185374a0Nature1854710196002063743750028-0836196010.1038/185374a0ukNatureNatureNATUREnatureNature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public./nature/journal/v185/n4710issueJournal homeArchiveCurrent issueAdvance online publicationPrivacy policySubscribeNature Publishing GroupCurrent issue185374a0Determination of Tetrahedral Aluminium in Mica by Infra-Red Absorption Analysis
AU  - LYON, R. J. P.Stanford Research Institute, Menlo Park, California.
AU  - TUDDENHAM, W. M.Kennecott Research Center, Kennecott Copper Corporation, Salk Lake City, Utah. October 1.CONTINUATION of studies1 relating the infra-red absorption spectra and the aluminium substitution for silicon in the chlorite minerals has shown that a similar correlation exists in almost all micas. Regardless of the species under study (that is, muscovite, lepidolite, biotite, phlogopite), the shape of the 9-10[micro] region of the absorption spectra depicts the amount of aluminium substitution for silicon (Y number) in the basal, tetrahedrally co-ordinated level in the mica structure. The micas may be natural or synthetic. Position of the maxima of absorption is again dependent upon the dominant type of ion in the octahedrally co-ordinated level, the maxima shifting to slightly longer wave-length with increasing atomic weight of the ions in this position.The aluminium content and distribution in a mica, therefore, can now be estimated by a rapid physical method. This is in marked contrast to experiences of previous workers2 who have had great difficulty with differential solubility methods in estimating the aluminium in micas and clays.
In the dioctahedral micas, the aluminium in the octahedral level does not influence the shape of the infra-red absorption pattern, although, because of its lighter mass effect, the position of the maxima is shifted to shorter wave-lengths than with iron-and magnesium-bearing trioctahedral analogues. At first thought, it is strange that such a correlation should exist between Y number and absorption spectra, regardless of the mica species present. The tetrahedrally co-ordinated basal level in the mica structure, however, is common to all species ; the ions differentiating the species from one another appearing in the octahedral and interlayer positions.
By the courtesy of people too numerous to mention in this short note, who have kindly sent us samples, we have been enabled to include the infra-red absorption curves of 22 analysed micas (Figs. 1 and 2). Members of the same mica species are listed in the vertical columns of curves, in order of decreasing Y number, so that horizontal comparison between micas with the same degree of aluminium substitution may be readily made. One of the muscovites and three of the biotites are not yet analysed, but have been placed in relative position as deduced from the shape of their curves.
Tig. 1. Centre section (8-5-110) of infra-red absorption curves arranged in order of decreasing aluminium substitution for silicon (Y number). Curves in each species column are displaced vertically. Micas with equivalent Y number may be compared horizontally. Missing number for the muscovite curve is awaiting completion f chemical analysis
Fig. 2. Centre section (8-5-11/[ast]) of infra-red absorption curves for the phlogopite and biotite series. Arrangement as for Fig. 1. Phlogopite curves (a) and (ft) have same Y number but are displaced laterally 0-2A[ast]. Curve (a) is a magnesian phlogopite whereas (6) has a 1/3 iron substitution. Three of the biotites are not analysed at the present time
As with the chlorite series, the pure tetrasilicic end member (F=0) (of at least the muscovite and lepidolite series) has a complex absorption pattern in the 9-1 Opt region. In addition to the normal broad absorption (or doublet) at 10-1-10-4, a pronounced doublet appears between 8-85 and 9-04(. With increasing aluminium substitution, as Y rises from 0-4 towards 0-6, this doublet between 9-8 and 10-3(JL becomes a triplet, with the short wave-length peak of this triplet becoming dominant as Y values increase above 0-9. The doublet at 9(gradually fades out as Y increases towards unity, leaving the simplified, single absorption at 9-7-9-8[ji.
Only one mica sample with Y value greater than 1 -2 could be obtained. It may be that this value of 1 -2 is again as critical in the broad groups of micas as it was in the chlorites1, marking some type of structural change.
Difficulty has been experienced in relating the minerals talc and pyrophyllite to this scheme. Both possess effectively a zero aluminium substitution (F=0), but their absorption spectra, which are very similar in pattern, consist of simple, single absorptions, suggesting a Y=2 situation as in the chlorite series. In addition, margarite, with a F=2, has an unusually complex pattern and is markedly displaced to higher wave-lengths, compared with the sep teamesite (Y =2) of the chlorite study.
It would be expected that the montmorillonite groups would show a similar correlation between Y number and infra-red absorption, although their lower degree of crystallinity would make the individual absorptions less distinct. Investigation of all these points is actively progressing.Tuddenham, , W. M., and Lyon, , R. J. P., Anal. Chem., 31, 377 (1959).ArticleISIYoder, , H. S., in [ldquo]Clays and Clay Minerals[rdquo], Monograph No. 2 (Pergamon Press, New York, 1959).