Cowlesite specimens from nine new occurrences in Northern Ireland and from eight localities previously reported in the literature, were crystal-chemically characterised. The chemical data for a total of 25 samples indicate a composition close to the stoichiometric formula for cowlesite and a very limited compositional range. X-ray powder diffraction spectra were performed on 8 samples. The crystallographic unit cell permit a proper indexing of the X-ray powder diffraction patterns as orthorhombic, unit cell constants close to a 23.3, b 30.6, c 25.0 Å and doubled with respect to those originally reported in literature. The powder pattern and the TG curve arc reported for an unidentified mineral species associated with cowlesite in one of the samples from Northern Ireland.

The subject of the paper is the old and well-known linear least squares method for determining the lattice dimension of a crystalline phase from the peak locations of a powder diffraction pattern. Approximations, limits, and extensions of the method are discussed with reference to the assignment of hkl indices to an experimental multiphase pattern.

Patterns useful for identification are obtained by automated diffractometer methods. The lattice constants from the experimental work are refined by least-squares methods; reflections are assigned hkℓ indices consistent with space group extinctions. Relative intensities, calculated densities, literature references, and other relevant data are included.