Synthesis of sodalite-group phases at ambient pressare with different starting materials

The purpose of this work is the synthesis of feldspathoids minerals belongings to the sodalite group. We studied the following nominal compositions: Na8Al6Si6O24Cl2 (sodalite s.s.), Na8Al6Si6O24SO4 (nosean), Na8Al6Si6O24CO3 (carbonato-nosean) and Na8Al6Si6O24S2 (lazurite), by the use of 3 different starting materials; their thermal stabilities as a function of X have been investigated as well. These compounds are important for different topics, such as: a) geo-fluid barometry, b) geo-thermometry, c) technological applications. However, even nowadays several crystal-chemical features are relatively few constrained (Tomisaka and Eugster, 1968; Deer et al., 2004). The first starting materials is a mixture of salts and an amorphous compound, i.e. a commercial synthetic zeolite Na-A (NaAlSiO4 • n H2O), transformed to a gel by chemical treatments. The second starting material used is a mixture of salts and a natural compound, i.e. a kaolinitic rock (calcinated at 700 °C). The third starting materials is a mixture of high-grade commercial reagents of oxides, silicates and salts. All the experiments were conducted at T between 700 and 900 °C. The run-products were analysed by powder diffraction (XRPD) method and successively refined by the Rietveld and Le Bail methods. The three syntheses run at 750 °C with oxides on the nominal a) sodalite s.s., b) nosean and c) carbonato-nosean compositions resulted to be composed by: a) sodalite + minor amounts of corundum and nepheline, b) nosean + significant amounts of corundum and nepheline and c) corundum and nepheline plus other minor undetermined phases, respectively. These run-products were used again for a new synthesis, but at 850 °C. The sodalite s.s. yield attained then was closed to 100%, whereas the nosean yield significantly increases up to 80-90%. The chemical system of zeolite Na-A gel + NaCl was studied at 700-850-900 °C. At 700° C there is the presence of sodalite s.s., nepheline, carnegieite and halite, whereas at 900 °C there are sodalite, carnegieite, nepheline and halite. By contrast the highest sodalite yield was achieved at 850 °C experiments, by significantly increasing the NaCl amount (4, 6 and 8 molar). The chemical gel system of zeolite Na-A gel + Na2CO3 investigated at 700° C again was mainly composed of nepheline, and carnegieite; the carbonatic phase(s) was totally absent. The chemical gel system zeolite Na-A gel + Na2SO4 crystallised nosean, nepheline and unknown solphatic phases at 850° C. The chemical system metakaolin + NaCl was investigated only at 850° C, producing sodalite in association with nepheline. In order to eliminate nepheline and so obtain a unique compound, we investigated the same chemical system at the same T, but increasing the NaCl amount. In this case, we carried out sodalite, without the occurrence of nepheline for a time interval of 120-144 h. The chemical system metakaolin + Na2SO4 shows the presence of nosean, in association with nepheline and unknown solphatic phases. The chemical system metakaolin + Na2S gave the synthesis of lazurite, always in association with nepheline and, as in the case of the above Na2SO4 system, unknown solphatic phases. The general results that can be depicted by this study are: a) the sodalite s.s. and nosean high yields can be achieved by re-cycling the lower T run-products at higher T or by increasing the salt buffer amount for a fixed T; b) the CO3 group was never detected, suggesting that it can be incorporated in the feldspathoid structure only at lower T, at least at room pressure and c) that a low-cost natural material as kaolinitic rock can be used to produce sodalitic phases. References: Deer W.A., Howie R.A., Zussman J. (2004). Rock-forming Minerals, vol. 4b. Tomisaka T., Eugster H.P. (1968). Min. Jap., 5: 249-275.