have performed a X-ray study on natural single crystals of both fluorapatite [FOW; a=9.3549(9)Å, c=6.874(1)Å, V=521.0(1)ų] and type-B carbonate fluorapatite [FRA; a=9.330(1)Å, c=6.898(1)Å, V=520.0(2)ų] The two crystals were mounted concurrently in a ETH-type DAC, with a single crystal of quartz as internal P calibrant and a 16:3:1 mixture of methanol:ethanol:water as P-transmitting medium. We used SINGLE program (Angel & Finger, 2011) on a Siemens P4 four-circle diffractometer and EosFit7c (Angel et al., 2014) to collect and fit the data, respectively. Our data [28 P points up to 7.05(4)GPa; V values fitted with a BM3-EoS] confirm that type-B CO3-substitution decreases the bulk modulus from KT=89.4(1.3)GPa [KTʹ=5.1(4)] to KT=86.3(1.2)GPa [KTʹ=5.0(4)]. This preliminary result well agrees with those ones previously found for fluorapatite. References Angel R.J. & Finger L.W. 2011. J.Appl.Crystallogr.,44,247-251. Angel R.J., Gonzalez-Platas J. & Alvaro M. 2014. Z. Kristallogr.,229,405-419. Brunet F., Allan D.R., Redfern S.A.T., Angel R.J., Miletich R., Reichmann H.J., Sergent J. & Hanfland M. 1999. Eur. J. Mineral.,11,1023-1035. Comodi P., Liu Y., Zanazzi P.F. & Montagnoli M. 2001. Phys.Chem.Minerals.,28,219-224. Liu X., Shieh S.R., Fleet M.E., Zhang L. & He Q. 2011. Am.Mineral.,96,74-80. Matsukage K.N., Ono S., Kawamoto T. & Kikegawa T. 2004. Phys.Chem.Minerals,31,580-584.