Thermodynamic Behavior of Analcime-Leucite Analogue Systems

G. Hovis (hovisguy@mail.lafayette.edu)¹, E. Rodrigues (beth@cnrs-orleans.fr)² & J. Roux (jroux@cnrs-orleans.fr)²

¹ Department of Geology, Lafayette College, Easton, PA 18042, USA

² CNRS-CRSCM, 1A, rue de la Ferollerie, 45071 Orleans Cedex 2, France

Only very limited solid solution exists between natural analcime (NaAlSi₂O₆.H₂O) and leucite (KAlSi₂O₆), nor can this series be metastably synthesized. In order to explore the thermodynamic behavior of this system, we have studied the chemical analogue systems analcime - Rb-leucite (RbAlSi₂O₆) and analcime - pollucite (CsAlSi₂O₆), where synthesis is indeed possible. Such systems also offered case studies of complete solid solution between hydrous and anhydrous end members involving structurally coupled water-alkali-vacancy substitutions, a type of thermodynamic "mixing" that to our knowledge has not been previously investigated. Data have been collected on a hydrothermally synthesized ten-member Rb-bearing series and also a ten-member Cs-bearing series. Unit-cell results based on room-temperature X-ray diffraction measurements display non-linear behavior as a function of composition, evidence of non-ideal thermodynamic behavior. Interestingly, however, the enthalpies of solution measured in 20.1 weight percent hydrofluoric acid at 50°C are continuous and linear functions of composition for both series. Non-ideality, therefore, is athermal in both cases. To conclude this project, we are in the process of characterizing the energetic effects of Al:Si non-stoichiometry in this system, given the substantial effects of such non-stoichiometry found in the nepheline-kalsilite system (Hovis & Roux, 1999).

Hovis GL, & Roux J, European Journal of Mineralogy, 11, 815-827, (1999).

Stability and Phase Relations of Ca[ZnSi₃]O₈, A New Phase with Feldspar Structure in the System CaO-ZnO-SiO₂

Alexandra L. Huber (ahuber @petro1.min.uni-muenchen.de) & K. Thomas Fehr

Institute of mineralogy, petrology & geochemistry, Ludwig-Maximilians-University, Munich, Theresienstr. 41/III D-80333 München, Germany

Zinc-feldspar, Ca[ZnSi₃]O₈, - a new phase - is located within the system CaO-ZnO-SiO₂ together with hardystonite, willemite, quartz and petedunnite [1,2,3].

Zinc-feldspar was synthesised hydrothermally within 35 days from an oxide mixture at 600°C/0.7 GPa. Ca[ZnSi₃]O₈ crystallises in spacegroup P-1, displaying lattice constants of a=8.121(1) Å, b=12.927(1) Å, c=7.206(1) Å, (alpha)=93.76(5)°, ß=116.120(7)°, (gamma)=84.368(7)°, V=675.7 ų, Z=2. Zn and Si are ordered on tetrahedral sites [4]. The high similarity to the structure of low albite is striking.

The stability of zinc-feldspar is limited to higher temperatures by reaction (1) 4 zinc-feldspar = willemite + 2 hardystonite + 7 quartz. It has a negative slope in the P-T-field, which can be expressed by the equation P=2.797(0.325)-0.0031(0.0004)T(°C). Two further reactions were determined by reversed experiments: (2) willemite + 2 hardystonite + 3 quartz = 4 petedunnite and (3) zinc-feldspar = petedunnite + quartz. Equation (3) is a double degenerated reaction. The intersection of the reactions (1), (2) and (3) generates an invariant point at about 530°C/1.2 GPa [5]. Due to the stability relations of the SiO₂ system, two further invariant points I(low-/high-quartz) and I(high-quartz/tridymite) are generated at 720°C/0.57 GPa and 896°C/0.016 GPa.

Generally ZnO is not included in feldspar analyses by electron microprobe. However, Zn-enriched feldspar can be observed in scarns, Pb-Zn hydrothermal vein deposits and in slags of lead blast furnaces. Naturally grown feldspar contains up to 0.18 wt% ZnO. In addition appreciable amounts of BaO were analysed in natural feldspars, too, implying a Ba[ZnSi₃]O₈ component beside Ca[ZnSi₃]O₈.

Rothkopf AL & Fehr KT, Terra Nova Abstr. Suppl., 10, 53, (1998).

Fehr KT & Hobelsberger B, Beiheft z. European J. Miner, 9, 98, (1997).

Essene EJ & Peacor DR, Amer. Miner, 72, 157-166, (1987).

Heuer M, Bente K, Steins M & Rothkopf AL, Z. Kristallogr. NCS, 213, 691-692, (1998).

Huber AL, Beiheft z. European J. Miner, 11, 99, (1999).

Rare Earth Element Partitioning between CaSi-Perovskite and Mg-Silicates Under Mantle Conditions: Comparison with Inclusions in Diamond

Mark T. Hutchison (mhutchis@lpl.arizona.edu)¹, Tibor Gasparik (gasparik@sbmp04.ess.sunysb.edu)² & Michael J. Drake (drake@lpl.arizona.edu)¹

¹ Lunar and Planetary Laboratory, University of Arizona, Tucson AZ 85721-0092, U.S.A.

² CHiPR, Dept. of Earth and Space Sciences, State University of New York, Stony Brook NY 11794, U.S.A.

Mantle minerals found as inclusions in diamonds from São Luiz, Brazil contain a wide range of rare earth element (REE) concentrations with originally perovskite structured CaSiO₃ (CaSiPvk) being by far the most REE-phyllic phase (Harte et al., 1999 and Hutchison et al., 2000). This observation is consistent with previous experimental results using mantle compositions (Kato et al., 1988) and measurements of analogous CaTiO₃ perovskite found in kimberlites (Mitchell and Reed, 1988). A systematic experimental framework of partitioning data between CaSiPvk and coexisting phases allowing better constraint of the details of depth and temperature of formation of the natural sample has however, yet to be constructed. Partitioning of 10 REE are thus being investigated, initially in the CMS system. Starting materials of diopside glass doped with 1000 ppm REE oxides and mixed with 10wt% each of ReO₂ to buffer fO₂ and PbO₂ flux, are being employed. Experiments on Stony Brook's USSA-2000 and the University of Arizona's Walker-style 1000 ton multi-anvil presses have been conducted at 16.5, 20 and 22 GPa with temperatures between 1400°C and 2200°C in order to crystallise CaSiPvk with ilmenite, garnet and perovskite structured Mg-silicates in addition to the CM phase. Results analysed by SIMS and EPMA show that with decreasing Ca content in the Mg silicate phase, partition coefficients with CaSiPvk increase significantly. Partitioning with MgSiPvk is particularly extreme, involving values approaching those found for natural Mg-silicates coexisting with CaSiPvk, corroborating their previously attributed lower mantle origin. Additionally, Ca content and REE partitioning of some São Luiz 'diopsides' (Hutchison, 1997) are consistent with results in the CM field suggesting that these samples may have formed as the CM phase. Further experiments are being carried out to constrain the effects of pressure and temperature within phase fields and investigate the effects of additional atomic components and oxygen fugacity.

Harte B et al., Geochem. Soc. Spec. Publ., 6, 125-153, (1999).

Hutchison MT, Ph.D. Thesis of the University of Edinburgh (CDRom), 660 pp., (1997).

Hutchison MT, Harris JW & Harte B, Science (submitted), (2000).

Kato T, Ringwood AE and Irifune T, Earth Planet. Sci. Lett., 89, 123-145, (1988).

Mitchell R and Reed S, Mineral. Mag., 52, 331-339, (1988).