Bismuth oxychloride

Structure

The crystal structure has a tetragonal symmetry and can be thought of as consisting of layers of , and ions, in the order Cl-Bi-O-Bi-Cl-Cl-Bi-O-Bi-Cl. This layered, graphite-like structure results in a relatively low hardness of bismoclite (Mohs 2–2.5) and most other oxohalide minerals.

The structure of bismuth oxychloride can be thought of as consisting of layers of , and ions (in the image Bi = grey, O = red, Cl = green). These ions are ordered as Cl–Bi–O–Bi–Cl–Cl–Bi–O–Bi–Cl, i.e., with alternating anions (, ) and cations (). The layered structure gives rise to the pearlescent properties of this material.

Focusing on the coordination environment of the individual ions, the bismuth centers adopt a distorted square antiprismatic coordination geometry. The Bi atom is coordinated to four Cl atoms, forming one of the square faces, each at a distance of 3.06 Å from Bi, and four O atoms forming the other square face, each at a distance of 2.32 Å from Bi. The O atoms are tetrahedrally coordinated by four Bi atoms.

Synthesis and reactions

BiOCl is formed during the reaction of bismuth chloride with water, i.e. the hydrolysis: : When heated above 600 °C, BiOCl converts to , called the "Arppe compound" which has a complex layer structure.

Use and occurrence

It has been used in cosmetics since the days of ancient Egypt. It is part of the "pearly pigment found in eye shadow, hair sprays, powders, nail polishes, and other cosmetic products". Owing to the plate-like structure of the BiOCl, its suspensions exhibit optical properties like nacre. In cosmetic its name is C.I. 77163.

BiOCl exists in nature as the rare mineral bismoclite, which is part of the matlockite mineral group.

An analogous compound, bismuth oxynitrate, is used as a white pigment.

References

References

1. "Handbook of Mineralogy". Mineralogical Society of America.
2. (1993). "The crystal structure of BiOCl". Zeitschrift für Kristallographie.
3. {{Greenwood&Earnshaw2nd
4. (2000). "Redetermination of the crystal structures of the 'Arppe compound' Bi₂₄O₃₁Cl₁₀ and the isomorphous Bi₂₄O₃₁Br₁₀". Acta Crystallographica Section B.
5. Völz, Hans G. ''et al.'' "Pigments, Inorganic" in ''Ullmann's Encyclopedia of Industrial Chemistry'', 2006 Wiley-VCH, Weinheim. {{doi. 10.1002/14356007.a20_243.pub2.
6. Carrasco, F. 2009. Diccionario de Ingredientes Cosmeticos(Paperback)
7. [https://www.mindat.org/min-683.html Bismoclite on Mindat.org]