Siderite - Spathic Iron, Chalybite Composition, Crystallization & Structure
Composition. Ferrous carbonate, FeCO3 = Carbon dioxide 37.9, iron protoxide 62.1, iron = 48.2. Manganese, magnesium and calcium may be present in small amounts.
Crystallization. Hexagonal-rhombohedral. Crystals usually unit rhombohedrons (same as cleavage form). Frequently with curved faces.
Structure. Usually cleavable granular. AT times botryoidal, compact and earthy. More rarely in crystals.
Physical Properties. Perfect rhombohedral cleavage (cleavage angle = 107°). H. = 3.5-4. G. = 4.5-5. Vitreous luster. Color usually light to dark brown. Transparent to opaque.
Tests. Difficultly fusible (4.5-5) Becomes strongly magnetic on heating. Heated in C.T. decomposes and gives a black magnetic residue. Soluble in hydrochloric acid with effervescence; solution gives with potassium ferricyanide a dark blue precipitate (test for ferrous iron). Recognized usually by its color and cleavage.
Varieties.
- Crystallized. In crystals or granular cleavable masses.
- Concretionary. In globular concretions.
- Clay Ironstone. Impure by admixture stratified bodies with coal formations, etc.
- Black-band Ore. An impure stratified deposit of siderite, containing considerable carbonaceous matter. Associated with coal beds.
Occurrence. Found in the dorm of clay ironstone and black-band ore in extensive stratified formations lying in clay or slate rocks and commonly associated with coal measures, These ores are the chief source of iron in Great Britain and are found in Staffordshire, York-shire and Wales. Clay ironstone is also abundant in the coal measures of western Pennsylvania and eastern Ohio, but it is not used to any great extent as an ore. Siderites also formed by the replacement action of ferrous solutions upon limestones. Siderite, in its crystallized form, is a common vein mineral associated with various metallic ores, as silver minerals, pyrite, chalcopyrite, tetrahedrite,, galena, etc.
Name. The original name for the mineral was spherosiderite, given to the concretionary variety and subsequently shortened to siderite to apply to the entire species. Spathic ore is a common name. Chalybite, used by some mineralogists, was derived from the Chalybes, who lived on the Black Sea, and were in ancient times workers in iron
Use. An ore of iron. Important in Great Britain, bur of very subordinate value in the United States.
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