Colour in minerals

Rubidium was discovered as a minor constituent of lepidolite by R. W. Bunsen and G. R. Kirchhoff in 1861 only a few months after their discovery of caesium (1860) in mineral spa waters. These two elements were the first to be discovered by means of the spectroscope, which Bunsen and Kirchhoff had invented the previous year (1859) accordingly their names refer to the colour of the most prominent lines in their spectra (Latin rubidus, deepest red caesius, sky blue). 

The vat dyes section of the Colour Index incorporates a subgroup called solubilised vat dyes. These are sodium salts of sulphuric acid esters of the parent leuco vat dyes, such as Cl Solubilised Vat Blue 6 (1.50). In contrast to the leuco compounds, the vat leuco esters dissolve readily in water at neutral pH. They have relatively low substantivity for cellulose and thus have been used mainly in continuous dyeing and printing. In the presence of an oxidant in mineral acid solution (sodium nitrite and sulphuric acid, for example) the leuco ester is rapidly decomposed and the insoluble vat dye regenerated. Thus application of a vat leuco ester represents a simpler (but more costly and less versatile) alternative to conventional dyeing methods via the alkaline leuco compound. 

Iron oxides in soils have in common that they are of extremely small crystal size and/or low crystal order. This, in combination with their low concentration (only tens g kg in most soils) explains why soil iron oxides have escaped identification for a long time in spite of their obvious existence as seen from the soil colour. In the past, therefore, Fe oxides in surface environments have been considered to be amorphous to X-rays and often called limonite , which mineralogically, is an obsolete term. Furthermore, in order to identify the clay minerals in soils properly, Fe oxides are usually removed before X-ray diffraction methods are applied (Alexander et al., 1939 Mehra Jackson, 1960). 

The kukersites mined from the Estonia deposit have a light chocolate-brown colour in dry condition (10YR 6/4, i.e., light yellowish brown Munsell Color Company 1994), with a conchoidal fracture oblique to the lamination. The density is less than 2.1 g/cm3, but varies depending on the content of clastic material and carbonate minerals versus organic matter. 

Trihydroxo-aquo-diammino-chromium, [Cr(NH3)2H20(0H)3], is formed from tetraquo-diammino-ehromic sulphate, [Cr(NH i)2(H30)4 2 (SO4)3, by decomposing it in aqueous solution with excess of ammonia or pyridine. It crystallises as a violet-coloured powder, insoluble in water but easily soluble in mineral acids.4... 

There has also been a range of developments leading to the removal of acidity, colour and minerals from clear juices such as apple. The product of such a combination of processes can be a clear, colourless carbohydrate syrup that can be used in a variety of food processes. There seems little doubt that the legal status of such a product is not fruit juice yet it is often, optimistically, so called. 

On the other hand, for the detection of resin oils in mineral oils or in the unsaponifiable parts of the above mixtures, use may be made of the colour reactions of section 8 and of determinations of certain physical and chemical characters, such as sp. gr., refractive index and rotation—which are higher with resin oils than with mineral oils—and iodine and Maumeni numbers, which are almost zero with mineral oils (see observations below). 

The substances used as colours in the arts and industries are of diverse nature and origin according to their source and composition they may be divided into two main classes, namely, mineral and organic, these being treated separately below. 

Natural clays are coloured due to pollutions of the clay minerals. After baking the colour is strongly affected by the Ca/Fe ratio. When the baking colour is not white, this is caused by metal ions with d-valence electrons (see glazings in the paragraph Fine ceramics and baking colour in the paragraph Coarse ceramics). 

Bis(octylphenoxy)dithiophosphate is a viscous liquid ranging in colour from light yellow to brown it dissolves well in mineral oils. 

Cations of the lanthanide elements also produce colours in some minerals through intra-electronic transitions within 4/orbitals (Adams, 1965 Bernstein, 1982). Absorption bands are usually sharp and weak, leading to pastel shades. Examples of such coloured minerals are monazite, bastnaesite, rhabdophane, xenotime, gadolinite, and certain apatites, calcites, scheelites and fluorites. As noted earlier, some rare earth-bearing minerals, notably fluorite and monazite, also display the alexandrite effect (Berstein, 1982 Schmetzer et al., 1980). 

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