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

灰长岩是各种类型的沉积物中普遍存在的一种次要但不可缺少的磁性矿物，研究的主要目的是阐明其热磁性质。在以往的研究中，大多报道辉长岩的居里温度为320℃。然而，一些含灰长岩的沉积物在接近400℃时表现出稳定的残余，为了解决这种不一致性，我们使用台湾南部二心池剖面的天然灰长岩样品进行了岩石磁性和x射线衍射（XRD）研究。XRD结果表明，在340℃的高温下，灰长岩仍能存活，但在超过340℃的高温下，很难用磁检测到灰长岩。我们还在氦气氛中应用了高温XRD方法。在加热到380℃时，证实了灰长铁矿的存在。在加热到340℃以上时，还发现了磁黄铁矿、黄铁矿和马氏铁矿的新形成。在380℃以上时，还观察到赤铁矿。这些结果与加热到400℃后，部分含灰长铁矿的沉积物仍能保持剩余磁化的事实相一致，但部分新形成的矿物可以显示从原始磁性矿物继承的磁化。我们现在正在考虑继续对这个主题进行进一步的调查，因为我们也发现我们的结果强烈依赖于样品的纯度。有必要通过使用几个不同的样品来检查结果的再现性。

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.