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A lodestone, or loadstone, is a naturally magnetized piece of the mineral magnetite. They are naturally-occurring magnets, which can attract iron. The property of magnetism was first discovered in antiquity through lodestones. Pieces of lodestone, suspended so they could turn, were the first magnetic compasses,〔 and their importance to early navigation is indicated by the name ''lodestone'', which in Middle English means 'course stone' or 'leading stone', from the now-obsolete meaning of ''lode'' as ‘journey, way’.〔: ‘Literally ‘way-stone’, from the use of the magnet in guiding mariners. ’〕 Lodestone is one of only two minerals that is found naturally magnetized; the other, pyrrhotite, is only weakly magnetic.〔 Magnetite is black or brownish-black, with a metallic luster, and has a Mohs hardness of 5.5–6.5 and a black streak. ==Origin== The process by which lodestone is created has long been an open question in geology. Only a small amount of the magnetite on Earth is found magnetized as lodestone. Ordinary magnetite is attracted to a magnetic field like iron and steel is, but does not tend to become magnetized itself; it has too low a magnetic coercivity (resistance to demagnetization) to stay magnetized for long. Microscopic examination of lodestones has found them to be made of magnetite (Fe3O4) with inclusions of maghemite (cubic Fe2O3), often with impurity metal ions titanium, aluminum, and manganese.〔 This inhomogeneous crystalline structure gives this variety of magnetite sufficient coercivity to remain magnetized and thus be a permanent magnet.〔〔〔 The other question is how lodestones get magnetized. The Earth's magnetic field at 0.5 gauss is too weak to magnetize a lodestone by itself.〔〔 The leading theory is that lodestones are magnetized by the strong magnetic fields surrounding lightning bolts.〔〔〔 This is supported by the observation that they are mostly found near the surface of the Earth, rather than buried at great depth.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Lodestone」の詳細全文を読む スポンサード リンク
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