Rom A mineral is a compound of different elements. There are a number of elements that are common in the earth's crust, and a mineral is a compound of these. Crystals are a type of mineral that has specific physical features, in particular flat faces that meet at regular angles. Crystals are a subset of minerals. A rock is a composite structure, usually with many different types of minerals in it. There are specific minerals that are most common in rocks, but the composition of any given rock will vary.

A polymorph is a mineral that converts from an unstable state to a stable state at a specific temperature. Graphite and diamonds in particular are crystals, formed in high-pressure, high-temperature environments. Both are made from carbon.

b) Diamond has a cubic structure to its molecules, while graphite has a hexagonal structure.

c) The different structures impact their perceived colours. The cubic structure of diamond reflects light in a way that allows diamonds to be clear, and to reflect light. In part, this is because diamonds can exist in very large crystals. Graphite in large crystals is rare, and the much smaller crystals allow graphite to be more of a silver-black colour, because light does not pass through them as easily.

3a) A pseudomorph is a special type of mineral or compound. The pseudomorph has stability like a polymorph, but achieves this stability in a different way. In a pseudomorph, one material is replaced by another, while maintaining the outward shape and structure of the mineral. The properties of the new material are often different than the older material, and the specimen will take on the properties of the new materials.

3b) Malachite is a blackish green colour, while azurite is a bluer colour. Both are formed from copper. The chemical formula for malachite is Cu2CO3(OH)2 . The chemical formula for azurite is Cu3(CO3)2(OH)2.

3c) The colour in each of these derives from the presence of copper. The amount of copper is one variable in the colour, since individual specimens of each will vary in colour. In malachite, the green comes from the greater degree of oxidation in the colour of the copper. Oxidation turns copper green. Lower levels of copper oxidation allows azurite to have a bluer colour.

4.a) Hyalite = green

Calcite = red, blue

Meionite = yellow-red

4b) The process of fluorescence occurs because the mineral has absorbed radiation, and it then emits that radiation. This is why fluorescent minerals change colour when they are subject to ultraviolet light, because the UV light allows for the radiation to be seen better. The light that is being emitted has a longer wavelength than would otherwise be visible, so the UV allows for this light to be seen, giving the impression of the mineral changing colour.

5. The form of the SiO4 silicates is a tetrahedron.

Fill in the following table, describing the structure of various types of silicates:

type of silicate

Structural arrangement of SiO4

mineral example

Neosilicates

Single tetrahedron. The tetrahedrons of SiO4 are unbounded to others; tetrahedral basic ionic units (Amethyst Galleries, 2014).

Zircon (Zirconium Silicate)

Sorosilicates

Double tetrahedron, linked by oxygen (Si2O7). Hourglass-like shape (Amethyst Galleries, 2014).

Bertrandite (Beryllium Silicate Hydroxide)

Cyclosilicates

Ring structure. Ratio of silicon to oxygen is 1:2.

Beryl

Inosilicates

Two types (single chain, double chain). In single chain, the tetrahedrons form infinite chains with other tetrahedrons by sharing two oxygens, with a ratio of silicon to oxygen of 1:3. In double chain, the ratio of silicon to oxygen is 4:11, and the chains form side by side.

Jadeite (Sodium Aluminum Silicate)

Amphiboles

Octahedrons with multiple geometries. Contain much more than SiO4.

Jade

Phyllosilicates

Sheet structure, formed by rings that link to other rings on a two-dimensional plane. Ratio of silicon to oxygen is 1:2:5, depending on the symmetry.

Talc, Mica

Tectosilicates

Framework; matrix-like composition of interconnected tetrahedrons in multiple directions. Ratio of silicon to oxygen is 1:2.

When pure silicon and oxygen: quartz.

6. a) An allochromatic mineral is one that has no colour of its own, but which derives colour from its impurities. An idiochromatic mineral is one that derives its colour from being able to absorb certain light rays.

6b) Smithsonite comes in a number of different colours, including green, purple and a light blueish sort of colour.

6c) Cinnabar has a reddish-brown colour. Cinnabar is derived from mercury, or mercury sulphide. Vermillion is the pigment that has traditionally been made from cinnabar, and this a reddish hue.

6d) Inclusions are minerals that...

There are also fluid inclusions. Inclusions alter the colour because they alter the way that light passes through the mineral. Emerald is typically coloured via inclusions. It derives its green colour from the inclusion of chromium or vanadium, which lend the mineral a green colour.
7.a Amethyst: Amethyst has a deep, rich, and distinct purple colour. Its structure appears similar to quartz in that it forms six-sided crystals, and often appears in geode forms. The crystals are generally clear, but when they are exceptionally dark can almost seem opaque.

b. Emerald: When polished, emeralds give off a rich and bright green hue. When they are unpolished, the green colour is equally as compelling and can in some ways be more so due to the opacity, but in its raw form the gem lacks the sheen and lustre that makes it so popular in precious stone jewellery.

c. Topaz: Topaz ranges in colour, but one of the more common is golden. The appearance of topaz can be special because of the way each crystal can exhibit gradations from a white base, becoming increasingly imbued with the yellow-gold hue, until the crystal becomes a deeper darker gold and more transparent.

Essay

Amethyst is a form of quartz which, unlike its pure SiO4 brethren, contains additional minerals or ore that can be called "impurities," but which also impart distinct colours in the exhibition of the crystal. In the case of amethyst, those impurities are generally iron but can also be manganese. After being exposed to solar radiation (or other forms of radiation), the colour purple will form (Byrne & Hilbert, 1997). The type of colour that forms in amethyst is known as a "colour center," as opposed to transition metal impurities, charge transfer, dispersion, and other methods of colour formation in minerals. A colour center is a change in the crystalline structure, sometimes referred to as a "defect," especially when it is compared with its relatively pure cousin quartz ("Gemstone Colours from Defects," n.d.). Other quartz family members that form their hues due to colour centers include smoky quartz and topaz. Each of these quartz family minerals also exhibits gradations in colour, within the same crystal specimen.

Colour gradations are due to factors such as variations in the temperature during radiation. In fact, excessive heat can turn amethyst into citrine, a deep yellow coloured quartz which is practically the opposite end of the colour spectrum from amethyst. The same minerals are present (the SiO4 and the ferric compounds), but the temperature is responsible for altering the expression of colour and the stability of that colour. The presence of iron, plus exposure to heat and irradiation, makes amethyst have a stable colour but the mineral does exhibit a good degree of colour gradation and variation between specimens. Heat is one of the most important elements in determining colour differences in amethyst, but the specific types of ferric compounds and how, when, and the duration of exposure to heat and irradiation also have a bearing on the formation of colour in each crystal formation and between amethysts that come from different regions.

Amethyst is an oxide mineral, its composition being silicon dioxide, which has the molecular formula of SiO2. It is a crystal, and a quartz. It is rhombohedral in nature, meaning that its crystals are in the shape of a rhombus.

Amethyst is typically used as a gemstone, and has been used that way for centuries. The amethyst stone also has connotations with preventing drunkenness, and as such has been used on drinking vessels in some cultures. Its primary use, however, is decorative in the form of jewellery. Its purple colour also lent it royal connotations in the past. It is the birthstone for February, which creates some demand. There are few commercial uses besides gem usage. Amethyst was once a rarer gem, and held more value, but in recent times has fallen from the list of top gemstones as it has become more common.

There are substantial amethyst deposits in the far south of Brazil, which is the world's leading producer. Uruguay, Zambia, Austria and Russia are all major world producers of amethyst. There are no official catalogues of statistics regarding overall global amethyst production, and the quality of amethyst can be variable -- the biggest producers do not necessarily produce the best quality gemstones. For the most part, gemstone production is clustered globally, so that regions producing a lot of amethyst also produce emerald, rubies and other major gemstones, these areas having been subject to the types of geologic conditions that bring about quartzes.

References

Amethyst Galleries (2014). The silicate class. Retrieved online: http://www.galleries.com/silicates

"Silicate Structures," (n.d.). Retrieved online: http://www.geo.umass.edu/courses/geo311/systematic%20min.pdf

Byrne, A. & Hilbert, D.R. (1997). Readings on Colour. MIT Press.

"Gemstone Colours…