Detection of cation ordering in silicate minerals

The decrease of molar extinction coefficients of absorption bands in the polarized spectra of piemontites with increasing Mn3+ ion contents ( 4.4.2), which is contrary to the Beer-Lambert law, eq. (3.7), indicates that Mn3+ ions are not located entirely in one site of the epidote structure ( 4.4.2.1). Most of the man- 

Intracrystalline Fe2+-Mg2+ distributions in natural and synthetic orthopyroxenes have been determined from intensities of absorption bands in their polarized spectra (Goldman and Rossman, 1977a Steffen et al., 1988). Molar extinction coefficients of crystal field bands centred at 10,500 to 11,000 cm-1 and 4,900 to 5,400 cm-1 originating from Fe2+ ions located in pyroxene M2 sites ( 5.5.4) enabled the iron contents to be estimated from the Beer-Lambert law equation, eq. (3.7). 

Other examples discussed later where changes of spectrum profiles across a solid-solution series correlate with cation ordering in the crystal structure include Ni-Mg olivines (Hu et al., 1990), in which Ni2+ ions are strongly ordered in the Ml sites ( 5.4.2.4), and Mg-Fe2+ orthopyroxenes mentioned earlier where strong enrichment of Fe2+ ions occurs in the very distorted M2 sites ( 5.5.4). 

The visible region consists of electromagnetic radiation the wavelengths of which lie between 400 nm and 700 nm (table 3.1). However, the visible region constitutes only a very small portion of the electromagnetic spectrum. Radiation of many wavelengths outside the visible region may be absorbed or scattered by matter without optical effects. 

Pleochroism is the property of a non-cubic crystal whereby certain wavelengths of polarized light are absorbed in different amounts in different crystallographic directions. Such differential absorption may produce different 

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