Limonite - Brown Hematite, Bog-iron Ore Composition & Structure

Composition. Fe4O3(OH)  or 2FeO3.3H2O = Oxygen 25.7, iron 59.8, water content of limonite varies widely and it is probable that the mineral is essentially an amorphous from of goethite with adsorbed and capillary water.

Crystallization. Noncrystalline.

Structure. In mammillary to stalactitic forms with radiating fibrous structure (Fig. B. pl. II); also concretionary; sometimes earthy.

Physical Properties. H. = 5-5.5 g. = 3.6-6. Submetallic luster. Color dark brown to nearly black. Streak yellowish brown.

Test. Difficulty fusible (5-5.5). Strongly magnetic after heating in R.F. Much water in C.T. (15 per cent). Characterized chiefly by its structure and yellow – brown streak.

Occurrence. Limonite is a common ore of iron and is always secondary in its origin, formend thorough the alteration or solution to limonite, the crystal from being at times preserved, giving limonite pseudomorphs. Sulphide veins are often capped near the surface, where oxidation has taken, place, by a mass of cellular limonite, which is know as gossans, 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 witch the contain. The iron is transported as a carbonate by the waters to the surface and then o often carried by the stream 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, there iron carbonate 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. This separation is also aided by the so-called “iron bacteria” which absorb the iron from the water and later deposit it again as ferric hydroxide. 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 asbog-rin 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 by circulating waters and transported by them to some favorable spot, and there the iron is slowly redeposite 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.

Such deposit are often of considerable size, and because of their greater purity are much more often mined than the bog-iron ores. Deposits of this 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, northwestern Connecticut, southeastern New York, and Tennessee and Georgia. Limonite deposits of various kings are found throughout the western country, but as yet they have not been extensively developed. Oolitic limonites or “minettes” occur in large quantities in Lorraine and Luxemburg and constitiue the most important iron ore deposits of Europe.

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 it’s occurrence with hematite, turgite, pyrolusite, calcite, siderite, etc.

Name.  Derived from the Greek word meaning meadow, in allusion to its occurrence in bogs.

Use. as an iron ore. As a pigment, in yellow ocher.