Stable Mineral Assemblages of Igneous Rocks : A Method of Calculation

This book represents the results of a lengthy study which Professor ALFRED RITTMANN began some thirty years ago. The relationship between the chemical and mineralogical composition of igneous rocks is established as far as is possible. Petrographers will appreciate that this problem is extremely complex, particularly since this relationship forms the basis of the classification and nomenclature of igneous rocks. The ingenious scheme of calculation of the CI.P.W. norm system is essentially chemical in nature. The compositions of the stoichiometrically ideal "normative minerals" do not correspond to those of the constituent minerals found in rocks. Although the "norm" is not intended to equal the "mode" or actual mineral composition of a rock, at least a qualitative agreement between the norm and the mode is desirable. For a number of rocks and rock groups, especially the leucocratic and silicic rocks, the deviation of the norm from the mode is generally within tolerable limits. For the melanocratic and highly subsilicic rocks, on the other hand, the CI.P. W. scheme of calculation too often yields results which fail to reflect the observed mineral composition. The anomalies produced in the cal­ culation of extremely subsilicic volcanic rocks have recently been briefly discussed by F. CHAVES and H. S. YODER, JR. (1971).

I. Introduction.- 1. Previous Methods of Calculation.- 2. Nomenclature of Volcanic Rocks.- 3. Systematics of Volcanic Rocks.- 4. Calculation of Stable Mineral Assemblages.- 5. Advantages of Knowing the Stable Mineral Assemblage.- II. Igneous Rock Facies.- 1. Stability of Igneous Minerals.- 2. The Concept of Mineral Facies.- 3. The Volcanic and Subvolcanic Facies.- 4. The Plutonic Facies.- 5. The Carbonatite Facies.- 6. Stable Minerals of the Various Facies.- 7. Mixed Facies.- III. Basic Principles of the Calculation.- 1. Unit of Calculation.- 2. Number of Atoms.- 3. Degree of Oxidation.- 4. Formation of Magnetite.- 5. Saturated Norm.- a) Calculation of Occasional Components.- b) Calculation of Common Accessories.- c) Calculation of Saturated Silicates.- IV. Igneous Rock Forming Minerals.- 1. Introductory Remarks.- 2. Chemically Simple Minerals and Accessories.- 3. Feldspars.- 4. Nepheline and Kalsilite.- 5. Leucite.- 6. Sodalite Group.- 7. Cancrinite.- 8. Scapolites.- 9. Pyroxenes.- 11. Olivines.- 11. Micas.- 12. Amphiboles.- 13. Melilites.- V. Use of the Key Tables.- 1. Steps Indicating Alternatives.- 2. Steps Indicating the Rules for Calculation.- 3. Graphical Solutions.- 4. Calculation of Complex Minerals.- 5. General Scheme of Calculation.- 6. Determination of the Name of the Rock.- VI. Heteromorphism and Systematics.- 1. Some Heteromorphic Igneous Rocks.- 2. Definition of Plutonic Rocks.- 3. Nomenclature of Magma Types.- VII. Comparison between the Stable Mineral Assemblage and the Mode.- 1. Analytical Errors and Omissions.- 2. Secondary Alteration.- 3. Quantitative Determination of the Mode.- 4. Masking Effect.- 5. Optical Determination of the Average Feldspar.- 6. Presence of Metastable Phases.- VIII. Keys for Calculation.- Key 1: Calculation of the Saturated Norm.- Key 2: Calculation of the Mineral Assemblage of the Volcanic Facies.- Key 3: Calculation of the Mineral Assemblage of the “Wet” Subvolcanic-Plutonic Facies.- Key 4: Calculation of the Mineral Assemblage of the “Dry” Subvolcanic-Plutonic Facies.- Key 5: Approximate Calculation of Carbonatites.- Key 6: Determination of Volume Percent and of the Name of the Rock.- Determination of the Rock Name.- IX. Examples.- Example No. 1: “Hawaiian Tholeiitic Basalt”.- Example No. 2: “Alkalic Basalt”.- Example No. 3: “High-alumina Basalt”.- Example No. 4: “Two Pyroxene Andesite”.- Example No. 5: “Cordierite-bearing Rhyolite”.- Example No. 6: “Hyalo-Pantellerite”.- Example No. 7: “Sodalite-nepheline-phonolite”.- Example No. 8: “Leuko-Hornblende-Olivinbasalt”.- Example No. 9: “Leucite-tephrite”.- Example No. 10: “Nephélinite mélilitique à olivine et kalsilite”.- Example No. 11: “Melilitith”.- Example No. 12: “Venanzite”.- X. Appendix.- 1. Key for Calculation of Strongly Altered Rocks (Sil).- 2. Keys for Calculation of the Mineral Assemblages of Ultramafic Rocks.- Key A: Probable Mineral Assemblages of Ultramafic Rocks in the “Dry” Plutonic Facies.- Key B: Probable Mineral Assemblages of Ultramafic Rocks in the “Wet” Plutonic Facies.- Key C: Probable Mineral Assemblages of Ultramafic Rocks in the Eclogite Facies.- 3. Tables 21–26 Containing Symbols, Number of Atoms, Abbreviations, Formulae and Molecular Weights, Saturated Norms of Mineral Components, Equations of Field Boundaries.- Comparison of the C.I.P.W. Norm with the Rittmann Norm..- C.I.P.W. Norms of Femic Minerals.- Average Feldspars.- C.I.P.W. Norm and Degree of Oxidation.- Colour Index.- Conclusions.- ALGOL Program for the Computation of the Rittmann Norm..- Purpose of the Programs.- Source Language.- Inclusion of the Nomograms.- Input Data.- The Meaning of the Identification Number.- Output Data.- Application of the Rittmann Norm Method to Petrological Problems..- The Problem of Classification of Volcanic Rocks.- The Problem of Nomenclature of Igneous Rocks.- Comparison of Two Magmatic Regions.- Comparison of Rock Series of Various Ages within a Single Magmatic Region.- Trends of Magmatic Differentiation Elucitated by the Rittmann Norm System.- Trends of Magma Evolution Displayed with the Aid of the Rittmann Norm System.- Comparison of Volcanic Rocks with Holocrystalline Plutonic Ejecta.- Comparison of Pitchstones and Their Residual Glasses.- References.