A recent thermodynamic model for the Na–Ca clinoamphiboles in the system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–O (NCFMASHO), is improved, and extended to include cummingtonite–grunerite and the orthoamphiboles, anthophyllite and gedrite. The clinoamphibole model in NCMASH is adopted, but the extension into the FeO- and Fe2O3-bearing systems is revised to provide thermodynamic consistency and better agreement with natural assemblage data. The new model involves order–disorder of Fe–Mg between the M2, M13 and M4 sites in the amphibole structure, calibrated using the experimental data on site distributions in cummingtonite–grunerite. In the independent set of end-members used to represent the thermodynamics, grunerite (rather than ferroactinolite) is used for FeO, with two ordered Fe–Mg end-members, and magnesioriebeckite (rather than ferritschermakite) is used for Fe2O3. Natural assemblage data for coexisting clinoamphiboles are used to constrain the interaction energies between the various amphibole end-members. For orthamphibole, the assumption is made that the site distributions and the non-ideal formulation is the same as for clinoamphibole. The data set end-members anthophyllite, ferroanthophyllite and gedrite, are used; for the others, they are based on the clinoamphibole end-members, with the necessary adjustments to their enthalpies constrained by natural assemblage data for coexisting clino- and orthoamphiboles. The efficacy of the models is illustrated with P–T grids and various pseudosections, with a particular emphasis on the prediction of mineral assemblages in ferric-bearing systems.

A new thermodynamic model for clino- and orthoamphiboles in the system Na2O?CaO?FeO?MgO?Al2O3?SiO2?H2O?O

Applications of the Mössbauer effect to silicate mineralogy—I. Iron silicates of known crystal structure

Thirty samples of amphibole-rich rock from the largest mined vermiculite deposit in the world in the Rainy Creek alkaline-ultramafic complex near Libby, Montana, were collected and analyzed. The amphibole-rich rock is the suspected cause of an abnormally high number of asbestos-related diseases reported in the residents of Libby, and in former mine and mill workers. The amphibole-rich samples were analyzed to determine composition and morphology of both fibrous and non-fibrous amphiboles. Sampling was carried out across the accessible portions of the deposit to obtain as complete a representation of the distribution of amphibole types as possible. The range of amphibole compositions, determined from electron probe microanalysis and X-ray diffraction analysis, indicates the presence of winchite, richterite, tremolite, and magnesioriebeckite. The amphiboles from Vermiculite Mountain show nearly complete solid solution between these end-member compositions. Magnesio-arfvedsonite and edenite may also be present in low abundance. An evaluation of the textural characteristics of the amphiboles shows the material to include a complete range of morphologies from prismatic crystals to asbestiform fibers. The morphology of the majority of the material is intermediate between these two varieties. All of the amphiboles, with the possible exception of magnesioriebeckite, can occur in fibrous or asbestiform habit. The Vermiculite Mountain amphiboles, even when originally present as massive material, can produce abundant, extremely fine fibers by gentle abrasion or crushing.

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The Composition and Morphology of Amphiboles from the Rainy Creek Complex, Near Libby, Montana

It is shown on theoretical grounds that the electronegativity of oxygen in silicates depends upon the Si-0 structure as well as upon the Al/Si ratio in the (Si/Al)O₄ tetrahedra. The more Si-O-Si links in a silicate structure, the larger the electronegativity of the oxygen atoms attached to metallic ions. That is, the effective electronegativity value of oxygen increases stepwise through the sequence: orthosilicate → pyrosilicate ($Si_{2}O_{7}^{6-}$) → metasilicate ($SiO_{3}^{2-}$) → double-chain silicate ($Si_{8}O_{22}^{12-}$) → phyllosilicate ($Si_{5}O_{10}^{6-}$) → tectosilicate. This means that the corresponding hypothetical acids increase in strength through the sequence and that the metal-oxygen bonds in the corresponding salts become more ionic throughout the series. It is demonstrated that the "base exchange" of cations between two oxysalts commonly takes place in such a way that the most electropositive metal combines with the most acidic anion, and the less electropositive metal with the less aci...

Chemical Bonds and Distribution of Cations in Silicates

Intracrystalline distribution in silicates has been a subject of interest among geochemists since the classical work of Goldschmidt (1954).1 The phenomenon of strong Fe2+ -Mg ordering in two of the common rock-forming ferromagnesian silicates, pyroxenes and amphiboles, was discovered in the early sixties through single-crystal X-ray diffraction study. Soon thereafter, the possibility of rapid determination of intracrystalline Fe2+-Mg distributions through the newly discovered technique of Mossbauer resonance spectroscopy attracted the attention of a number of mineralogists and petrologists, because such distributions are related to the thermodynamic mixing properties of the Fe2+ and Mg-end member components, which are necessary for phase equilibrium calculations, and also to the cooling history of rocks. The purpose of this work (Parts I and II) is to critically review and synthesize the various contributions made in this field in the last two decades. We also include some of our own results, which are presented here for the first time. The crystal-chemical details, which are necessary to understand the atomic forces governing the Fe2+-Mg distribution in ferromagnesian silicates, as well as the experimental techniques commonly utilized to determine Fe2+ -Mg distribution, are reviewed in Part I (Ghose). Part II (Ganguly) deals with the thermodynamics and kinetics of Fe2+-Mg order-disorder and their application to geologic problems. Hopefully, this review will stimulate further interest in the subject and provide directions for future work.

John C. Rabbitt

Papers

A New study of the anthophyllite series

The U.S. Geological Survey; 75 years of service to the nation, 1879-1954.