Hutchison / Jeffrey Chemical Methods of Rock Analysis

3. Auflage 2012
ISBN: 978-0-08-098480-3
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 379 Seiten

Reihe: Pergamon Series in Analytical Chemistry

A practical guide to the methods in general use for the complete analysis of silicate rock material and for the determination of all those elements present in major, minor or trace amounts in silicate and other rocks that are routinely, commonly or occasionally determined by methods that are considered to be essentially chemical in character. Such methods include those based upon spectrophotometry, flame emission spectrometry and atomic absorption spectroscopy, as well as gravimetry, titrimetry and the use of ion-selective electrodes. Separation stages are described in full, using precipitation, solvent extraction, distillation, and ion-ex procedures as appropriate. The third edition has been fully revised and updated.

Autoren/Hrsg.

Hutchison, D.

Jeffrey, P G

Leseproben

CHAPTER 1

The Composition of Rock Material

Publisher Summary

This chapter discusses the composition and analysis of rock materials. The ability to undertake a good analysis depends upon the skill of the analyst in making separations and in completing his determinations gravimetrically or titrimetrically, although for manganese, a visual comparison of colors provided an early example of the use of colorimetric method. Some effort by a number of analysts is devoted in devising new schemes of rock analysis based upon spectrophotometric methods with complexometric titration for the determination of calcium and magnesium. The extensive introduction of spectrophotometric methods to silicate rock analysis has been followed by the use of other instrumental methods. Emission (optical) spectrography became a valuable additional technique in many rock analysis laboratories. The recent introductions to the rock analysis laboratory include ß-probe analysis, X-ray fluorescence, inductively coupled plasma emission, direct reading emission, atomic absorption, and atomic fluorescence spectroscopy. The difficulties inherent in collecting and determining all the silica by the classical method can be avoided by using a combined gravimetric and photometric method.

Since early times man has speculated upon the origin and composition of the earth and the great variety of rocks and minerals of which it is composed. For many of the eminent chemists of the eighteenth and nineteenth centuries, the uncharacterised minerals provided the challenge that led to the identification and subsequent isolation of the elements missing from the periodic table. By the end of the nineteenth century, Berzelius, Lothar Meyer, Lawrence Smith and others had laid the foundations of the classical scheme of silicate rock analysis as we know it today and, by the end of the century methods for the determination of all elements present in major amounts had been proposed and evaluated. By 1920, when Washington had issued the third edition of his book, “Manual of the Chemical Analysis of Rocks”(1) and Hillebrand his “The Analysis of Silicate and Carbonate Rocks”(2) (itself a revised and enlarged version of earlier texts), interest in silicate rock analysis had spread to those elements present in only minor amounts. Barium, zirconium, sulphur and chlorine - elements that could all be determined gravimetrically by well-established procedures - were soon added to the list of major components required for a “complete analysis”. Elements such as titanium, vanadium and chromium were recognised as essential components of certain silicates, and new procedures were devised for their determination.

The interest in the minor components of silicate rocks has continued almost without a break to the present day, extending to elements at lower and lower concentration as more and more sensitive techniques have become available.

As with other well-defined applications of classical analytical chemistry, the ability to undertake a good analysis depended upon the skill of the analyst in making his separations and in completing his determinations gravimetrically or titrimetrically, although for manganese a visual comparison of colours provided an early example of the use of a colorimetric method. The general sensitivity of photometric methods, coupled with the improvements in the design of instruments available from about 1950 onwards has resulted in a considerable extension in the use of such methods. At first this extension was limited to the minor and trace components such as titanium, phosphorus and fluorine, but this was later extended also to those elements present in major amounts - silicon, iron and aluminium.

Some considerable effort by a number of analysts has been devoted to devising new schemes of rock analysis based upon spectrophotometric methods, with complexometric titration for the determination of calcium and magnesium. Most of the early schemes suffered from some disadvantage - some of the procedures were analytically unsound, some required the services of an exceptionally skilled analyst, and most if not all were too inflexible to be applied to a wide range of rocks without modification.

Although many chemists regarded these early schemes for “complete analysis” of silicate rocks by spectrophotometry with suspicion, the prospect of obtaining large numbers of such analyses cheaply and rapidly has been welcomed by many geologists. Unfortunately this enthusiasm has not always been accompanied by an understanding of the chemistry (and the errors!) of the processes involved, or of the difficulties in making precise spectrophotometric measurement. The ease with which agreement between duplicate results can be obtained is often taken as an indication of the accuracy of the determination. What is all too often forgotten is that the “rapid” (sometime approximate) analyses, valuable in a series of similar analyses for comparative studies, may later be used by other workers and then given equal weight with analyses obtained by more rigorous methods.

The extensive introduction of spectrophotometric methods to silicate rock analysis was followed by the use of other instrumental methods. Emission (optical) spectrography, became a valuable additional technique in many rock analysis laboratories. In some of these it became the practice to make a qualitative examination of all silicate rocks prior to chemical analysis. This served to identify elements of interest that might subsequently warrant determination by other means. It also gave the analyst a guide to the approximate values that he could expect to find. Emission spectrography has provided the geologist with his dream of large numbers of rapid, cheap analyses - at least for the minor and trace components of silicates. Attempts to use it for obtaining “complete analyses”(3) have not been widely followed.

More recent introductions to the rock analysis laboratory include ß-probe analysis, x-ray fluorescence, inductively coupled plasma emission, direct reading emission, atomic absorption and atomic fluorescence spectroscopy.

One of the most tedious of the determinations in the classical scheme for the complete analysis of silicate rocks is that of the alkali metals, involving a difficult decomposition procedure and a number of subsequent separation stages. It is therefore easy to see why the use of flame photometry was widely adopted, even before the difficulties associated with its use were properly understood and defined.

Gravimetric methods and the separation of the alkali metals soon became unnecessary. The determination of the rarer alkali metals, previously seldom attempted and even more rarely successfully achieved, was now possible on a routine basis. Calcium, strontium and barium, elements with characteristic flame emission, were also determined by this technique, although rather less readily than sodium and potassium, also with less enthusiasm on the part of the rock analyst with the availability of other techniques.

Schemes of rapid rock analyses usually included titrimetric procedures for calcium and magnesium, although difficulties were sometimes encountered in the presence of much manganese. In recent years atomic absorption spectroscopy has provided an acceptable alternative technique for both calcium and magnesium, as well as for manganese, iron and many other elements at major, minor and trace levels - now rivaling spectrophotometry in the extent of its application.

The difficulties inherent in collecting and determining all the silica by the classical method can be avoided by using a combined gravimetric and photometric method.(4) The major part of the silica is recovered following a single dehydration with hydrochloric acid, and is then determined by volatilisation with hydrofluoric acid in the usual way. The minor fraction that escapes collection is determined in the filtrate by a photometric molybdenum-blue method. Atomic absorption spectroscopy may also be used to determine the minor fraction of silicon.

Geochemical Reference Material

Geochemical reference material in the form of distributed samples has been available for so long that it is now difficult to see how rock analysts can manage without them. The need for such material has grown with the availability of it. The number is now so large (Table 1), the compositions so variable and the compositional information so detailed, that no book of this kind can do justice to any kind of evaluation of the data relating to them.

TABLE 1

STANDARD GEOCHEMICAL REFERENCE MATERIALS

The first materials to be available as reference samples were those prepared primarily for industrial and commercial use. Both the National Bureau of Standards (USA) and the Bureau of Analysed Samples (UK) had prepared a number of sample materials of prime interest to the ceramic industry which were also of use to rock analysts and geochemists. These included alkali felspars, clays and refractories. Such samples are still available and are widely used. As befits sample materials prepared primarily for industrial and commercial use, the major interest was in their major constituents and those minor constituents of importance in the use of large tonnages of these materials.

The widespread adoption of instrumental analysis in industry introduced the widespread need for “standard” or “reference” samples by which such methods could be...