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Amphiboles: Crystal Chemistry

Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada, frank_hawthorne@umanitoba.ca

Roberta Oberti

Istituto di Geoscienze e Georisorse, Consiglio Nazionale delle Ricerche, I-27100 Pavia, Italy, oberti@crystal.unipv.it

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
This chapter provides an introduction to the crystal structure, crystal chemistry and chemical composition of the amphiboles. It is not an exhaustive treatment; it is intended as an introduction to the material discussed in the following chapters. More extensive discussion of many points is given in Hawthorne (1981, 1983a), although all later developments are discussed in some detail here. Published crystal-structure refinements are listed in Appendix I.

	CHEMICAL FORMULA

 
The general chemical formula of the amphiboles can be written as

where

A = Na, K, , Ca, Li;

B = Na, Li, Ca, Mn²⁺, Fe²⁺, Mg;

C = Mg, Fe²⁺, Mn²⁺, Al, Fe³⁺, Mn³⁺, Ti⁴⁺, Li;

T = Si, Al, Ti⁴⁺;

W = (OH), F, Cl, O^2–.

Minor elements such as Zn, Ni²⁺, Co²⁺, V³⁺, Sc, Cr³⁺ and Zr are also observed as C cations. In a mineral group as chemically complicated as the amphiboles, there are many problems connected with (1) the measurement of chemical composition, and (2) calculation of the chemical formula.

	SOME ASPECTS OF CHEMICAL ANALYSIS

 
Chemical composition
The chemical composition of an amphibole is most commonly produced by electron microprobe analysis (EMPA). Instrumentation is very reliable and data reduction (including matrix corrections) are accurate. The main source of error is almost certainly errors in standards, a problem that can be dealt with in a simple but tedious fashion by cross-analyzing all standards. A more serious problem involves the components that cannot be analyzed (or analyzed accurately) by EMPA. Of particular relevance with regard to amphiboles are FeO vs. Fe₂O₃, Li₂O and H₂O, all of which occur commonly as highly variable constituents in amphiboles. We will focus on microbeam methods of analysis where possible as these avoid heterogeneity and contamination problems.

Li₂O and H₂O.
Li can be analyzed by Secondary-Ion Mass . . . [Full Text of this Article]

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