By Professor Dr. Dr. h. c. Alfred Rittmann (auth.)

This ebook represents the result of a long examine which Professor ALFRED RITTMANN all started a few thirty years in the past. the connection among the chemical and mineralogical composition of igneous rocks is demonstrated so far as is feasible. Petrographers will have fun with that this challenge is intensely complicated, really due to the fact this courting kinds the root of the class and nomenclature of igneous rocks. The creative scheme of calculation of the CI.P.W. norm approach is basically chemical in nature. The compositions of the stoichiometrically perfect "normative minerals" don't correspond to these of the constituent minerals present in rocks. even though the "norm" isn't meant to equivalent the "mode" or genuine mineral composition of a rock, at the very least a qualitative contract among the norm and the mode is fascinating. For a couple of rocks and rock teams, specially the leucocratic and silicic rocks, the deviation of the norm from the mode is usually inside of tolerable limits. For the melanocratic and hugely subsilicic rocks, nonetheless, the CI.P. W. scheme of calculation too usually yields effects which fail to mirror the saw mineral composition. The anomalies produced within the cal­ culation of tremendous subsilicic volcanic rocks have lately been in short mentioned by means of F. CHAVES and H. S. YODER, JR. (1971).

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The amount of augite (450) in the above example is determined by the fact that all available Wo will enter into it. 2. Chemically Simple Minerals and Accessories These minerals can be calculated according to their stoichiometrical formulae for two reasons: either they are very pure and their composition actually corresponds to their formulae so closely that even if they occur IV. Igneous Rock Forming Minerals 36 in considerable amounts no significant error will be committed, or they are present in so small amounts that the error, introduced by neglecting their impurities, will be negligible as far as it concerns the determination of their quantity.

G. melilitites) some Fe3+ may be replaced by AI, corresponding to mixtures of Mto and Hc. In the following calculations, magnetite, ilmenite and hercynite will be given separately. They may be added, if the actual opaque mineral is known to be titanomagnetite, ulvospinel or AI-bearing magnetite. Chromite will not appear as such but as magnetite, because Cr has been added to Fe 3 +. However, a glance at the numbers of atoms will immediately show if sufficient Cr is available to justify the denomination chromite instead of magnetite.

Olivines The composition of rock forming olivines corresponds to the formula (Mg, Fe}zSi0 4 so closely, that olivine may be calculated stoichiometrically. e. if, after the calculation of the pyroxene, Hy* has been left over. Opx is converted into olivine according to the equation: 4 Hy - 1 Q = 3 01. The amount of olivine depends upon the relative amounts of the remaining Hy* and the remaining silica deficiency - AQ*. 25 Hy*, then 01 = 31A Q*I . 75 Hy*. 25 Hy* . 11. Micas Micas, being hydrated silicates, are stable only under "wet" igneous conditions.

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