Smoky quartz

Smoky quartz is a brownish grey, translucent variety of quartz that ranges in clarity from almost complete transparency to an almost-opaque brownish-gray or black crystals. The color of smoky quartz is produced when natural radiation, emitted from the surrounding rock, activates color centers around aluminum impurities within the crystalline quartz. 

Smoky quartz

General

Category Tectosilicates, quartz variety

Formula SiO2

Strunz classification 04.DA.05

Dana classification 75.01.03.01

Crystal system α-quartz: trigonal trapezohedral class 3 2; β-quartz: hexagonal 622

Space group Trigonal 32

Unit cell a = 4.9133 Å, c = 5.4053 Å; Z=3

Identification

Colour Brown to grey, opaque

Crystal habit 6-sided prism ending in 6-sided pyramid (typical), drusy, fine-grained to microcrystalline, massive

Twinning Common Dauphine law, Brazil law and Japan law

Cleavage {0110} Indistinct

Fracture Conchoidal

Tenacity Brittle

Mohs scale hardness 7 – lower in impure varieties (defining mineral)

Lustre Vitreous – waxy to dull when massive

Streak White

Diaphaneity Transparent to nearly opaque

Specific gravity 2.65; variable 2.59–2.63 in impure varieties

Optical properties Uniaxial (+)

Refractive index nω = 1.543–1.545

nε = 1.552–1.554

Birefringence +0.009 (B-G interval)

Pleochroism weak, from red-brown to green-brown

Melting point 1670 °C (β tridymite) 1713 °C (β cristobalite)

Solubility Insoluble at STP; 1 ppmmass at 400 °C and 500 lb/in2 to 2600 ppmmass at 500 °C and 1500 lb/in2

Other characteristics lattice: hexagonal, Piezoelectric, may be triboluminescent, chiral (hence optically active if not racemic)

Characteristics

Irradiation of colorless quartz with ionizing radiation (e.g., gamma rays) leads to the formation of lattice defects or color centers. In the case of natural irradiation, radioactive minerals or elements in the surrounding rock are responsible for the formation of the color centers.

Color

The color of smoky quartz is due to the presence of color centers, defects in the crystal lattice that are produced by the presence of an ion of To the Al^3+ and another monovalent cation replacing a Si^4+. This substitution is ubiquitous in smoky and colorless quartz crystals, which contain an average of about 500 ppm aluminium. However, this substitution alone does not directly produce the colour; the effect of radiation from neighbouring radioactive minerals is necessary. 

Since, as stated, colorless quartz usually contains sufficient aluminium, it is very easy to transform colorless quartz (rock crystal) into smoky quartz by artificial irradiation. Quartz irradiated in this way will have a colour proportional to the dose received. Quartz treated in this way retains no traces of radioactivity. The combination of irradiation and heat treatment makes it possible to obtain quartz with shades other than smoky. Smoky quartz exhibits dichroism when examined in polarized light, unlike dark-toned quartz coloured by inclusions. Many quartz crystals that would initially pass as colorless show a slight smoky tint when examined against a white background.

Varieties

Morion is a very dark brown to black opaque variety. Morion is the German, Danish, Spanish and Polish synonym for smoky quartz. The name is from a misreading of mormorion in Pliny the Elder.

Cairngorm is a variety of smoky quartz found in the Cairngorm Mountains of Scotland. It usually has a smoky yellow-brown colour, though some specimens are greyish-brown. It is used in Scottish jewellery and as a decoration on kilt pins and the handles of sgianan-dubha (anglicised: sgian-dubhs or skean dhu). The largest known cairngorm crystal is a 23.6 kg (52 lb) specimen kept at Braemar Castle.

Deposits

Smoky quartz is very common, even in the form of good centimeter-sized crystals, which can occasionally even exceed one meter. It is found in different types of deposits, especially in miarolitic cavities of pegmatites. It also appears in the so-called alpine veins, and even in sedimentary environments. In the pegmatite of the Pedra Alta mine, in Goiabeira, Minas Gerais (Brazil), crystals of almost two meters in length have been found, some of which are preserved in the Museum of Natural History in Paris. 

In the Mont Blanc massif, in Chamonix (France), cavities in the gneisses with perfectly transparent smoky quartz crystals are relatively frequent, with the usual prismatic morphology but also with the one known locally as gwindel, with tabular crystals displaced between them in a slightly spiral manner, highly prized by collectors. 

In Spain, smoky quartz is relatively common in miarolitic cavities of some granites. In the Porriño quarries (Pontevedra), where pink granite is mined, smoky quartz crystals up to 40 cm long have been found in the miarolitic cavities, although they usually do not exceed 15 cm. 

Crystals up to decimetric in size have also appeared in various granite quarries in the Cabrera pluton (Madrid) and in the quarries around Sils and Riudarenes (Girona). The crystals from the Sils pond area were already known in the 18th century as "black topazes of Catalonia" or simply "topazes", since the color is not exactly smoky, but brown.

Uses

Smoky quartz is common and was not historically important, but in recent times it has become a popular gemstone, especially for jewelry. Smoky quartz, when perfectly transparent and of a pleasing color tone, can be used as a gem of little value. It has also been used to carve works of art, less frequently than rock crystal, since large, transparent crystals are rarer. Sunglasses, in the form of flat panes of smoky quartz, were used in China in the 12th century. 

Occasionally, it is sold under the name "smoky topaz," which, however, is an unauthorized trade name. 

Smoky quartz can be produced relatively easily from aluminum-containing rock crystals (colorless quartz) through artificial irradiation, which is done in large quantities, especially with quartz from Arkansas (USA). However, smoky quartz produced by irradiation does not pose a danger, as this process does not produce radioactive isotopes. 

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