- © The Mineralogical Society Of America
Classification is always a highly contentious issue, particular when dealing with a group of minerals as structurally and chemically complicated as the amphiboles, and when trying to satisfy the needs and different scientific philosophies of crystallographers, mineralogists, petrologists and geochemists. As a result, the classification of the amphiboles has long been a work in progress. Leake (1968) provided a classification for calcic amphiboles, and this was expanded into the IMA (International Mineralogical Association) classification of Leake (1978). An IMA Subcommittee on Amphibole Classification was formed, and eleven years and approximately 1200 pages of memos later, Leake et al. (1997) reported on the current classification, as modified by Leake et al. (2003) to incorporate new discoveries in amphibole compositions in the intervening years. Here, we will (1) describe the current IMA-approved classification scheme, (2) discuss some shortcomings of the current scheme, and (3) discuss some of the features that may be considered as desirable in any new scheme of amphibole classification that may emerge in the future. We make no apology for dealing with these issues here; if the community who deals with amphiboles is to get the classification that it wants, that community has to be prepared to involve itself in the process of developing such a classification.
THE CURRENT CLASSIFICATION SCHEMES
The first thing to note is that the above title refers to classification schemes. There are two distinct classification schemes:
Hand-specimen and thin-section classification;
Classification by chemical formula.
Virtually all attention has focused on classification (2) and classification (1) seems to have escaped the attention of most critics of amphibole classification. Here, we will describe both schemes which are given by Leake et al. (1997, 2003).
HAND-SPECIMEN (FIELD) CLASSIFICATION OF AMPHIBOLES
To quote from Leake et al. (1997), “for amphiboles of which the general nature only is known, for instance from optical …