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

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 degrees and 530 degrees C indicative of some new ferrimagnetic mineral phase generation. Both a drop (between 540 degrees and 590 degrees C) on the heating cycle and a dramatic increase (from 590 degrees C to 520 degrees C) 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 700 degrees C 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 410 degrees-490C. 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 690 degrees C during incremental thermal treatments. The mineral transformation processes were confirmed by conventional optical microscopic observation, X-ray diffractometry and Mossbauer 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 temperature-dependent 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.