Rock magnetic properties related to thermal treatment of siderite: Behavior and interpretation

Abstract

Abstract. Detailed analyses of rock magnetic experiments were conducted on the oxidation products of high-purity natural crystalline siderite that were thermally treated in air atmosphere. Susceptibilities increase sharply between 400 ø and 530øC indicative of some new ferrimagnetic mineral phase generation. Both a drop (between 540 ø and 590øC) on the heating cycle and a dramatic increase (from 590øC to 520øC) on the cooling cycle occurred and are well consistent with the characteristic of magnetite. A distinct Hopkinson-type susceptibility peak indicates that hematite is the terminal product if siderite is heated to 700øC over and over. It has been revealed in detail that the original inverse magnetic susceptibility fabric contributed by the crystalline anisotropy of siderite in siderite-bearing specimens is changed to a normal magnetic fabric during incremental heating over 410ø-490øC. This is a result of dominant contributions from the distribution anisotropy of newly transformed ferromagnetic minerals. A strong chemical-viscous remanent magnetization could be produced during siderite oxidation in an external field. Rock magnetic experimental results show that magnetite, maghemite, and hematite are the transformation products of high-temperature oxidation of siderite in air. Maghemite was not completely inverted to hematite even at temperature as high as 690øC during incremental thermal treatments. The mineral transformation processes were confirmed by conventional optical microscopic observation, X-ray diffractometry and M6ssbauer spectroscopic analyses. These results indicate that the rock magnetic methods used here are reliable and highly sensitive in detecting very small magnetic phase changes in rocks. We conclude that these temperaturedependent variations of magnetic properties can be used as criteria for identification of siderite in rocks and sediments. Furthermore, it is clear that great care should be exercised in thermal demagnetization of siderite-bearing rocks in paleomagnetic, magnetic anisotropy, and rock magnetic studies.

High-resolution enviromagnetic records of the last deglaciation from Dali Lake, Inner Mongolia

“…3j, m, p), which indicates the dominance of (titano-)magnetite as the main magnetic carrier. The peak or hump at around 450-500°C may result from the neo-formation of magnetite grains from iron-containing minerals silicates, clays or carbonates (Deng et al, 2001;Zhang et al, 2014;Pan et al, 2000). This is consistent with significant increases of magnetic susceptibility in the cooling curves compared with their heating counterparts (insets in Fig.…”

Magnetostratigraphy of the Xiaolongtan Formation bearing Lufengpithecus keiyuanensis in Yunnan, southwestern China: Constraint on the initiation time of the southern segment of the Xianshuihe–Xiaojiang fault

“…The magnetic susceptibility increases moderately from room temperature to~300°C, which is likely due to the gradual unblocking of fine-grained (near the superparamagnetic/single-domain (SP/SD) boundary) ferrimagnetic particles . The abrupt increase of magnetic susceptibility above~400°C with a hump at 500°C followed by a quick decrease to zero may reflect the existence of siderite (Pan et al, 2000). T c of~580°C demonstrates the presence of magnetite.…”

Changes to magnetic minerals caused by frictional heating during the 1999 Taiwan Chi-Chi earthquake

“…In addition, Hirono et al (2006bHirono et al ( , 2007 reported high magnetic susceptibilities in the black gouge within each of the major fault zones, and proposed two possible explanations: (1) ferrimagnetic grains were crushed into submicrometer size by shearing, while the total concentration of magnetic mineral did not change or (2) magnetite or maghemite grains were newly formed by thermal decomposition of paramagnetic phases such as siderite (Pan et al, 2000).…”