Ballen Quartz in Jänisjärvi Impact Melt Rock with High Concentrations of Fe, Mg, and Al: EPMA, EDS, EBSD, CL, and Raman Spectroscopy

Abstract

Ballen quartz is a spherical aggregate that is typical for impactites. Previous studies of ballen quartz in different impact structures have revealed the presence of Fe, K, Al, Mg, and Ca in the contact zone between individual ballen only. In the present study, we describe ballen quartz found in a sample from a Jänisjärvi impact structure with a high concentration of Al2O3 (up to 4.56 wt.%), FeO (6.43 wt.%), and MgO (2.12 wt.%) in individual ballen. Microbeam methods: EPMA (Electron Probe Microanalysis), EDS (Energy-Dispersive Spectroscopy), BSE (Back-scattered Electrons), EBSD (Electron Backscatter Diffraction), CL (Cathodoluminescence), and Raman spectroscopy were used to understand the atypical compositions of ballen quartz and its formation processes. Ballen quartz in our sample consists of unaltered quartz (domain A), which was formed as a result of shock heating, and hydrothermally altered quartz (domain B). The abundance of Fe, Mg, and Al in domain B is associated with submicron-sized inclusions of chlorite or other Fe-Mg minerals. Misorientations in subgrains in ballen quartz type A and B reach 1° and 7°, respectively. The micrometer-sized orientational inhomogeneity in ballen quartz type B is comparable in size to the micrometer-sized blocks separated by the grid of microfractures. The new data obtained expand the diversity of the ballen quartz studied and could be used for an understanding the mechanisms of their formation.