CHALYBITE  , a

mineral species consisting of iron carbonate (FeCO3) and forming an important ore of iron . It was early known as spathose iron, spathic iron or steel ore . F . S . Beudant in 1832 gave the name siderose (from viSrlpos, iron), which was modified by W . Haidinger in 1845 to siderite . Chalybite (from xh))ui//, XaXv(3os, Lat. chalybs, steel) is of slightly later date, having been given by E . F . Glocker in 1847 . The name siderite is in common use, but it is open to objection since it had earlier been applied to several other species, and is also now used as a group name for meteoric irons . Chalybite crystallizes in the rhombohedral system and is isomorphous with calcite; like this it possesses perfect cleavages parallel to the faces of the primitive rhombohedron, the angles between which are 730 0' . Crystals are usually rhombohedral in habit, and the primitive rhombohedron r {loos is a common form, the faces being often curved as represented in the figure .

Acute rhombohedra in

combination with the basal pinacoid are also frequent, giving crystals of octahedral aspect . The mineral often occurs in cleavable masses with a coarse or fine granular texture; also in botryoidal or globular (sphaerosiderite) and oolitic forms . When compact and mixed with much clay and sand it constitutes the well-known clay ironstone . Chalybite is usually yellowish-grey or brown in colour; it is translucent and has a vitreous lustre . Hardness 31; sp. gr . 3.8 . The double refraction ((a —E=o•241) is stronger than that of calcite . When pure it contains 48.2 % of iron, but this is often partly replaced isomorphously by man- ganese, magnesium or calcium: the varieties known as oligon-spar or oligonite, sideroplesite and sidero- dote contain these elements respec- tively in large amount . These varieties form a passage to ankerite Crystal of Chalybite . (9.v.) and mesitite, and all are referred to loosely as brown-spar . Chalybite is a common gangue mineral in metalliferous veins, and well-crystallized specimens are found with ores of copper, lead, tin, &c., in Cornwall, the Harz, Saxony and many other places . It also occurs alone as large masses in 'veins and beds in rocks of various kinds .

The clay ironstone so extensively worked as an ore of iron occurs as nodules and beds in the

Coal Measures of England and the United States, and the oolitic iron ore of the Cleveland district in Yorkshire forms beds in the Lias . The mineral is occasionally found as concretionary masses (sphaerosiderite) in cavities in basic igneous rocks such as dolerite . (L . J .