Abstract
New in-situ high-temperature X-ray powder diffraction data on the normal-incommensurate phase transition in Co-åkermanite (Ca2CoSi2O7) are presented. Evidence for the phase transition is found in the abrupt change in the thermal expansivity of the c lattice parameter at 220° C. In addition, the c lattice parameter exhibits premonitory effects of the phase transition through the leveling out of the thermal expansivity at temperatures from 87 to 220° C.
The c/a ratio for X2ZSi2O7 incommensurate melilites is shown to be constant for a wide variety of compositions. Correlation of the trends in mean atomic positions with c/a ratio and the sigmoidal variation in the c/a ratio for Ca2CoSi2O7 provide insight into the atomic displacements occurring in the melilite structure as a function of temperature. Three temperature regimes are discussed in detail (i.e. temperatures well above T c, temperatures just above T c, and temperatures below T c). The atomic displacements occurring in each temperature regime are correlated with the changes in anisotropy and orientation of the atomic displacement ellipsoids for each site, as well as spectroscopic evidence for the increasing number of Z- and T-site environments.
Similar content being viewed by others
References
Armbruster T, Röthlisberger F, Seifert F (1990) Layer topology, stacking variation, and site distortion in melilite-related compounds in the system CaO-ZnO-GeO2-SiO2. Am Mineral 75:847–858
Brown NE, Weigel R (1994) High temperature (323–1323 K) furnace design for a transmission powder diffractometer. J Appl Cryst 27:151–154
Christie OHJ (1962) On sub-solidus relations of silicates. IV. The system åkermanite sodium gehlenite and gehlenite-sodium gehlenite. Norsk Geol Tidskr 42:31–44
Hagiya K, Ohmasa M, Iishi K (1993) The modulated structure of synthetic Co-Åkermanite, Ca2CoSi2O7. Acta Cyst B49:172–179
Hemingway BS, Evans HT Jr, Nord GL Jr, Haselton HT Jr, Robie RA, McGee JJ (1986) Åkermanite: Phase transition in heat capacity and thermal expansion, and revised thermodynamic data. Can Mineral 24:425–434
Iishi K, Fujino K, Furukawa Y (1989) Single crystal growth of åkermanites, Ca2Mg1-xCoxSi2O7) with modulated structure. N Jb Miner Mh 1989:219–226
Iishi K, Fujino K, Furukawa Y (1990) Electron microscopy studies of åkermanites (Ca1-xSrx)2CoSi2O7 with modulated structures. Phys Chem Minerals 17:467–471
Iishi K, Mizota T, Fujino K, Furukawa Y (1991) Heat Capacity anomalies at normal-incommensurate transition of åkermanite solid solution (Ca,Sr)2(Mg,Co,Zn,Fe)Si2O7. Phys Chem Minerals 17:720–725
Ito J, Peiser S (1969) Distorted tetrahedra in Strontium Copper åkermanite. J Res Nat Bur Stand 73A:69–74
Kimata M (1980) Crystal chemistry of Ca-melilites on X-ray diffraction and infrared absorption properties. N Jb Miner Abh 139:43–58
Kimata M (1983a) The crystal structure of Co-åkermanite, Ca2Co-Si2O7, compared with the mineralogical behavior of Mg cation. N Jb Miner Abh 146:221–241
Kimata M (1983b) The structural properties of synthetic Sr-åkermanite, Sr2MgSi2O7. Z Kirstallogr 163:295–305
Kimata M (1984) The structural properties of synthetic Sr-gehlenite, Sr2Al2SiO7. Z Kirstallogr 167:103–116
Kimata M (1985) The structural properties and mineralogical significance of synthetic Sr2MnSi2O7 melilite with 4-ccordinated manganese. N Jb Miner Mh 1985:83–96
Kimata M, Ii N (1981) The crystal structure of synthetic åkermanite, Ca2MgSi2O7. N Jb Miner Mh 1981:1–10
Kimata M, Ii N (1982) The structural properties of synthetic gehlenite, Ca2Al2SiO7. N Jb Miner Abh 144:254–267
Kimata M, Ohashi H (1982) The crystal structure of synthetic gugiate, Ca2BeSi2O7. N Jb Miner Abh 143:210–222
Li Z, Chan S-K, Ghose S (1990) Elastic properties of the incommensurate phase of åkermanite, Ca2MgSi2O7. Phys Chem Minerals 17:462–466
Louisnathan SJ (1969) Refinement of the crystal structure of hardingstonite. Z Kirstallogr 130:427–437
Louisnathan SJ (1970) The crystal structure of synthetic soda melilite. Z Kristallogr 131:314–321
Merwin LH, Sebald A, Seifert F (1989) The incommensurate-commensurate phase transition in åkermanite, Ca2MgSi2O7, observed by in-situ 29Si MAS NMR spectroscopy. Phys Chem Minerals 16:752–756
Nelmes RJ (1969) Representational Surfaces for thermal Motion. Acta Crystallogr A25:523–526
Röthlisberger F (1989) Zusammenhang zwischen Chemismus, Stabilität und struktureller Variation der Melilite. Doctoral Dissertation, Universität Bayreuth
Röthlishberger F, Seifert F, Czank M (1990) Chemical control for the commensurate-incommensurate phase transition in synthetic melilites. Eur J Mineral 2:585–594
Seifert F, Czank M, Simons B, Schmahl W (1987) A normal-incommensurate phase transition in iron-bearing åkermanites. Phys Chem Minerals 14:26–35
Smith JV (1953) Reexamination of the crystal structure of melilite. Am Mineral 38:643–661
Swainson IP, Dove MT, Schmahl WW, Putnis A (1992) Neutron Powder diffraction study of the åkermanite-gehlenite solid solution series. Phys Chem Minerals 19:185–195
Touloukian YS, Kirby RK, Taylor RE, Lee TYR (1977) Thermophysical Properties of Matter. 13, Plenum PubIishing Corporation, New York
Webb SL, Ross CR II, Liebertz J (1992) Thermal expansion and spontaneous strain associated with the normal-incommensurate phase transition in melilites. Phys Chem Minerals 18:522–525
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Brown, N.E., Ross, C.R. & Webb, S.L. Atomic displacements in the normal-incommensurate phase transition in Co-åkermanite (Ca2CoSi2O7). Phys Chem Minerals 21, 469–480 (1994). https://doi.org/10.1007/BF00202277
Received:
Revised:
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/BF00202277