Journal ArticleDOI
Thermal alteration of the magnetic mineralogy in ferruginous rocks
Ann M. Hirt,Andreas U. Gehring +1 more
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The Northampton ironstone contains the paramagnetic minerals siderite and berthierine and a trace of the ferrimagnetic mineral magnetite as mentioned in this paper, and the magnetic fabric of this sedimentary rock, as defined by the anisotropy of magnetic susceptibility, is controlled by bedding compaction but is inverse (i.e., the maximum susceptibility axes are normal to the bedding plane).Abstract:
The Northampton ironstone contains the paramagnetic minerals siderite and berthierine and a trace of the ferrimagnetic mineral magnetite. The magnetic fabric of this sedimentary rock, as defined by the anisotropy of magnetic susceptibility, is controlled by bedding compaction but is inverse (i.e., the maximum susceptibility axes are normal to the bedding plane). This inverse fabric is attributed to the dominating presence of siderite. The rocks were incrementally heated to 600°C, and the magnetic fabric was measured at room temperature and at near liquid nitrogen temperature after each heating step; cooling the samples in liquid nitrogen enhances the paramagnetic contribution to the magnetic fabric. After heating the samples above 250°C, the room temperature magnetic fabric became normal with the minimum susceptibility axes perpendicular to the bedding plane. The low-temperature magnetic fabric remains inverse until the samples are heated to about 500°C. After heating to higher temperatures a normal fabric was observed at both room temperature and low temperature. Rock magnetic evidence shows that magnetite is created during the heating. The start of mineralogical changes in the rock was detected by electron spin resonance spectroscopy, powder X ray diffraction, and acquisition of isothermal remanence. Berthierine starts to break down at approximately 250°C and is totally oxidized by 500°C. The creation of magnetite in the heating experiments corresponds with this mineralogical change. The change in the room temperature magnetic fabric is associated with the decomposition of berthierine, part of which alters to magnetite. The new magnetite phase has a normal magnetic fabric which overprints the inverse magnetic fabric of siderite.read more
Citations
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Journal ArticleDOI
Rock magnetism and the interpretation of anisotropy of magnetic susceptibility
TL;DR: The conventional rules for the interpretation of magnetic susceptibility (AMS) in terms of microstructure and deformation are subject to numerous exceptions as a result of particular rock magnetic effects.
Journal ArticleDOI
Anisotropy of magnetic susceptibility (AMS): magnetic petrofabrics of deformed rocks
TL;DR: In this paper, the main factors determining the magnetic fabrics of tectonized rocks include mineral-physics properties, crystal symmetry, mineral abundance, mineral-abundances, tectonic symmetry and crystal orientation-distribution, strain or stress, kinematic history and certain tectono-metamorphic processes (e.g., diffusion, crystal plasticity, dynamic recrystallization, particulate flow, neomineralization).
Journal ArticleDOI
Magnetic mineral diagenesis
TL;DR: In this paper, the authors provide a treatment for the full range of diagenetic environments, including oxic, manganiferous, ferruginous, and methanic.
Journal ArticleDOI
Is this magnetic fabric normal? A review and case studies in volcanic formations
TL;DR: In this paper, a review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics, often encountered in magnetite-rich volcanic rocks, are due to abnormal preferred orientations or distributions of magnetite grains.
Journal ArticleDOI
Magnetic anisotropy, rock fabrics and finite strain in deformed sediments of SW Sardinia (Italy)
Catalina M. Lüneburg,Stephan A. Lampert,Hermann Lebit,Ann M. Hirt,Martin Casey,William Lowrie +5 more
TL;DR: In this paper, a comparative study has been made in order to elucidate the AMS-strain relationship by investigating mineral preferred orientations and the responsible microstructural deformation processes.
References
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Journal ArticleDOI
Single‐domain particles in rocks and magnetic fabric analysis
David K. Potter,A. Stephenson +1 more
TL;DR: In this article, it was shown that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro- or ferrimagnetic particles in rocks, in particular foliated rocks can actually be lineated.
Journal ArticleDOI
Bituminous shales and oolitic ironstones as indicators of transgressions and regressions
Anthony Hallam,M. J. Bradshaw +1 more
TL;DR: The evidence from bituminous shales and ironstones supports a Jurassic eustatic model characterised by moderately rapid rises and falls of sea level, separated by longer phases of stillstand.
Journal ArticleDOI
Electron spin resonance in natural kaolinites containing Fe (super 3+) and other transition metal ions
R. E. Meads,P. J. Malden +1 more
TL;DR: In this paper, a number of natural kaolinites from a variety of world sources have been investigated using electron spin resonance at both X-band and Q-band, and the results show systematic differences some of which are related to the crystailinity of the material, some to the presence of transition metal ions other than Fe 3+ and hole-trapping defects.
Journal ArticleDOI
Inverse magnetic fabric in carbonate-bearing rocks
TL;DR: Inverse magnetic fabrics due to magnetite are also encountered in weakly deformed limestones as mentioned in this paper, and a mineralogical model based on the property of single-domain grains to have a zero susceptibility parallel to their long axis is proposed.