Anisotropy of magnetic susceptibility in alkali feldspar and plagioclase

Abstract

S U M M A R YFeldspars are the most abundant rock-forming minerals in the Earth's crust, but their magnetic properties have not been rigorously studied. This work focuses on the intrinsic magnetic anisotropy of 31 feldspar samples with various chemical compositions. Because feldspar is often twinned or shows exsolution textures, measurements were performed on twinned and exsolved samples as well as single crystals. The anisotropy is controlled by the diamagnetic susceptibility and displays a consistent orientation of principal susceptibility axes; the most negative or minimum susceptibility is parallel to [010], and the maximum (least negative) is close to the crystallographic [001] axis. However, the magnetic anisotropy is weak when compared to other rock-forming minerals, 1.53 × 10 −9 m 3 kg −1 at maximum. Therefore, lower abundance minerals, such as augite, hornblende or biotite, often dominate the bulk paramagnetic anisotropy of a rock. Ferromagnetic anisotropy is not significant in most samples. In the few samples that do show ferromagnetic anisotropy, the principal susceptibility directions of the ferromagnetic subfabric do not display a systematic orientation with respect to the feldspar lattice. These results suggest that palaeointensity estimates of the geomagnetic field made on single crystals of feldspar will not be affected by a systematic orientation of the ferromagnetic inclusions within the feldspar lattice.

Crystal Mush Flow in Small Concentrically Expanded Pluton (Castle Crags Pluton; Klamath Mountains, CA, USA)

“…The additional gradual decrease of magnetic susceptibility above the Curie temperature of magnetite most probably represents small amounts of paramagnetic biotite and amphibole (Figure S2 and Text S1) that has been documented by microstructural analysis. The calculation based on evolving modal composition of all three rock types, single‐crystal magnetic properties of constituent mineral phases (Biedermann et al, , ; Martín‐Hernández & Hirt, ; Nye, ), and an extreme assumption, that all individual AMS ellipsoids/tensors are aligned with maximum axes parallel to each other, showed insignificant contribution of paramagnetic phases to the bulk AMS (Text S1). Additional analyses of the hysteresis loops, isothermal remanent magnetization acquisition (IRM) curves, and DC demagnetization curve measurements were performed in order to further constrain the magnetic mineralogy and possible effect of magnetostatic interactions in the CCG granite types.…”

Magnetic Anisotropy in Single Crystals: A Review

“…In feldspar, Si is partially replaced by Al, and cations are incorporated into the holes of the framework in order to maintain charge balance. Despite their abundance, only few studies discuss the magnetic anisotropy of quartz or feldspar [47,[50][51][52]120,121], because they are weakly magnetic, and their contribution to anisotropy is outweighed by small amounts of iron-bearing mafic minerals [121]. The few data available for quartz agree that the most negative susceptibility is aligned with [001], and susceptibility is isotropic in the (001) plane (Figure 1g).…”

“…Interestingly, even in crystals with positive mean susceptibility, the magnetic anisotropy is dominated by a diamagnetic component with the maximum (or least negative) susceptibility close to [001], and minimum (or most negative) sub-parallel to [010] (Figure 1h). This is related to a preferred plane for electrons along the bases of the SiO 4 or AlO 4 tetrahedra in the (010) plane [121]. Because feldspars can be monoclinic or triclinic, symmetry may require that one principal susceptibility is parallel to [010], or there may be no symmetry constraints.…”

Influence of static alternating field demagnetization on anisotropy of magnetic susceptibility: Experiments and implications

“…Comparison of contributions from field-induced anisotropy to anisotropy in NRM states for all four sample groups, and typical anisotropies of single crystals. Single crystal anisotropies were taken from Ballet and Coey [1982], Beausoleil et al [1983], Belley et al [2009], Biedermann et al [2014a, 2014b, 2015a, 2015b, 2016a, Borradaile et al [1987], Ferr e et al [2005], Finke [1909], Haerinck et al [2013], Lagroix and Borradaile [2000], Martin-Hernandez and Hirt [2003], and Wiedenmann et al [1986].…”