A method of indirect analysis of X-ray diffraction patterns of homogeneous hydrated microcrystalline silicates is introduced. This method is applied to a sodium beidellite of Rupsroth, Bavaria, Germany, hydrated with two water layers to determine all its structural characteristics: namely, the average dimension and size distribution of coherent domains, stacking mode of the layers along the c* direction, levels and number per unit cell of water molecules. A critical analysis on the `concept of homogeneity' of hydrates and a discussion on the conditions of its application to lamellar microcrystalline silicates with a few numbers of layers are made.

Abstract. NiS20 3. 6H20 , M r = 463.02, orthorhombic, Pnma, a = 9-282 (5), b = 14.444 (9), c = 6.803 (3) A, Z--4, V=912.1A 3,D x--2.03,D m--1.92gem -3 (flotation), g(Cu Kct) = 73.27 cm -l. The title compound is isostructural with MgS20 3. 6H20. The structure was refined by a full-matrix least-squares method to an R (excluding zero-weight data) of 0.056 for 248 reflexions and an R (including zero-weight data) of 0-072 for 266 reflexions. The S-S and S--O bond distances in the $2 O2-anion are 2.015 (7) and 1.465 (I0) A respectively. The bond angle O-S-O is 110.2 (6) °. The H atom positions were found from a difference Fourier synthesis.

Ammonium thiosulphate is monoclinic with cell dimensions a = 10"282 (9), b = 6-541 (6), c = 8"766 (9) A,/3 = 95-49 (9) ~, space group C2, Cm or C2/m. Powder data are given. Chemical characterizationThe crystals of ammonium thiosulphate of purity>_97% (RT), stabilized with 1% sodium sulphite, were obtained from FLUKA. Crystal dataThe hygroscopic crystals were placed in Lindemann tubes and examined by the oscillation and Weissenberg methods with Cu K~ radiation. It was verified that the crystals belong to the monoclinic system. Systematic absences in hkl with h + k = 2n + 1 and hOl with h = 2n + 1 indicate that the space group is C2, Cm or C2/m. Lattice parameters (Table 1) were calculated by the least-squares method from the d spacings (Table 2) obtained from photographically recorded powderpattern measurements. The radiation used was Cu K~I (2= 1.54051 A,), and the temperature was 298 K. The program CELCIUS (Tegenfeldt, 1967) was used for the calculation. Indexing was based on preliminary cell dimensions obtained from the single-crystal photographs. Experimental density was determined by flotation. Table 1. Crystal data for (NH4)2S2O 3 Monoclinic C2, Cm or C2/m a= 10-282 (9), b=6"541 (6), c= 8"766 (9) A /3 = 95-49 (9) ° V=589"5 A 3, Z=4 Dx = 1"670, D,,, = 1"679 g cm-3

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Barium thiosulphate monohydrate is orthorhombic, space group Pbcn, with cell dimensions a = 20.062 (4), b = 7.192 (1), c = 7.386 (1) Å. Powder data are given and discrepancies in relation to the X‐ray Powder Data File are discussed.