What molecule am I?
Topaz is an aluminosilicate with varying amounts of fluoride and hydroxide ions. The 3-D image (shown) is a portion of its crystal unit cell in which the small gray spheres are aluminum, the small black sphere is silicon, the large red spheres are oxygen, and the large blue spheres can be fluorine or hydroxide. A naturally occurring gemstone, topaz is the birthstone for November.
Pure crystalline topaz is colorless; but impurities in the natural stone cause it to appear in a wide range of colors. Common varieties are yellow to brown (iron), blue (chromium or titanium), and pink to red (manganese). Topaz is one of the hardest natural materials, with a rating of 8 on the 1–10 Mohs scale in which diamond is defined as 10.
A very early mention of topaz in the chemical literature appeared in 1881 when geologist Alexis A. Julien at Columbia University (New York City) wrote an extensive treatise on carbon dioxide in the fluid cavities of the mineral. Julien describes the occurrence of CO2 in topaz samples from several locations worldwide; but he does not seem to arrive at any definite conclusions.
More than a century later, M. S. Hampar and J. Zussman at the University of Manchester (UK) described the thermal decomposition of topaz. With the use of X-ray diffraction, thermal analysis, and electron microscopy, they found that at ≈1000 °C topaz begins to form silica (SiO2) and mullite [(Al2O3)3–4(SiO2)2]. The authors noted that the presence of fluoride reduced the decomposition point of the silica–alumina system, which is normally in the range of 1700–1800 °C.
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