Empirical relationships between magnetic fabrics and deformation have long served as a fast and efficient way to interpret rock textures. Understanding the single crystal magnetic properties of all minerals that contribute to the magnetic anisotropy of a rock, allows for more reliable and quantitative texture interpretation. Integrating information of single crystal properties with a determination whether or not mineral and magnetic fabrics are parallel may yield additional information about the texture type. Models based on textures and single crystal anisotropies help assess how the individual minerals in a rock contribute to the rock’s anisotropy, and how the individual anisotropy contributions interfere with each other. For this, accurate and reliable single crystal data need to be available. This review paper discusses magnetic anisotropy in single crystals of the most common rock-forming minerals, silicates and carbonates, in relation to their mineralogy and chemical composition. The most important ferromagnetic minerals and their anisotropy are also discussed. This compilation and summary will hopefully lead to a deeper understanding of the sources of magnetic anisotropy in rocks, and improve the interpretation of magnetic fabrics in future structural and tectonic studies.

Numerous studies have revealed genetic similarities between Tethyan ophiolites and oceanic “proto-arc” sequences formed above nascent subduction zones. The Semail ophiolite (Oman–U.A.E.) in particular can be viewed as an analogue for this proto-arc crust. Though proto-arc magmatism and the mechanisms of subduction initiation are of great interest, insight is difficult to gain from drilling and limited surface outcrops in marine settings. In contrast, the 3–5 km thick upper-crustal succession of the Semail ophiolite, which is exposed in an oblique cross section, presents an opportunity to assess the architecture and volumes of different volcanic rocks that form during the proto-arc stage. To determine the distribution of the volcanic rocks and to aid exploration for the volcanogenic massive sulfide (VMS) deposits that they host, we have remapped the volcanic units of the Semail ophiolite by integrating new field observations, geochemical analyses, and geophysical interpretations with pre-existing geological maps. By linking the major-element compositions of the volcanic units to rock magnetic properties, we were able to use aeromagnetic data to infer the extension of each outcropping unit below sedimentary cover, resulting in a new map showing 2100 km2 of upper-crustal bedrock. Whereas earlier maps distinguished two main volcanostratigraphic units, we have distinguished four, recording the progression from early spreading-axis basalts (Geotimes), through axial to off-axial depleted basalts (Lasail), to post-axial tholeiites (Tholeiitic Alley), and finally boninites (Boninitic Alley). Geotimes (“Phase 1”) axial dykes and lavas make up ∼55 vol % of the Semail upper crust, whereas post-axial (“Phase 2”) lavas constitute the remaining ∼45 vol % and ubiquitously cover the underlying axial crust. Highly depleted boninitic members of the Lasail unit locally occur within and directly atop the axial sequence, marking an earlier onset of boninitic magmatism than previously known for the ophiolite. The vast majority of the Semail boninites, however, belong to the Boninitic Alley unit and occur as discontinuous accumulations up to 2 km thick at the top of the ophiolite sequence and constitute ∼15 vol % of the upper crust. The new map provides a basis for targeted exploration of the gold-bearing VMS deposits hosted by these boninites. The thickest boninite accumulations occur in the Fizh block, where magma ascent occurred along crustal-scale faults that are connected to shear zones in the underlying mantle rocks, which in turn are associated with economic chromitite deposits. Locating major boninite feeder zones may thus be an indirect means to explore for chromitites in the underlying mantle.

Pyroxenes constitute an important component in mafic igneous and metamorphic rocks. They often possess a prismatic habit, and their long axis, the crystallographic c axis, helps define a lineation in a textured rock. Anisotropy of magnetic susceptibility (AMS) serves as a fabric indicator in igneous and metamorphic rocks. If a rock's AMS is carried by pyroxenes, it can be related to their crystallographic preferred orientation and degree of alignment. This requires knowing the intrinsic AMS of pyroxene single crystals. This study provides a comprehensive low‐field and high‐field AMS investigation of chemically diverse orthopyroxene and clinopyroxene crystals in relation to crystal structure, chemical composition, oxidation state of Fe, and the possible presence of ferromagnetic inclusions. The paramagnetic anisotropy, extracted from high‐field data, shows clear relationships to crystallographic directions and Fe concentration both in clinopyroxene and orthopyroxene. In the diopside‐augite series, the intermediate susceptibility is parallel to b, and the maximum is at 45° to the c axis. In aegirine, the intermediate axis remains parallel to b, while the maximum susceptibility is parallel to c. The AMS of spodumene depends on Fe concentration. In enstatite, the maximum susceptibility aligns with c and the minimum with b, and in the case of hypersthene, the maximum susceptibility is normal to the exsolution lamellae. Magnetite inclusions within augite possess a ferromagnetic anisotropy with consistent orientation of the principal susceptibilities, which dominates the low‐field anisotropy. These results provide better understanding of magnetic anisotropy in pyroxenes and form a solid basis for interpretation of magnetic fabrics in pyroxene‐bearing rocks.

Anisotropy of magnetic susceptibility (AMS) is often used as a proxy for mineral fabric in deformed rocks. To do so quantitatively, it is necessary to quantify the intrinsic magnetic anisotropy of single crystals of rock-forming minerals. Amphiboles are common in mafic igneous and metamorphic rocks and often define rock texture due to their general prismatic crystal habits. Amphiboles may dominate the magnetic anisotropy in intermediate to felsic igneous rocks and in some metamorphic rock types, because they have a high Fe concentration and they can develop a strong crystallographic preferred orientation. In this study, the AMS is characterized in 28 single crystals and 1 crystal aggregate of compositionally diverse clino- and ortho-amphiboles. High-field methods were used to isolate the paramagnetic component of the anisotropy, which is unaffected by ferromagnetic inclusions that often occur in amphibole crystals. Laue imaging, laser ablation-inductively coupled plasma-mass spectrometry, and Mössbauer spectroscopy were performed to relate the magnetic anisotropy to crystal structure and Fe concentration. The minimum susceptibility is parallel to the crystallographic a*-axis and the maximum susceptibility is generally parallel to the crystallographic b-axis in tremolite, actinolite, and hornblende. Gedrite has its minimum susceptibility along the a-axis, and maximum susceptibility aligned with c. In richterite, however, the intermediate susceptibility is parallel to the b-axis and the minimum and maximum susceptibility directions are distributed in the a-c plane. The degree of anisotropy, k′, increases generally with Fe concentration, following a linear trend: k′ = 1.61 × 10⁻⁹ Fe – 1.17 × 10⁻⁹ m³/kg. Additionally, it may depend on the Fe²⁺/Fe³⁺ ratio. For most samples, the degree of anisotropy increases by a factor of approximately 8 upon cooling from room temperature to 77 K. Ferroactinolite, one pargasite crystal and riebeckite show a larger increase, which is related to the onset of local ferromagnetic (s.l.) interactions below about 100 K. This comprehensive data set increases our understanding of the magnetic structure of amphiboles, and it is central to interpreting magnetic fabrics of rocks whose AMS is controlled by amphibole minerals.