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Dicalcium aluminate ([Ca.sub.2][Al.sub.2][O.sub.5]) was prepared in a piston cylinder apparatus at 1250 [degrees] C and 2.5 GPa. The compound is orthorhombic with space group symmetry /2mb, a = 5.2281(1) [Angstrom], b = 14.4686(2) [Angstrom], c = 5.4004(1) [Angstrom] (Z = 4, [D.sub.calc] = 3.481 g/[cm.sup.3]), and belongs to the brownmillerite structure family. Main building units are (1) layers of perovskite type corner connected Al[O.sub.6]-octahedra perpendicular to [010], and (2) zweier single chains of Al[O.sub.4]-tetrahedra running parallel [100]. The alternate stacking of the layers and sheets of single chains results in a three dimensional network in which the calcium ions are incorporated for charge compensation. The present structure is the first example for an alkaline earth aluminate with zweier single chains of tetrahedra.

Oxygen isotope exchange between natural analcime and water vapor at low pressure is studied by [sup.17]O NMR, which can distinguish Si-O-Si and Si-O-Al sites and provide information on site specific exchange kinetics and mechanisms. Si-O-Al sites in analcime are found to exchange faster than Si-O-Si sites at lower temperature (400 [degrees] C). At higher temperature (500 [degrees] C), the exchange rates for the two types of site are similar, suggesting that the exchange process for Si-O-Si has a higher activation energy, as predicted by published theoretical calculations. The overall exchange reaction appears to be controlled largely by reaction at sites, not diffusion. Static [sup.17]O spectra show an anomalously high ratio of Si-O-Si to Si-O-Al sites, suggesting the presence of a few percent of Al-O-Al sites, which are indeed resolved and observed in triple quantum magic-angle spinning (3QMAS) spectra.

Single-crystal structure refinements are presented of the holotype of crystal peprossiite-(Ce) (Monte Cavalluccio) and of a new sample from Cura di Vetralla (Viterbo, Italy) with slightly different composition, together with new EMP-SIMS chemical analyses. These results allow us to propose a new unit formula: [[REE.sub.1-x-y][(Th,U).sub.x][Ca.sub.y]] [([Al.sub.3]O).sub.2/3][([B.sub.4-z][Si.sub.z])][O.sub.10] with x - y + z = 1/3 (Z = 1) for the peprossiite group. Lattice constants for the holotype crystal are: a = 4.612(1), c = 9.374(3) [Angstrom], V = 172.6 [[Angstrom].sup.3], Z = 1, space group P[bar]6 2m. The crystal structure was solved by Patterson methods and refined to [R.sub.obs] = 1.8% ([R.sub.all] = 2.2%) for 706 unique reflections in the 2[Theta]-range 6-136 [degrees]. Lattice constants for the thorian peprossiite-(Ce) from Cura di Vetralla are: a = 4.596(3), c = 9.309(16) [Angstrom], V = 170.3 [[Angstrom].sup.3], and the structure was refined to [R.sub.obs] = 2.9% and [R.sub.all] = 3.0% for 271 unique reflections in the 2[Theta]-range 4-80 [degrees] The topology of the tetrahedral layer and the site of the inter-layer cation (REE) in peprossiite resembles that of dioctahedral micas. The main difference lies in the presence of layers of pyramids instead of layers of octahedra typical of mica. In peprossiite, Al is coordinated by five O atoms in a nearly square-pyramidal arrangement, the base of which is formed by pairs of apical O atoms from two layers of tetrahedra related by a mirror plane. Three of these pyramids share their apical O forming [Al.sub.3]0 groups with occupancy of 2/3 according to the structure refinement. A model is proposed that explains the apparent disorder in the pyramidal layer of peprossiite by the stacking within a triple cell (with a' = a [square root of] 3 and a ^ a' = 30 [degrees]) of three ordered layers randomly translated by [+ or -] a.

We present [sup.17]O MAS NMR data for crystalline calcium dialuminate (grossite), Ca[Al.sub.4][O.sub.7] and monoaluminate, Ca[Al.sub.2][O.sub.4]. The first of these contains an oxygen tricluster site and serves as a model compound for sites of this type in aluminosilicate glasses. Tricluster site NMR parameters are distinct from those of bridging O atoms (Al-O-Al), allowing partial resolution in triple quantum MAS NMR spectra. Such spectra for calcium aluminosilicate glasses are consistent with the presence of a small fraction of tricluster sites. Observed chemical shifts for non-bridging oxygen (NBO) atoms in an impurity phase in the Ca[Al.sub.2][O.sub.4] sample are distinct from those for NBO in Ca-aluminosilicate glasses, indicating that the latter are primarily bonded to Si, not Al.