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Understanding Our Planet Through Chemistry

Digesting Rocks

But how do you take a rock and purify a portion of it for mass spectrometry, and how do you analyze a rock sample on an instrument that only analyzes liquids? In most cases, before a rock's chemical composition can be determined, it must pass through both a physical and a chemical preparation to free the element(s) of interest from the rock and present them in a dissolved or liquid form.

Initially, fist-size pieces of rock are broken down to pea-size fragments using a crusher with steel jaws. A pulverizer grinds this coarse material into a powder as fine as flour.

Next, the powder is further broken down, or decomposed, by using either an acid treatment or fusion. During this chemical decomposition, the weighed sample of powdered rock releases its elements into solution.

Because most rocks are composed of a combination of many types of minerals, each having different chemical and physical properties, digestion is accomplished by using a combination of acids. Most commonly used is a mixture of hydrofluoric, nitric, hydrochloric, and perchloric acids, which will decompose all but the most resistant minerals. The acids are heated with the sample powder in Teflon containers, on a hot plate, or in a specially designed microwave oven.

In the fusion technique, a powdered inorganic reagent (known as a flux) is mixed with the rock powder and heated above the melting point of the flux; the molten flux then attacks the sample and decomposes it into a uniform melt. The melt may then be poured into a mold and cooled for methods that require a uniform solid such as X-ray fluorescence spectrometry (click here to preview the discussion of XRF from Chapter IIb) or dissolved in a diluted acid to create a liquid solution. The higher temperatures (500 to 1,200°C) and caustic nature of the molten chemicals used for fusions increases the efficiency of the decomposition as compared to acid techniques and renders most minerals soluble. Each form of sample decomposition, acid or flux, has its advantages and disadvantages that must be considered. In addition, the importance of safety and simplicity must not be ignored.

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