SOLVING CRYSTAL STRUCTURES OF NANOMINERALS USING HIGH-RESOLUTION XRD, PAIR DISTRIBUTION FUNCTION (PDF), AND TEM METHODS

New magnetic nanominerals of luogufengite (ε-Fe₂O₃), valleyite (Ca₄(Fe,Al)₆O₁₃) and xuite (Ca₃Fe₂[(Al,Fe)O₃(OH)]₃) have been identified using high-resolution XRD and transmission electron microscopy methods. High-resolution powder XRD can solve peak overlapping problem for a sample with multiple phases. Mineral phases with different average sizes can be easily distinguished based on their peak shapes (broadness). Measured Curie temperatures for valleyite and luogufengite are 645 and 519 K, respectively. This unique magnetic property of luogufengite with large coercivity may explain the observed unusually high-remnant magnetization in some igneous and metamorphic rocks and even Martian rocks with high-remnant magnetization.

Determination of the atomic-scale structures of certain fine-grained minerals using single-crystal X-ray diffraction (XRD) has been challenging because they commonly occur as submicron and nanocrystals in the geological environment. Synchrotron powder diffraction and scattering techniques are useful complementary methods for studying this type of minerals. Pair distribution function (PDF) method is based on total scattering from X-ray or neutron, and is sensitive to study the local structure with high quality anisotropic distribution parameters (ADPs). We have re-investigated moganite structure intergrown with quartz nano-crystals using high-resolution synchrotron X-ray diffraction (XRD) and X-ray/neutron pair distribution function (PDF) analyses. Our study for the first time reports the anisotropic atomic displacement parameters (ADPs) of a natural moganite. Neutron PDF result confirms the occurrence of OH over some of the tetrahedral sites. The combined methods can provide high-quality structural parameters of moganite nanomineral, including its ADPs and extra OH position at the surface. This approach can be used as an alternative means for solving the structures of crystals that are not large enough for single-crystal XRD measurements, such as fine-grained and nanocrystalline minerals formed in various geological environments.