Study of thermomagnetic identification of greigite as an indispensable carrier to natural remanent magnetization of sediments

云英岩是一种常见于各类沉积物中的微量但不可缺少的磁性矿物，其热磁性质是研究云英岩的主要目的。在以前的工作中，大多数报道的钙铝榴石的居里温度为320 ℃。然而，一些含云英岩的沉积物在接近400摄氏度时显示出稳定的剩磁。为了解决这个矛盾，我们对台湾南部二仁溪剖面的天然云英岩样品进行了岩石磁性和X射线衍射（XRD）研究，XRD结果表明云英岩在340 ℃的高温下仍然存在，但在该温度以上很难检测到磁性。我们还应用了高温XRD方法在氦气气氛中。在加热到380 ℃时，证实了云英岩的存在。在340 ℃以上的加热过程中，我们还发现了磁黄铁矿、黄铁矿和白铁矿的新形成。在380 ℃以上也观察到赤铁矿。这些结果与某些含云英岩的沉积物在加热到400 ℃后仍能保持剩磁的事实是一致的。然而，一些新形成的矿物可以显示出原始磁性矿物的磁性继承。我们现在正在考虑继续对这个话题进行进一步的研究，因为我们也发现我们的结果强烈依赖于样品的纯度。有必要通过使用几种不同的样品来检查结果的重现性。

The main target of the research is to elucidate thermomagnetic property of greigite which is generally found as a minor, but indispensable magnetic mineral in various types of sediments. The Curie temperature of greigite has been reported as 320 degrees C in most of the previous works. However some of the greigite-bearing sediments show stable remanence close to 400 degrees C. To solve this inconsistency, we conducted rock magnetic and X-ray diffraction (XRD) studies using natural greigite sample form Erhjen-chi section, southern Taiwan.XRD results show that greigite has survived by heating up to 340 degrees C, but difficult to detect magnetically above that temperature. We also applied a high-temperature XRD method in helium atmosphere. The existence of greigite was confirmed during the heating up to 380 degrees C. We also found neo-formation of pyrrhotite, pyrite, and marcasite by the heating above 340 degrees C. Hematite was also observed above 380 C. These results are consistent with the fact that some of the greigite-bearing sediments can hold remanent magnetization after the heating up to 400 degrees C. Some of neo-formed minerals could show the magnetization inherited from the original magnetic minerals, however.We are now thinking to continue further investigation on this topic, because we also found that our result was strongly dependent upon the purity of the sample. It is necessary to check reproducibility of the results by using several different samples.