Topaz

The mineral topaz, also known by the misleading trade names Finder's diamond, Killiecrankie diamond, Mogok diamond, Saxon diamond and Slave diamond, is a frequently occurring island silicate with the chemical composition Al2^[6][(F,OH)_2|SiO4]. The fluorine (F) and hydroxide (OH) ions indicated in the round brackets may each represent each other in the formula (substitution, diadochy), but are always in the same quantitative ratio to the other constituents of the mineral. An F-free OH analogue is known as a synthetic compound.

Topaz crystallizes in the orthorhombic crystal system and develops mostly well formed and partly very extensive crystals with short to long prismatic or columnar habit and glass-like luster on the surfaces. In pure form it is colourless and transparent. Through multiple light refraction due to lattice defects or polycrystalline formation, however, it can also appear white and take on a yellow, pinkish red to brownish red, violet, light blue and light green colour through foreign admixtures, whereby the transparency decreases accordingly.

With a Mohs hardness of 8, topaz belongs to the hard minerals and serves as a reference value on the scale according to Friedrich Mohs, which goes up to 10 (diamond). Similar to diamond, however, it is also brittle and after the base surface (001) very easy to cleave with perfectly smooth fracture surfaces, which is often already indicated by cleavage cracks in the stone. Irregularly fractured topazes have uneven to faintly conchoidal fracture surfaces.

Etymology and history

According to the Roman naturalist Pliny the Elder, the name topaz comes from the island of Topazos (probably St. John's Island), located in the Red Sea. In fact, however, it was not topaz that was mined there, but olivine, which was long confused with topaz. Another explanation traces the name back to the Sanskrit word tapas, which means "fire" or "glow".

In 1740, a topaz known as the Braganza diamond was placed in the Portuguese crown in the belief that it was a genuine diamond.

Classification

Already in the meanwhile outdated, but still common 8th edition of the mineral systematics according to Strunz the topaz belonged to the mineral class of the "silicates and germanates" and there to the division of the "island silicates with anions foreign to the tetrahedron (Neso-subsilicates)", where it formed as eponym the "topaz group" with the system no. VIII/B.02 and the further members andalusite, boromullite, kanonaite, krieselite, kyanite, mullite, sillimanite and yoderite.

The 9th edition of Strunz's mineral systematics, valid since 2001 and used by the International Mineralogical Association (IMA), also classifies topaz in the division of "isosilicates (nesosilicates)". However, this is further subdivided according to the crystal structure, so that the mineral is to be found according to its structure in the subdivision "island silicates with additional anions; cations in [4]er, [5]er and/or only [6]er coordination", where it forms only together with krieselite the "topaz group" with the system no. 9.AF.35.

The system of minerals according to Dana, which is mainly used in English-speaking countries, also classifies topaz in the class of "silicates" and there in the division of "insular silicates: SiO4 groups and O, OH, F and H2O". Here it is to be found as eponym of the "topaz group" with the system no. 52.03.01 and the further member krieselite within the subdivision of the "island silicates: SiO4 groups and O, OH, F and H2O with cations only in [6] coordination".