The X-ray powder method for determining minerals has been applied to the important rock-forming mineral group of the pyroxenes in this thesis. The purpose of the investigation was to seek the relationship between the variations of the intensities and positions of the reflections in the powder diagram and the variations in optical properties and chemical composition. For that purpose a number of pyroxenes from different localities were investigated optically, chemically and röntgenographically. The orthopyroxenes. — The optical examination of the orthopyroxenes indicates, that the variation of the optical properties is related to the chemical composition (see Table 1). A difference between plutonic and volcanic orthopyroxenes lies in the size of the optic axial angle 2 V; this appears to be smaller with volcanic orthopyroxenes between En80 and En15 than with plutonic orthopyroxenes (see fig. 5). Further a lamellar structure can be observed in the plutonic orthopyroxenes (see figs. 2 and 3) while the volcanics do not have these lamellae but often show zoning (see fig. 1). It is seen from chemical investigation of the orthopyroxenes that both the plutonic and volcanic orthopyroxenes show about the same variation in Al- and Ca-atomic proportions (see Table 3). It is quite possible that a part of the Ca content of the plutonic orthopyroxenes is present in exsolved diopside lamellae according to the hypothesis of Hess and Philips (1938). The orthopyroxenes can be distinguished from the clinopyroxenes by X-ray powder diagrams on the ground of their characteristic reflection pattern. These powder diagrams are made by means of a camera with a diameter of 9 centimeters and FeK\u03b11 radiation (\u03bb = 1.93597 Å). All powder diagrams of the orthopyroxenes are classed as one group (Group A, see fig. 6). The variation in the relative distance between the reflections 10 31 and 0 6 0 appears to be connected with the chemical composition. These distances are measured very accurately in millimeters by means of a Cambridge Universal Measuring Machine and plotted against the chemical composition in fig. 8. Through the influence of Al and Ca, the Mg content cannot be determined unequivocally from this diagram. Therefore also X-ray powder photographs are made of a mixture of 70 % orthopyroxene and 30 % quartz (see fig. 9). The relative distance between quartz reflection 2 1 3 1 and pyroxene reflection 0 6 0 in millimeters and the distance between quartz reflection (2 0 2 3) (3 0 3 1) and pyroxene reflection 11 3 1 in millimeters depend on the chemical composition which can be seen in figs. 10 and 11, respectively. In fig. 10 two curves are shown, one for orthopyroxenes with an atomic proportion of Al of about 0.010 and one for those with an atomic proportion of Al of about 0.050 in BVI position. In fig. 11 two curves can be seen which are related to orthopyroxenes with an atomic proportion of Ca of about 0.020 and those with an atomic proportion of Ca of about 0.060. One may determine the chemical composition of an orthopyroxene from these three diagrams (figs. 8, 10 and 11). For that purpose one should measure three relative distances. In each diagram one can find two values for the Mg content. From these, a total of six values, three will lie close to each other; the average of these three values indicates the Mg content. With this Mg content one can determine the Al and Ca contents in the diagrams. This röntgenographic method meets with difficulties when there do not occur certain proportions of Al and Ca in the orthopyroxene. Then there may be present two groups of three Mg's which lie close together (see Table 9). In such cases of doubt one must use the optical method to determine the Mg content. By substitution of Fe for Mg, Nz changes strongly, the unit cell dimensions do not, however, and neither do the relative distances. The Al and Ca contents then may be determined by the röntgenographic method. By substitution of Al and Ca for Mg, the unit cell dimensions change strongly and with them the relative distances between the reflections, which are very sensitive. The variation in the relative distance between the reflections mentioned has been explained by means of a crystal model of enstatite (see figs. 12 and 13). This variation results from the substitution of Fe, Al and Ca for Mg and of Al for Si. The substitution of Fe for Mg increases the unit cell dimensions only slightly so that the shape of the unit cell also changes little. The substitution of Ca for Mg has a great influence on the a- and the c dimension, which both become much greater. The substitution of Al for Mg and of Al for Si strongly decreases the b dimension. These changes in the unit cell occur because all substituting ions have a different ionic radius from Mg and moreover because in the structure of enstatite two kinds of Mg ions occur with altogether different positions and which are linked with the tetrahedra in very different ways. Since the relative distance in millimeters between certain reflections depends on the camera and radiation used, in Tables 7a, 7b and 7c these distances are stated for a few types of camera and radiation. In addition the differences between the lattice spacings of these reflections are given in Ångström units. The clinopyroxenes. — In this thesis the optical investigation on clinopyroxenes consists of a description of the specimens, both macroscopieally and microscopically and a determination of 2 V and Z \u039b c. For a few clinopyroxenes the values of Nz and Nx have also been determined. The described clinopyroxenes are subdivided in a number of groups; this classification is based upon the chemical composition (see p. 224). It turned out that the optical properties of the röntgenographically investigated clinopyroxenes do not differ much from the data mentioned in the literature about this group of minerals (see fig. 20 and Table 10). The chemical investigation is restricted to the analysis of a few clinopyroxenes; the results are stated in Table 11. On the basis of difference in position and intensity of certain reflections in the X-ray powder diagrams a classification in four groups has been established for the clinopyroxenes. Group B 1 (figs. 21 and 23) The group includes, hedenbergite, diopside, augite and diallage. Group B 2 (figs. 21 and 23) Pigeonite belongs to this group. Group B 3 (figs. 21 and 22) This group includes, aegirite and jadeite. Group B 4 (figs. 21 and 22) Spodumene belongs to this group. No sharp limits can be drawn between these groups and transitions may exist between some of these groups, as between groups B 1 and B 2 and also between groups B 1 and B 3. Through lack of clinoenstatite and ferrosilite samples we could not check whether any more groups may be distinguished. Of each of these groups the principal features are discussed on p. 245. Each group has its own characteristic reflection pattern; the similarity between these patterns, however, is great enough to conclude that all the investigated clinopyroxenes have a similar structure. The grouping of the X-ray powder diagrams agrees in the main with the classification of the pyroxenes according to the chemical composition. The chemical composition of the different clinopyroxenes of the groups B 1 and B 2 may be determined by a combined optical and röntgenographic investigation. This combination is necessary because the substitution of Fe for Mg has practically no influence on the dimensions of the unit cell, but it does have on the refractive indices. On the other hand the substitution of Ca for Mg strongly influences the shape of the unit cell. For the different clinopyroxenes of groups B1 and B 2 the variation of the relative distance in millimeters between the reflections 2 2 0 and 2 2 1, the reflections 2 2 1 and 3 1 0 and the reflections 1 3 1 and 2 2 1 is plotted against the chemical composition in figs. 25 and 26. From these diagrams one may determine the chemical composition by measuring the relative distances mentioned, on the X-ray powder diagrams. In figs. 27, 28 and 29 the relation between the chemical composition and the difference between the lattice spacings of the reflections in question in Å can be seen. Further Tables 16a, 16b and 16c indicate the distances between these reflections for a few types of camera and radiation. The X-ray powder diagrams of the alkali pyroxenes can be distinguished from those of the other pyroxenes, while they also show great mutual differences. It may be noted, however, that transitions between these pyroxenes always are possible. The powder diagram of spodumene has its own character, so that this pyroxene can be distinguished very simply from the other pyroxenes by the röntgenographic method. The X-ray investigation on clinopyroxenes is not yet completed, because much can still be done, for instance in the jadeite-diopside-aegirite field.