They are four important iron ores, magnetite, haematite, limonite and siderite, and one of less but still considerable importance, pyrite or pyrites.
Siderite, or spathic iron ore, FeCO 3, crystallizes in the rhombohedral system and contains 48.28% of iron.
The clayey siderite of the British coal measures is called " clay band," and that containing bituminous matter is called " black band."
About two-thirds of the ore mined is clayey siderite.
The most important British ore deposit is the Lower Cleveland bed of oolitic siderite in the Middle Lias, near Middlesborough.