FREEZE DRYING AS A METHOD FOR PREPARING RANDOMLY ORIENTED CLAY MOUNTS FOR X-RAY DIFFRACTION (XRD) ANALYSIS

Methods used to evaluate oriented clay mounts by X-ray diffraction (XRD) are well known, but randomly oriented sample preparation methods are less accessible. Randomly oriented clay powders can be prepared by spray drying (Hillier, 1999) and chopping loose powder (“razor-tamped surface” method; Zhang et al., 2003). We propose a new method for preparing random clay mounts by freeze drying.

To test the methodology, ~ 0.5-1.5 g of relatively pure illite and kaolinite samples were sonicated in an NaHMP solution and centrifuged (771 x g; 3 minutes), and ~ 10-15 mL were poured off from each sample to isolate the finest sediment. The suspended clays were frozen overnight in a standard freezer, and subsequently freeze dried using a Labconco FreeZone 2.5 freeze dryer, producing powders arranged with a stringy texture.

Double-sided tape was placed in the base of XRD sample holders, and freeze-dried clays were vertically pressed onto the tape with a knife blade. The use of double-sided tape and a knife blade for tamping was derived from Zhang et al. (2003), who incorporated these components into their chopped-powder method. Data were collected using a Bruker D8 Advance X-ray diffractometer and analyzed with MDI Jade software.

Preliminary results from randomly oriented samples match visually with their respective PDF-4 relational database (RDB-MIN2021) patterns. The modeled, best-fit preferred orientation (PO) value in Jade is within 0.05 of 1.0 for the freeze-dried samples, which appears to compare well with the chopped-powder method. In contrast, the PO value for an oriented kaolinite sample is much lower (~ 0.3), indicating that freeze drying successfully induces random orientation.

For those who have access to a laboratory freeze dryer, this method eliminates the tedious sample prep associated with chopping a powdered surface, and excludes multiple variables involved in building and operating a spray dryer. Furthermore, the freeze-drying method has the potential to work with small sample volumes that would otherwise be inadequate for loose-powder tamping or spray drying (e. g. clays extracted from sandstone). We hope that this freeze-drying methodology will provide new opportunities for bulk clay mineral analysis by XRD.