IMPROVED METHODS FOR PREPARING RANDOMLY ORIENTED CLAY SAMPLES FOR X-RAY DIFFRACTION ANALYSIS BY FREEZE DRYING IN POLYVINYL ALCOHOL

Freeze drying is a promising method for producing random clay mounts for X-ray diffraction (XRD) analysis due to its effectiveness and simplicity compared to spray dried (Hillier, 1999) or chopped powder (Zhang et al., 2003) approaches. Initial samples were prepared in water (Maithel et al., 2022), but the fragile nature of the freeze-dried clays made them susceptible to orientation and dislodging during sample loading and analysis. Additionally, X-ray peaks from the underlying double-sided mounting tape were prominent in XRD data from thin samples.

Clays extracted from sandstone and powdered kaolinite were sonicated and frozen in a standard freezer or liquid nitrogen. The samples were prepared in 1%, 5%, and 10% polyvinyl alcohol (PVA) solutions, frozen, then dried using a Labconco FreeZone 2.5 freeze dryer. Dried samples were then separated using knife blades and loaded into XRD sample holders, with double-sided tape used to secure the samples to the holder backings. One kaolinite sample was freeze dried into a thin disc that fit directly into the sample holder without knife separation. XRD data were collected using a Bruker D8 Advance X-ray diffractometer and analyzed with MDI Jade software.

We observed that clays dried in PVA exhibit a firmer texture and retain greater volume after drying than preparations in water. Initial results indicated that the modeled, best-fit preferred orientation (PO) values for freeze-dried kaolinite were highest for the disc (~1.05), followed by the knife-separated, 5% PVA (~0.95), 10% PVA (~0.87), and 1% PVA (~0.84). Clays extracted from sandstone also showed PO values in this range, and samples with very low ratios of mineral to PVA scattered X-rays only from the minerals and PVA, with no scattering from the mounting double-sided tape.

Freeze drying clays in a PVA solution preserves random orientation better than water, increasing their utility for XRD analysis. These results can likely be attributed to the dried PVA's firm structure, which prevents clay orientation during sample loading. We aim to refine these methods by collecting additional data from other clay minerals and clays extracted from sandstone.