Amethyst Crystal Structure
by Yumain Hussain
Molecular structure of amethyst consist of chains of silicon tetrahedrons with six-sided pointed long pyramids. Most of the amethyst mined from different places and sources show remarkable differences in the molecular structure.
Amethyst is one of the purple varieties of common quartz. It derives its name from Greek word ‘amethustos’ meaning not drunk. The ancient Greeks believed that drinking from amethyst cup would not intoxicate them. Amethyst looks really stunning when used in jewelry. The royal purple color of Amethyst has added beauty to the crowns of monarchs and queens right from antiquity. Belonging to macro crystalline variety Amethyst individual crystals can be seen through naked eyes. It is related to Citrine which is the yellow variety of Quartz and also to colorless rock crystal.Molecular structure is defined as a way molecules attach to each other. While it is not possible to see the individual atoms, one can see the result of how they attach to each other in whole crystals. Quartz is long crystal with six sides. The external appearance of quartz crystal show regular arrangement of atoms. The angles of the sides of crystal are constant because it is made up same atom that is one atom of silicon for every two atoms of oxygen. The molecular formula is also similar SiO2. Amethyst belongs to silicate mineral category which has structures containing silica tetrahedra (a tetrahedron with silicon at the centre surrounded by 4 oxygen anions).
|Figure: 1. Tetrahedron of Amethyst crystal depicting silica at the centre surrounded by four anions at the corners of the triangular pyramid.Source: http://ist-socrates.berkeley.edu/~ep2/wisc/|
It has trigonal crystal system consisting of chains of silicon tetrahedrons with six-sided pointed long pyramids
|Figure 2: Amethyst Trigonal system with six-sided pointed long pyramids.
Each tetrahedron is linked to other tetrahedral making cross-linked framework.
|Figure 3. Amethyst Crystals showing cross-linked framework
Source: making matter minerals and gemstones.htm
Sometimes it also forms druzies which are crystalline crusts showing pointed termination filling the fissures and fractures within the rocks. It is happens due to the molecular structure. Quartz is generally colorless in its pure form. It is only through impurities it gains color. Amethyst crystal has excessive iron within its structure. These iron atoms are aligned rather than scattered. When light passes through, the crystal appears purple. It is the low level radiation emission from the nearby rocks causes alignment of iron within the Amethyst crystal. It is common to find phantom quartz shapes composed of other minerals visible inside the amethyst crystal. It is basically the earlier stage of the crystal formation. These shapes form a coating on the surface giving a distinct color. The crystal than starts crystallization with a phantom crystal outline within itself.
|Figure 4. Amethyst elongated crystal showing phantom crystal outline
Most of the amethyst mined from different sources and areas show different molecular structure and occurrence. Mexican amethyst from Vera Cruz are in the form of pale, clear and prismatic crystals. They are double terminated and found grown on light colored host rock. These quartz show phantom shapes that is clear quartz interior and amethyst colored exterior. Guerrero mines in Mexico have amethyst stones which are dark purple prismatic crystals radiating outward from a common attachment point. These also show phantom shapes with purple interior and white quartz exterior. Mexican stones are considered valuable than their counterparts found in other areas. Many amethyst crystals also occur as massive or pyramidal druzy clusters. In Brazilian mines the quartz occurs with agate. Formed as druzy crusts these are lined inside volcanic vugs or rock pockets. They are light to medium violet colored only at the top of the crystal. Some of these vugs were formed from trees which withered away during volcano eruptions several million years ago. Maraba in Brazil have large crystals of pale to medium color. Uruguay mines have amethyst crystals occurring as large geodes or gas bubbles and pipes. These are dark to medium colored formed in druzy crusts lining the inside of the geodes and pipes. The crystals are fully colored with agate as base. A significant quantity of large sized amethyst is found in Zambia and South Africa. Though these are not very attractive, the interior color and clarity are excellent making them as an object of desire for jewelry. Some amethyst crystal with parallel growth clusters are found in some places in Italy. These are similar to those found in Mexican mines. German mines have amethyst crystal grown together with colorful agates forming a light colored druzy crust. Ural Mountains in Russia are another zone where very clear and dark variety of amethyst are mined and cut into exclusive gemstones. There are several places in Canada and USA which produce amethyst. These are located inside fissures of rocks mostly as druzy clusters .They display a wide range of colors from bluish violet to brownish purple. For example-Thunder Bay mines of Canada show unusual reddish hematite inclusions just below the quartz surface. Clusters are formed with druzy crusts lining the fissures of metamorphic rock. Amethyst crystals are formed as flat hexagon with alternate purple and clear quartz sections formed by numerous crystals twinning in planes. The Colorado mines have amethyst druzy clusters inside fissures of sandstone. These are found often on top of green fluorite crust. North Carolina mines produce druzy clusters of amethyst with blue-violent tint. No two amethyst crystals are similar in color or composition. One crystal may show pale pink to lavender shade near the base to a deep dark purple at the point where as the other crystal may show uniform purple color from base to top. This happens due to process called crystal zoning which is layers of different colors caused by changing compositions of the fluids that formed the amethyst.
Amethyst is graded according to the high, medium and low categories based on crystal structure and clarity of colour. However it is always rounded cut to highlight the color. Almost all amethyst stones are lab- heated to intensify the dark color. This is a permanent process usually irreversible in normal circumstances. Due to its different crystal shapes and stunning colors Amethyst is an attractive item for the mineral collectors.