FREEZE DRYING: AN EFFECTIVE METHOD FOR PREPARING RANDOM X-RAY DIFFRACTION MOUNTS FROM SMALL QUANTITIES OF CLAYS

Freeze drying in polyvinyl alcohol (PVA) has shown preliminary effectiveness as a method for preparing randomly oriented clay samples for X-ray diffraction (XRD) analysis (Maithel et al., 2023). This method is simpler than the chopped-powder (Zhang et al., 2003) and spray-dried (Hillier, 1999) approaches, and it works with a fraction of the sample mass that those methods require. XRD patterns from spray-dried samples were compared with equivalent freeze-dried preparations to confirm the methodology's validity further.

Two multi-mineral, clay-rich samples from Southern California were ground in a McCrone Micronizing Mill: one from a dry lake bed in the Panamint Valley and one from San Timoteo Canyon near San Bernardino. Bulk laboratory-grade kaolinite was also prepared for this comparison. Small masses (~0.1 and 0.2 g) of each powdered sample were combined with 1 ml of a 5% PVA solution, frozen overnight in a standard freezer, and then freeze dried using a Labconco FreeZone 2.5 freeze dryer. The freeze-dried samples were separated into small pieces with knife blades, back loaded into the sample holders, and analyzed using a Bruker D8 Advance X-ray diffractometer. Equivalent splits were milled and spray dried at The James Hutton Institute in Scotland and mailed back to us for XRD analysis.

Results from all freeze-dried samples were visually similar to their spray-dried counterparts. Moreover, the 002 illite scattering peaks were noticeably more distinct in freeze-dried preparations of the Panamint and San Timoteo samples. Preferred orientation (PO) values for the illite were observed to be greater than 0.9 for all samples (both spray and freeze dried), with the ~0.1 g freeze-dried preparations exhibiting the highest values and slightly less preferred orientation.

While spray drying is the standard for producing randomly oriented powder sample mounts, it requires a substantial amount of material – at least 3 grams – due to sample loss during the spray-drying process. In contrast, the freeze-drying method produces comparable XRD patterns from a sample of as little as 0.1 g. Furthermore, freeze drying yields near-random orientations in clay minerals which are particularly susceptible to preferred orientation. This method will provide new opportunities for XRD analysis where only small samples are available, such as clay minerals extracted from sandstones and other rocks.