English: Lodestone from the Cretaceous of Arkansas, USA

Magnetite is Fe3O4, a moderately common iron oxide mineral and the # 2 iron ore mineral. It has a metallic luster, dark gray to black color and streak, a hardness of 6 on the Mohs Hardness Scale, has no cleavage, is moderately heavy for its size, and often forms octahedral crystals. Magnetite is readily identified - a magnet will stick to it, hence the name.

Two pieces of magnetite will not stick to each other, nor will they attract objects such as staples, paper clips, or nails. Lodestone is a "variety" of magnetite that actually does attract iron-bearing objects - lodestone is referred to as a "natural magnet". Published analysis of lodestone samples has shown that this material is actually not a variety of magnetite - it's a rock having intergrown magnetite with polarity and maghemite (= Fe2.67O4, iron oxide with iron vacancies in the molecular structure); hematite may be present (Fe2O3, also iron oxide). Titanomagnetite-bearing rocks may also be lodestones. The degree of magnetization can vary within a sample. Lodestones have a slightly different color than magnetite samples and they only occur at or near the land surface - usually at elevationally high outcrops. Lightning strikes are thought to be the magnetization charging mechanism.

Supposedly, being exposed to a strong magnet / magnetic field for a while can make magnetite act like a lodestone - in other words, artificial lodestone.

This sample comes from central Arkansas' Magnet Cove Complex, a mid-Cretaceous ring dike complex that includes calciocarbonatite, a rare, calcitic igenous rock ("igneous limestone"). Metal nails stick to this specimen and so can be referred to as lodestone. I suspect it is completely natural lodestone.

Geologic unit: Magnet Cove Complex, Arkansas Alkaline Province, late Albian to Cenomanian, mid-Cretaceous, 96-102 Ma

Locality: unrecorded / undisclosed site at or near Magnet Cove, northern Hot Spring County, central Arkansas, USA

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Photo gallery of lodestone: www.mindat.org/gallery.php?min=5805

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Photo gallery of magnetite: www.mindat.org/gallery.php?min=2538

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Locality info.: www.mindat.org/loc-3407.html

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See info. at: en.wikipedia.org/wiki/Lodestone and www.mindat.org/min-2533.html

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Lodestone-specific info. mostly summarized from:

Wasilewski & Ketetschka (1999) - Lodestones: nature's only permanent magnet - What it is and how it gets charged. Geophysical Research Letters 26: 2275-2278.