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

Detailed analyses of rock magnetic experiments were conducted on the oxidat ion products of high-purity natural crystalline siderite that were thermall y treated in air atmosphere. Susceptibilities increase sharply between 400 degrees and 530 degrees C indicative of some new ferrimagnetic mineral phas e generation. Both a drop (between 540 degrees and 590 degrees C) on the he ating cycle and a dramatic increase (from 590 degrees C to 520 degrees C) o n the cooling cycle occurred and are well consistent with the characteristi c of magnetite. A distinct Hopkinson-type susceptibility peak indicates tha t hematite is the terminal product if siderite is heated to 700 degrees C o ver and over. It has been revealed in detail that the original inverse magn etic susceptibility fabric contributed by the crystalline anisotropy of sid erite in siderite-bearing specimens is changed to a normal magnetic fabric during incremental heating over 410 degrees-490C. This is a result of domin ant contributions from the distribution anisotropy of newly transformed fer romagnetic 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 t ransformation products of high-temperature oxidation of siderite in air. Ma ghemite was not completely inverted to hematite even at temperature as high as 690 degrees C during incremental thermal treatments. The mineral transf ormation processes were confirmed by conventional optical microscopic obser vation, X-ray diffractometry and Mossbauer spectroscopic analyses. These re sults indicate that the rock magnetic methods used here are reliable and hi ghly 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 sedime nts. Furthermore, it is clear that great care should be exercised in therma l demagnetization of siderite-bearing rocks in paleomagnetic, magnetic anis otropy, and rock magnetic studies.