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Please help in identification

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Hi Guys~

Can anyone on the forum help me with the identification of the object shown below? I have found some other ones, all found in limestone rock. To me, it appears that they had fallen into the limestone when it was in a plastic state. Some are buried several inches deep, some almost on the surface. All are very low in magnetism, but all have definite geometric shapes with what appears to be a fusion crust on the surface.





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Limonite cubes.


Limonite is an amorphous mineral that is normally found as a pseudomorph of another mineral, more often than not it is found in the shape of pyrite. Authigenic limonite, which is any limonite that is not a pseudomorph, has no crystalline form and is often found with colloidal silica, organic material, phosphates, and clay materials. A limonite sample can contain varying amounts and combinations of any one of those materials. Limonite occurs commonly in deposits of secondary origin, those formed by the alteration of minerals containing iron. For example, iron rust and bog iron are both considered to be limonite because “limonite” can be used as a generic term for undifferentiated hydrated oxides (often hydrated goethite.) Because it contains 12% to 14% of water by weight, limonite is a hydrate. Most samples are normally light brown to dark brown in color, but it can sometimes be yellowish brown. The streak of authigenic limonite is also variable depending on the content of the sample, but it is most commonly dark brown to brownish yellow. The hardness also varies with the composition of the limonite sample, but the luster of the specimen is almost always earthy. Limonite has been found to have a similar composition to goethite, but the main differences are the color and the fact that limonite is amorphous, and goethite has a crystal structure. In fsome limonite samples, the main constituent is microcrystalline goethite, which may contain ochreous yellow ferric oxide. Other names for limonite are: marsh ore, brown hematite, meadow ore, and brown ironstone.

Her is a sample from Northern Nevada


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