Minerals and Gemstones

Beauty Macroscopic & Microscopic

Index © M. Hewat 1998 Help

Several natural gemstones have the structure of ¶common quartz or silica SiO2. Amethyst colours vary from "Rose-de-France" to the deep purple of "Siberian" amethyst, and other variations of quartz range from golden orange to sherry coloured Citrine, the green of Vermarine, to the pink and smokey quartzes. A fine example of clear rock crystal is the 32 cm diameter "crystal ball" at the Smithsonian Institution . Fire Agate consists of microscopic quartz crystals that contain layers of tiny inclusions of other minerals, which produce a firelike iridescence. Opal contains microscopic amorphous close-packed silica spheres; white light is refracted and split into its constituent colours because the spheres are comparable in size to the wavelength of light.

¶Feldspars or alkali aluminium silicates (Alkali)Al2Si2O8 are the most common minerals in the earth's crust. Feldspar gemstones have romantic names like Moonstone which reflects light in a distinctive shimmering phenomenon known as adularescence. Amazonite is an opaque light green to blue-green feldspar with a distinctive mottled or striated appearance. The structure consists of zeolite-like nets of (Al,Si)O4 corner connected tetrahedrae, with channels occupied by the blue alkali earth metals.

¶Beryl or beryllium aluminum silicate Be3Al2Si6O18 provides many well known gems, including "sea-green" Aquamarine - coloured by the heat of volcanoes, Emerald - green from chromium content, and the most rare of all, Red Beryl from the Wah Wah Mountains of Utah. Here the blue SiO4 corner connected tetrahedrae form alternate layers with green BeO4 tetrahedrae and purple AlO6 octahedrae. Note the zeolite-like channel surrounded by SiO4 tetrahedrae.

¶Chrysoberyl or beryllium aluminate BeAl2O4 is very hard. Alexandrite is the most famous variety; it was found on the birthday of Czar Alexander II of Russia. Cat's eye is translucent yellow-brown-green chrysoberyl containing fine silk-like inclusions that produce a distinctive chatoyance. The green BeO4 tetrahedrae sit at the apex of groups of three edge connected AlO6 octahedrae.

¶Garnet or calcium aluminum silicate Ca3Al2(SiO4)3 occurs in almost all colours except blue, but garnets are generally dark brown or purple-red. (The colours used to illustrate these structures have nothing to do with the colours of the gemstones of course; gemstone colours are mainly due to defects or impurities !). Bohemian Garnet was very popular in the jewelry of the late 19th century. In this structure the dark blue sodium cations are sitting in holes in the corner connected net of AlO6 octahedrae and SiO4 tetrahedrae.

¶Jadeite or sodium aluminum silicate NaAl(Si2O6) may occur with many diffrent colours including white, green, yellow, red, orange, violet, and black jade. It is also very hard and tough. The finest green Imperial Jade, especially prized in China for carving, is very rare and expensive. Again the blue sodium cations sit in holes in an edge connected net of silica and alumina.

¶Olivine or Peridot is magnesium iron silicate (Mg,Fe)2(SiO4) and ranges from yellow to green. The Navajo Reservation in New Mexico and Arizona is a major supplier. Chains of green (Mg,Fe) octahedrae bridged by silica tetrahedra are interleaved between layers of brown (Mg,Fe) octahedrae.

¶Pyrite or fools gold is just iron sulphide FeS2. Pyrite was polished by the Native Americans in the early times and used as a mirror. Today, it is used as an ornamental stone, and sometimes as gemstone. The usual co-ordination polyhedrae drawn here do not emphasise the most interesting aspect of the structure: in fact the sulphur atoms occur as homopolar pairs, and ¶the pyrite structure is best described as a distorted rock salt packing of Fe and S-S pairs.

Pyrite is often wrongly called ¶Marcasite. Although Marcasite has the same FeS2 composition as Pyrite, it is a quite different mineral, and is not suitable for gem use because it powders and eventually disintegrates upon exposure to air. Again, the co-ordination polyhedrae are misleading: ¶the marcasite structure can be described as a distorted BCC packing of Fe and S-S homopolar pairs; this is very clear from the 3D VRML drawing.

¶Ruby is the best known form of the mineral Corundum Al2O3, which is second only to diamond in hardness. Red Rubies from the Mogok area of Burma are the most prized of gemstones, and stones over 2 carats are rare and expensive. Large Blue Sapphires, which are also corundum, are similarly rare and expensive, but smaller stones are more common and popular. Corundum is industrially important as an abrasive, and ruby for fine bearings and lasers ! In corundum, pairs of face-sharing AlO6 octahedrae are linked by edge sharing, stacking along the c-axis. The octahedrae are needed to satisfy the valence of Al+++, and the face, edge or corner sharing arrangements are then required to satisfy the formula Al2O3.

¶Zircon or zirconium silicate Zr(SiO4) was often used as a substitute for diamond in jewlery. Zircons in nature may be blue, yellow, green, red, brown or orange as well as colourless. Reddish-brown zircons are often heat-treated to produce blue or golden yellow gems.

¶Topaz or hydroxy-aluminium silicate Al2(SiO4)(OH)2 is often found in huge crystals. Natural pink and red material is rare, and fine orange-yellow Imperial Topaz is relatively scarce. Common colorless material is irradiated and heat-treated to produce blue gems resembling aquamarine. This is another alumino-silicate with linked SiO4 tetrahedrae and AlO6 octahedrae, but some of the alumina oxygen are actually hydrated. Since hydrogen atoms are difficult to see with x-rays (neutrons are needed), hydrogen atoms were often missed in early mineral structures.

¶Spinel or magnezium aluminium silicate MgAl2O4 has often been confused with ruby and sapphire, and some famous large "rubies," such as the Black Prince's Ruby and the Timur Ruby in the British Crown Jewels are actually spinels ! The spinel structure is similar to that of magnetite Fe3O4, with Mg++ occupying the terahedral holes and Al+++ the octahedral sites.

You might like to look at some pictures of real gemstones to see the macroscopic beauty that is produced by these microscopically beautiful structures.

Now that we have seen some of the wonders of the natural world, lets look at some equally wonderful inventions of man, and in particular the structures of the new high temperature superconductors.

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