A lodestone, or loadstone, is a naturally magnetized piece of the mineral magnetite. They are naturally-occurring magnets, which can attract iron. Ancient people first discovered the property of magnetism 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'. 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.
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. Recent research has found that only a variety of magnetite with a particular crystalline structure, a mixture of magnetite and maghemite, has sufficient coercivity to remain magnetized and thus be a permanent magnet. One theory suggests 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.
One of the first references to lodestone's magnetic properties was made by 6th century BCE Greek philosopher Thales of Miletus, whom the ancient Greeks credited with discovering lodestone's attraction to iron and other lodestones. The name "magnet" may come from lodestones found in Magnesia.
The earliest Chinese literary reference to magnetism occurs in a 4th-century BC book called Book of the Devil Valley Master (鬼谷子): "The lodestone makes iron come or it attracts it." The earliest mention of a needle's attraction appears in a work composed between 20 and 100 AD (論衡): "A lodestone attracts a needle." Medieval Chinese navigators were using lodestone compasses by the 12th century.
Based on his discovery of an Olmec hematite artifact in Central America, the American astronomer John Carlson suggests that "the Olmec may have discovered and used the geomagnetic lodestone compass earlier than 1000 BC," thereby predating "the Chinese discovery of the geomagnetic lodestone compass by more than a millennium". Carlson speculates that the Olmecs, for astrological or geomantic purposes, used similar artifacts as a directional device, or to orient their temples, the dwellings of the living, or the interments of the dead.
- Hurlbut, Cornelius Searle; W. Edwin Sharp; Edward Salisbury Dana (1998). Dana's minerals and how to study them. John Wiley and Sons. p. 96. ISBN 0-471-15677-9.
- Bowles, J. F. W.; R. A. Howie; D. J. Vaughan; J. Zussman (2011). Rock-forming Minerals: Non-silicates: oxides, hydroxides and sulphides, Volume 5A, 2nd Ed.. UK: Geological Society of London. p. 403. ISBN 186239315X.
- Du Trémolet de Lacheisserie, Étienne; Damien Gignoux; Michel Schlenker (2005). Magnetism: Fundamentals. Springer. pp. 3–6. ISBN 0-387-22967-1.
- Dill, J. Gregory (Jan–Feb 2003). "Lodestone and Needle: The rise of the magnetic compass". Ocean Navigator online. Navigator Publishing. Retrieved 2011-10-01.
- Merrill, Ronald T.; Michael W. McElhinny; Phillip L. McFadden (1998). The Magnetic Field of the Earth. Academic Press. p. 3. ISBN 0-12-491246-X.
- Needham, Joseph; Colin A. Ronan (1986). The Shorter Science and Civilization in China. UK: Cambridge Univ. Press. pp. 6, 18. ISBN 0-521-31560-3.
- "Lodestone". Mirriam-Webster online dictionary. Mirriam-Webster, Inc. 2009. Retrieved 2009-06-12.
- Wasilewski, Peter; Günther Kletetschka (1999). "Lodestone: Nature's only permanent magnet - What it is and how it gets charged". Geophysical Research Letters 26 (15): 2275–78. Bibcode:1999GeoRL..26.2275W. doi:10.1029/1999GL900496. Retrieved 2009-07-13.
- Brand, Mike; Sharon Neaves; Emily Smith (1995). "Lodestone". Museum of Electricity and Magnetism, Mag Lab U. US National High Magnetic Field Laboratory. Retrieved 2009-06-21.
- Keithley, Joseph F. (1999). The Story of Electrical and Magnetic Measurements: From 500 B.C. to the 1940s. John Wiley and Sons. p. 2. ISBN 0-7803-1193-0.
- Paul Hewitt, "Conceptual Physics". 10th ed. (2006), p.458
- Li Shu-hua, “Origine de la Boussole 11. Aimant et Boussole,” Isis, Vol. 45, No. 2. (Jul., 1954), p.175
- Carlson, John B. (1975) "Lodestone Compass: Chinese or Olmec Primacy?: Multidisciplinary analysis of an Olmec hematite artifact from San Lorenzo, Veracruz, Mexico”, Science, 189 (4205 : 5 September), p. 753-760, DOI 10.1126/science.189.4205.753. p. 753–760
- Lodestone Compass: Chinese or Olmec Primacy?: Multidisciplinary analysis of an Olmec hematite artifact from San Lorenzo, Veracruz, Mexico - Carlson 189 (4205): 753 - Science
|Look up lodestone in Wiktionary, the free dictionary.|