Cubic crystal system
CUBIC CRYSTAL SYSTEM
a1=a2=a3
α1=α2=α3=90°
It is also called the isometric crystal system. It is the most complex in terms of symmetry, but has the simplest coordinate system – three equal crystallographic axes perpendicular to one another.
CRYSTAL FORMS:
HALITE, NaCl – crystallises in the form of a hexahedron, a cube. It is usually colourless or white, but it can be differently coloured depending on impurities and defects in the structure.
Halite has great health value for the human body. We consume it every day as table salt in the household.
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FLUORITE, CaF2 – in nature it most commonly occurs in the form of a cube or octahedron, but it can crystallise in combinations of different forms of the cubic system.
It can be differently coloured, and individual crystals may shimmer in all shades of blue, purple, green and orange-red.
It is most impressive when illuminated with ultraviolet light, because then it shows the effect of fluorescence – it glows with excited light.
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DIAMOND, C – often crystallises in the form of an octahedron.
It is the hardest mineral in nature, and because of its hardness it is used as a tool for cutting, drilling, grinding and polishing.
In jewellery, it is often cut in the brilliant form, which maximally emphasises its high lustre and dispersion of light – “fire”, making it an extremely valuable and popular gemstone.
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GALENA, PbS – often occurs in forms of the hexahedron, although combinations of forms can be numerous.
It cleaves along cube faces, so when broken, the surface of the crystal consists of many tiny “steps”. It represents the most important source of lead and is often silver-bearing.
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PYRITE, FeS2 – crystallises in the form of a hexahedron or typically in the form of a pentagonal dodecahedron, the so-called pyritohedron.
It is golden-yellow in colour, which makes it easy to confuse with gold, “fool’s gold”.
The name pyrite comes from the Greek word πυρίτης, pyrites, meaning “of fire”, because of the sparks that appear when it is struck with a steel object.
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GARNET – a group of minerals from the nesosilicate group.
They can crystallise in various holohedral forms of the cubic system, often as a rhombic dodecahedron or deltoidal icositetrahedron.
Because of its deep colour, especially red, purplish-red, orange or green, it is a popular gemstone.
In geology it is important for interpreting the genesis of metamorphic rocks, geothermobarometry.
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SPHALERITE, ZnS – often crystallises in the form of a tetrahedron.
It has a very high lustre and can be differently coloured: yellow, orange, reddish, greenish, red or brown. Shiny black varieties are rich in iron.
It is the most important ore of zinc.
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