LIMONITE MINERAL FACTS Nevada Turquoise gem stones
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Limonite Mineral Facts:

Chemical Formula: Fe4O3(OH)6
Hydrated oxide of iron, often impure. The analyses of limonite range between wide limits, largely because of the great quantities of impurities mixed with it. The principal impurities are clay, sand, phosphates, silica, manganese compounds and organic matter. Once an important iron ore known in the mining trade "brown hematite" or "bog iron ore".

Colors: Various shades of brown; when earthy, red, yellow or brownish-yellow.

Hardness: 5.0 to 5.5

Density: 3.6 to 4.0

Cleavage: None.

Crystallography: Monoclinic
Limonite occurs in mammillated or stalactitic forms, having a radiating fibrous structure, resembling that of hematite ; also in dull earthy  or massive condition and in concretions and psuedomorphs.

Luster:. Submetallic to earthy, opaque.

Above: Limonite Pseudomorph after Pyrite

Composition, Structure and Associated Minerals:
Limonite is a common ore of iron and is always secondary in its origin, formed through the alteration or solution of previously existing iron minerals. Pyrite is often found altered
to limonite, the crystal form being at times preserved, giving limonite pseudomorphs.
Sulfide veins are often capped near the surface, where oxidation has taken place, by a mass of cellular limonite, which is known as gossan, or an iron hat. Iron minerals existing in the rocks are among the first to undergo decomposition, and their iron content is often dissolved by percolating waters through the agency of the small amounts of carbonic acid which they contain.
The iron is transported as a carbonate by the waters to the surface and then often carried by the streams finally into marshes and stagnant pools. There, under the effect of the evaporation of the water and its consequent loss of the carbonic acid, which served to keep the iron carbonate in solution, and through the agency of the reducing action of carbonaceous matter present, the iron carbonate is changed to an oxide, which separates from the water and collects first as an iridescent scum on the surface of the water, and then later
sinks to the bottom. In this way, under favorable conditions, beds of impure limonite can be formed in the bottom of marshes and bogs.



Such deposits are very common and are known as bog-iron ores, but, because of the foreign materials deposited along with the limonite, are seldom of sufficient purity to be worked. Limonite deposits are also to be found in connection with iron bearing limestones. The iron content of the limestone is gradually dissolved out by circulating waters and transported by them to some favorable spot, and there the iron is slowly redeposited as limonite, gradually replacing the calcium carbonate of the rock. Or, by the gradual weathering and solution of the limestone, its iron content may be left in the form of residual masses of limonite, lying in clay above the limestone formation.  Banded iron deposits are often of considerable size, and because of their greater purity are much more often mined than the bog-iron ores. Limonite is the coloring material of yellow clays and soils, and mixed with fine clay makes what is known as yellow ocher. Limonite is commonly associated in its occurrence with hematite, turgite, pyrolusite, calcite, siderite, etc.

Identification and Diagnostics
In its chemical properties limonite resembles goethite, from which it can be distinguished only with great difficulty except when the latter is
in crystals. From uncrystallized varieties of goethite it can usually be distinguished only by quantitative analysis, although in pure specimens
the streaks are different. Strongly magnetic after heating in reducing flame. Yields much water in closed tube (15 per cent). Characterized
chiefly by its structure and yellow-brown streak.

Occurrence, Localities and Origins:
Limonite is the usual result of the decomposition of other iron-bearing minerals. Consequently, it is often found as pseudomorphs.
The varieties recognized are: compact, the stalactitic and other fibrous forms; ocherous, the brown or yellow earthy, impure variety; bog iron, the porous variety found in marshes, pseudomorphing leaves, etc., and brown clay ironstone, the compact, massive or nodular form. In almost all cases where large beds of the ore occur the material has been deposited from ferriferous water rich in organic substances. One of the commonest types of occurrence is "gossan." In the production of this type of ore, those portions of veins carrying ferruginous minerals are oxidized under the influence of oxygen-bearing waters, forming a layer composed largely of limonite which covers the upper portion of the veins and hides the original vein matter. Gossan ores derived from chalcopyrite and pyrite are common in all regions in which these minerals occur. Another type of limonitic iron ore comprises those found in clays derived from limestones by weathering. In such deposits the ore occurs as nodules and in pockets in the clay. Ores of this type are common in the valleys within the Appalachian Mountains.
Bog iron ores occur in swamps and lakes into which ferruginous solutions drain.
At times, bog iron ores are formed as beds on the floors of some lakes, as in Sweden, where a layer of 7 inches thick accumulated in twenty-six years. The deposition of the iron compound from the stream water flowing into the lake may be caused by minute organisms. Some limonite beds are true chemical precipitates.
 The iron may come from pyrite or iron silicates in the drainage basins of the lakes or swamps. When carried down it is oxidized by the air and sinks to the bottom.
Localities. The mineral occurs abundantly and in many different localities.
Deposits of the bog iron type are to be found chiefly along the Appalachian Mountains, from western Massachusetts as far south as Alabama. These ores have been of considerable importance in western Massachusetts, at Salisbury and Kent, Conn.; at many points in New Jersey, southeastern New York, Pennsylvania, Michigan, Tennessee, Alabama, Ohio, Virginia and Georgia. Limonite deposits of various kinds are found throughout the western part of the US, but as yet they have not been extensively developed. Although containing less iron than hematite, on account of its cheapness, and the ease with which it works in the furnace, limonite was once an important ore of this metal. The earthy varieties are used as pigments in cheap paints.

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