Challenges in powder XRD-based Rietveld analyses of Carbonates and Sulfates from Lechuguilla Cave (New Mexico, USA)

Lechuguilla Cave, located near Carlsbad in New Mexico, USA, displays an extraordinary spectrum of speleothems formed by unusually diverse secondary cave minerals, among them various carbonates, sulfates, oxides, silicates, and halides.

Various samples of mineral assemblages taken in the context of a study on unusual speleothems formed by either barite and/or celestine were analyzed using powder XRD-based Rietveld analyses (BRASS software [1]). The samples were crushed and ground thoroughly and filled into flat sample holders. However, when celestine, barite and/or calcite constituted the dominant part of the samples, we encountered strong textures and often more than one preferred crystallographic orientation.

Some samples were re-ground several times, but no significant improvement was achieved, and sample deterioration commenced, as indicated by additional broadening of reflections. Accordingly, the samples had to be analyzed after normal grinding.

The issues could be resolved by using several instances of the same phase, which had most parameters in common except for scale factors, preferred orientations and, sometimes, peak broadening parameters. This approach had additional merits compared to multi-texture models with a single phase, since the relative amounts of the orientational variants could be determined.

This procedure also resulted in better matches of patterns and the retrieval of additional significant information, such as the co-occurrence of two different dolomites and Mg-calcite with various magnesium concentrations. In one case, two different Mg-calcites were present, which could only be identified by peak shoulders.

[1] J. Birkenstock, T. Messner & R.X. Fischer (2025). BRASS 2.4, Universität Bremen, Germany, www.brass.uni-bremen.de