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In focus: Pyrite |
| Chemical Composition: FeS2 | |
| Group: Sulphide | |
| Colour: Pale brass yellow | |
| Streak: greenish black | |
| Hardness: 6-6.5 | |
| Density: 4.9-5.2 | |
| Luster: Metallic | |
| Cleavage: Poor |
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Pyrite is an iron sulphide with a distinctive a pale brass-yellow colour. The name comes from Greek word "Pyr" meaning 'fire' because it gives off a characteristic spark when it is struck sharply. Pyrite is looks similar to gold and this visual similarity has caused many to mistakenly belive they are about to become rich. For this reason, Pyrite is often referred to as the "Fool's Gold". Ironically, small quantities of actual gold are sometimes found in Pyrite. In fact, auriferous (containing gold) Pyrite is a valuable ore from which gold is mined. |
Pyrite is one of the most widely distributed sulphide minerals, occurring in a variety of environments. It can be found in in igneous rocks as a accessory mineral (mineral that occurs with other minerals), in sedimentary rocks, especially in black shales (fine-grained sedimentary rocks whose original constituents were clays or muds) formed under stagnant, anaerobic conditions, and as nodules. It is also present in metamorphic rocks, especially slates (fine-grained, homogeneous, sedimentary rocks composed of clay or volcanic ash), where it frequently forms well shaped cubic crystals. Pyrite is a common mineral in hydrothermal sulphide veins. It is also a replacement mineral in fossils.
The largest deposits of Pyrite can be found in the USA, Peru, Germany, Russia, Spain, and South Africa.
Interestingly, Pyrite has never been used as a significant source of iron, although Pyrite contains a high percentage of this element. Instead, iron oxides such as hematite and magnetite are the primary iron ores. However, Pyrite is used in the production of sulfur dioxide, e.g. for the paper industry, and in the manufacture of sulphuric acid, though such applications are declining in importance.
Pyrite exposed to the atmosphere reacts with oxygen and water to form sulphuric acid, resulting in acid mine drainage Acid mine drainage (AMD), or acid rock drainage (ARD), refers to the outflow of acidic water from (usually) abandoned metal or coal mines. At the same time pyrite oxidizes either to iron sulphate or to the hydrated oxide known as limonite.
Pyrite is a metamorph of Marcasite, which means that these two minerals share the same chemical structure but differ in their physical structure. as a result it is difficult to distinguish between these two minerals except by examining crystal forms. Macasite is also a slightly lighter colour and has lower density. Marcasite is metastable relative to pyrite and will slowly be changed to pyrite if heated or given enough time.
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