Showing papers in "Contributions to Mineralogy and Petrology in 1966"

The iron-titanium oxides of salic volcanic rocks and their associated ferromagnesian silicates

Abstract: The co-existing microphenocrysts of magnetite and ilmenite together with the ferromagnesian silicates in salic volcanic rocks have been analysed with the electron microprobe. The temperatures and oxygen fugacities of the oxide equilibration have been estimated from the curves of Buddington and Lindsley (1965). The co-existing ferromagnesian silicate phenocrysts are either iron-rich olivine, or orthopyroxene or biotite and amphibole; for each of these groups of phenocrysts, the oxide equilibration data are specific and fall on three distinct curves, parallel to experimental oxygen buffer curves. Many of the investigated rhyolites were quenched at temperatures near 900°C, which may represent liquidus temperatures for those with sparse phenocrysts, and also the intrusion temperature of water-undersaturated granites. The composition of the biotite phenocrysts, which are Al-poor and Ti-rich, taken in conjunction with the oxide data, suggest that two Lassen dacites precipitated biotite at a water fugacity of approximately 400 bars. The composition of the later crystallizing ferromagnesian silicates, particularly the pyroxenes which show a wide range in Fe/Mg ratio, is strongly influenced by the prior crystallization of the oxide phases. If the biotite phenocrysts are typical of acid liquids, then they are incapable of generating by fractionation a peraluminous residual liquid; rather they would tend to make a liquid peralkaline.

Ultramafic inclusions in basaltic rocks from Hawaii

Abstract: Ultramafic inclusions and the enclosing basaltic rocks were collected from a number of localities in the Hawaiian Islands; these and other specimens were studied by standard petrographic techniques and with an electron microprobe. Emphasis was on determination of mineral assemblages, mineral compositions, and variations in composition. Sixty-eight inclusions and thirteen basaltic rocks are described, with partial chemical analyses (Ti, Al, Cr, Fe, Mn, Ni, Mg, Ca, Na, K) of olivines, orthopyroxenes, clinopyroxenes, and some feldspars and other minerals. Inclusions range from dunite to anorthosite, and basaltic hosts range from olivine nephelinite to olivine tholeiite. The inclusions are separable into three categories, which correlate with three groups basaltic hosts: Lherzolite inclusions are relatively poor in Fe, and the component minerals have limited ranges of composition. In Hawaii, lherzolite inclusions occur preferentially in extremely undersaturated hosts (olivine nephelinite, nepheline basanite, and ankaratrite). Other varieties of inclusions (dunite, wehrlite, feldspathic peridotite, pyroxenite) are relatively rich in Fe, and the component minerals have wider ranges of composition. These inclusions, together with gabbro, occur preferentially in hosts which are but moderately undersaturated (alkaline olivine basalt, hawaiite, and ankaramite). The sparse inclusions in nearly-saturated basalt (olivine tholeiite) are petrographically distinct from those in the other two categories. These correlations suggest that the inclusions and the enclosing basaltic rocks are genetically related. As the three suites of inclusions differ chemically, mineralogically, physically, and texturally, more than one origin is probable.

The sillimanite-potash feldspar isograd in Western Maine, U.S.A.

Abstract: The techniques of electron probe microanalysis and x-ray diffractometry have been utilized in a study of the sillimanite-potassium feldspar isograd in western Maine. The isograd reaction is theoretically a discontinuous one, calling for the nearly instantaneous loss of muscovite and crystallization of sillimanite and orthoclase, with a small contribution of albite from the pre-existing plagioclase. In fact, muscovite coexists with orthoclase, sillimanite, and plagioclase for a distance of at least seven miles from the isograd (marked by the initial coexistence of orthoclase and sillimanite). In this assemblage, muscovite has an extremely narrow range of composition, about an average of Ms93.5Pg6.5. A possible explanation for the divariant character of the isograd reaction is that, during dehydration, PH2O slowly increased from initial values less than Ptotal + rock strength, under conditions of low permeability, the actual value of PH2O being controlled by a buffer assemblage and local conditions of P and T. An alternative explanation postulates the flattening of thermal gradients following the onset of fractional melting.

Zones of impact metamorphism in the crystalline rocks of the Nördlinger Ries crater

Abstract: Fragments of crystalline basement rocks in suevite, a tuff-like breccia found in the area of the Nordlinger Ries crater, display a continuous gradation of metamorphism. This is interpreted as an effect of the shock waves caused by meteoritic impact, the degree of metamorphism reflecting a radial gradient of pressure and temperature diminishing outward from the point of impact.

Composition of plagioclase and associated minerals in some schists from Vermont, U.S.A., and South Westland, New Zealand, with inferences about the peristerite solvus

Abstract: Two suites of regionally metamorphosed semi-pelitic schists were studied in order to investigate the paragenesis of low temperature plagioclase, from which something may be inferred as to the nature of the peristerite solvus at the temperatures and pressures of formation of these rocks: one from the Gile Mountain Formation in the Hanover and Mt. Cube quadrangles, eastern Vermont, U.S.A.; the other from the Alpine schists along the Haast River, South Westland, New Zealand. Plagioclase, muscovite, biotite, chlorite, carbonate, and garnet compositions were determined with an ARL EMX electron probe microanalyzer. The variation in plagioclase composition with increasing grade in the Vermont schists suggests that the peristerite solvus is asymmetrical with a near vertical albite-rich side and a sloping oligoclase-rich side. The top of the solvus appears to lie slightly above the temperature expressed by the almandine isograd in these schists. The compositions of the coexisting albite and oligoclase in the New Zealand rocks suggest a lower geothermal gradient than in Vermont, creating a different pattern of variation in plagioclase composition. Distribution diagrams of Mg, Ti, and AlIV for muscovite-biotite and chlorite-biotite pairs in both suites of rocks support the hypothesis that the plagioclase relations observed represent equilibrium.

Quantitative analysis of clay minerals and their admixtures

Abstract: Clay minerals are of non uniform composition and particle size. Also their identification and nomenclature have given much confusion. Examples are given. Quantitative analysis of clay minerals from deposits and soils, by X-ray, thermo- and infrared analysis is treated. Examples are given of the large variability in the results; even for X-ray and electron microscope pure- and 85 to 90% chemical pure samples of kaolinite a well defined clay mineral. They are caused mainly by varying conditions of crystal growth from which result differences in isomorphous replacements, structure, ordering and strain. An amorphous weathering substance coating the mineral particles (Beilby layer) upsets in particular quantitative analyses of the finer kinds of clay minerals. Clay minerals from soils have, as compared to those from pure deposits, in particular formed by hydrothermal action, only poor characteristics of small intensity. Examples are given. Quantitative analyses are further hindered by specific characteristics for a certain mineral being masked by those of other minerals which usually occur in the same sample. Examples are given.

Reappraisal of the metamorphic facies concept

Abstract: Eskola's concept of metamorphic facies, now 50 years old, is reappraised in the light of current knowledge and usage among petrologists. Facies should be defined solely in terms of observable geologic criteria. LikeEskola, we continue to view each facies as a set of mineral assemblages that approximate equilibrium within a definite range of temperature; but this is inference and must be excluded from the definition of facies. Mutual boundaries between facies are transitional. Division into subfacies has proved unacceptable to many writers, and has led to confusion in the physical interpretation of metamorphic parageneses. We propose henceforth not to recognize subfacies. Eleven facies are recognized in this paper, and their terminology has been adapted as nearly as possible to current general usage: The pressure-temperature regime of a metamorphic terrane can be discussed in terms of the facies there represented, with detailed gradients inferred from sequences of mineral assemblages in basic, pelitic and calcareous rocks. The general simplicity of metamorphic mineral assemblages and their tendency to recur in space and time suggests a simple relation between such theoretically independent pressure variables as load pressureP l fluid pressureP j partial pressure of water, $$P_{{\text{H}}_{\text{2}} {\text{O}}}$$ . As a first approximation we accept for non-carbonate, hydrous assemblages a model in which $$P_{{\text{H}}_{\text{2}} {\text{O}}}$$ ≃P f ≃P l during mineral reactions.

Major and trace element relationships in granodiorites and granites from Australia and South Africa

Abstract: Averages and concentration ranges for 32 elements are reported for 4 gneissic granites, 20 granodiorites and 8 leucogranites from the Snowy Mountains, N.S.W., Australia and for 17 coarse porphyritic, 9 medium- and 8 finegrained granites from South-Western Cape Province, South Africa. Similar data for 27 elements is reported for 52 K-feldspars, 21 biotites and 4 muscovites from the Cape Granites. Individual determinations, petrographic descriptions and sample localities are given in Kolbe (1965b) and Kolbe and Taylor (1965).

Orthorhombic amphibole in some metamorphic reactions

Abstract: The following reactions involving gedrite were experimentally investigated at 500, 1,000 and 2,000 bars H2O pressure: It was found, that the equilibrium conditions for the reaction (1), that is also the conditions for the beginning of the hornblende-hornfels facies, are 562±8° C at 2,000 bars, 548±12° C at 1,000 bars and 530±10° C at 500 bars H2O pressure, when the atomic ratio Fe2++Fe3+/Fe2++Fe3++Mg is approximately 0.44 in the system. The upper stability of gedrite in the presence of quartz is 755±15° C at 2,000 bars, 704±14° C at 1,000 bars and 663±11° C at 500 bars H2O pressure.

Experiments on preferred orientation of platy minerals

Abstract: Polycrystalline aggregates of phlogopite, talc, and brucite have been grown hydrothermally from their constituent oxides at 300–600° C, 3–5 kb, and compressed 10–30% in short-term experiments (typically 30 minutes). Under hydrostatic conditions, approximately random orientation of crystals results. When the specimen is strained at high temperature, either during or after growth of the minerals, a preferred orientation of basal planes normal to the axis of compression results. Since a similar result is obtained by straining at room temperature after growth of the minerals, the mechanism of orientation is probably mainly mechanical rotation after formation. Microscope examination showed that the preferred orientation is most marked in coarser grains of the aggregates. A second kind of foliation is defined in some specimens by closely spaced, narrow domains within which coarse grains are slightly rotated. These domains occur in conjugate sets symetrically oriented at about 45° to the axis of compression. They are interpreted as shear domains and are geometrically similar to incipient strain-slip cleavage in foliated rocks. The experiments may represent likely behavior in geological situations where the temperature or time scale precludes recrystallization during deformation, but they are probably not directly revelant to cases of axial-plane cleavage where reorientation through an influence of stress or strain during recrystallization is believed to have occurred. No unequivocal indication of the latter process was obtained in the experiments.

The layered basic and ultrabasic intensives of the Giles Complex, Central Australia

Abstract: A series of layered basic and ultrabasic intrusions in Central Australia is similar to other folded basic complexes. The Australian examples were emplaced after a period of highgrade metamorphism and have subsequently been rotated. Chemical investigation of several of the intrusions shows that cryptic variation is present, and in at least two cases intrusions were initially of different bulk composition. It is suggested that the parent magma was partially fractionated prior to final emplacement. The relative deficiency of ultrabasic rocks in this complex (less than 10%) when compared with other complexes is attributed to this process.

Exsolution in ternary feldspars

Abstract: It is shown that the microstructures of the antiperthites of the South Norwegian anorthosites strongly suggest an exsolution origin. In addition to the normal exsolution reactions responsible for most antiperthites, exsolution of potash feldspar in plagioclase may occur by a discontinuous mode of precipitation.

Recent dolomitization of quaternary biocalcarenites from fuerteventura (Canary Islands)

Abstract: Quaternary marine and eolian biocalcarenites in the supratidal breaker and spray zone along the Barlovento coast of the peninsula Jandia, Fuerteventura (Canary Islands) are dolomitized by percolating brines with a high Mg/Ca ratio resulting from evaporation of seawater on the sediment's surface. Only fragments of calcareous algae primarily consisting of high-magnesian calcite are replaced by a cryptocrystalline variety of dolomite. Dolomite also occurs in large euhedral crystals in intraparticle and interparticle pore spaces. In the marine biocalcarenite dolomite has a composition of Ca56Mg44(CO3)2. It is well ordered.

Staurolite-quartzite bands in kyanite quartzite at big rock, Rio Arriba County, New Mexico

Abstract: Concordant “igneous-looking” bands of ferruginous bulk composition occur in a highly aluminous Precambrian metasedimentary series composed predominantly of kyanite quartzite. The bands consist of quartz, staurolite, and magnetite (partially martitized) with accessory amounts of muscovite, chlorite (pseudomorphous after biotite), chloritoid, apatite, and monazite. Quartz is found in three types (I–III) differing in appearance as well as in origin. Staurolite, in combination with quartz-II, shows peculiar radial sieve textures caused by mimetic crystallization after preexisting chloritoid rosettes. The chloritoid has been largely consumed, either by a reaction with hypothetical former kyanite to produce staurolite+ quartz with rock composition unchanged, or, possibly, by metasomatic introduction of oxygen (oxidation) to yield staurolite+quartz+magnetite; the remaining chloritoid, however, persisted in stable equilibrium with the other minerals of the rock. The staurolite quartzites are thus considered to represent original sedimentary bands which have undergone several stages of recrystallisation and (possibly) metasomatic modification during their metamorphic history. Their “igneous aspect” results from annealing crystallisation during a late static, i.e. postdeformational, thermal event of regional metamorphism.

Major element composition of the Loch Coire migmatite complex, Sutherland, Scotland

Abstract: Analyses of Moine metasediments and migmatites in the Loch Coire area of northern Scotland are used to delineate the fields of composition of the major rock groups. An assessment is made of the relative merits of the hypotheses of isochemical metamorphism and metasomatism in the genesis of the migmatites. Within the migmatites the Fe″/Fe‴ ratio is constant, a feature not shown by the metasediments. This is interpreted as evidence of permeation of the migmatites by metasomatising fluids. Granitic sheets which are common in the migmatites have a very variable and predominantly sodium-rich composition more easily reconciled with metasomatism than a simple magmatic or anatectic origin. Attention is also drawn to the sodic nature of the large mass of Loch Coire granite which occurs in the centre of the migmatite area and the possibility is considered that the uprise of sodic magma provided sodium-rich fluids for permeation of the rest of the complex.

On the widespread occurence of oxidized chlorites in the pennine zone of the western Italian Alps

Abstract: Oxidized chlorites, having optical properties quite similar to those of biotite, stilpnomelane or iron-rich vermiculite, are described from the schistes lustres and other similar metamorphites of the Pennine Zone of the western Italian Alps. Optical, x-ray, chemical, DTA and infrared spectroscopic data are furnished. From the results of the chemical study it seems reasonable to conclude that the chlorite has undergone an internal oxidation. Possible petrogenetic implications of its occurenee are discussed in some details.

Sedimentpetrographische und geochemische Untersuchungen an Karbonatgesteinen des Zechsteins

Abstract: Introduction and methods applied: some new DTA-methods (quantitative determination of gypsum, determination of grain-size of carbonates, analysis of small amounts of iron in the dolomite lattice and of magnesium in calcite) and improved DTA-data on some minerals (clay minerals, Fe- and Al-hydroxides, pyrite, quartz, fluorite), occuring in the residue of carbonate rocks and not admitting solution by acids, are given.

Der petrogenetische Werdegang der Klinopyroxene in den tertiären Vulkaniten der Hocheifel

Abstract: The course of crystallization of basalt forming clinopyroxenes in the olivine-basalt-hawaiite-mugearite-trachyte-and basanitoid (= basanite)-series from the Hocheifel area (Western Germany) has been presented by the author in two earlier papers (Hucken-holz, 1965 a, 1965 b). The present paper deals with the evolution of clinopyroxenes from the ankaramites in the same area. The ankaramites are richer in olivines and clinopyroxenes than the normal olivine basalts and may be a product of accumulation of olivines and clinopyroxenes in an olivine basalt magma. The first clinopyroxene formed in the ankaramites is a greenish chromian salite Ca47.1 Mg40.9 Fe12.0 which is preserved in the cores of the phenocrysts. Strongly zoned brown titansalite Ca45 Mg42Fe13 surrounds the chromian salite (HF 5) or appears in independent microphenocrysts (HF53). The groundmass clinopyroxene is titansalite Ca45Mg37Fe18 with a small optic axial angle. Phenocrysts of nickel-rich olivine Fe12–14 and chrome spinell were formed together with chromium clinopyroxene. The same minerals together with orthopyroxenes have been observed in the olivine basalts, hawaiites and basanitoids (= basanites). This paragenesis indicates high pressure and high temperature in the alkali basalts of the Hocheifel during an initial stage of magmatic evolution. With the decrease of pressure, mineral reaction occurred with the alkali basalt melt. The high pressure clinopyroxenes of the ankaramites and basanitoids (= basanites) were changed to a larger extent than the clinopyroxenes of the olivine basalts and hawaiites or were completely destroyed (HF 53) because they are in contact with the hot magma for a longer period of time. This favours the reaction between the solids and the melt, and the high pressure garnet and/or enstatite components in the clinopyroxenes are substituted by Ca-Tschermak’s and titanaugite molecules.

Die Beziehungen zwischen der chemischen Zusammensetzung, Lichtbrechung und Dichte einiger koexistierender Biotite, Muskowite und Chlorite aus granitischen Tiefengesteinen

Abstract: From 19 granitic rock samples the coexisting biotites, muscovites, and chlorites were separated by means of a magnetic separator as well as through hand-picking under the microscope.

Experimental anatexis and genesis of migmatites: A reply

Abstract: Recentlyvon Platen applied the results of his laboratory experiments to a migmatite studied byHarme. Some comments are presented upon that re-interpretation.

Die autometamorphe retrograde Umwandlung von Biotiten in Chlorite und Muskowite in sauren Tiefengesteinen

Abstract: The autometamorphic transformation of iron-rich biotite to chlorite and muscovite with the by-products hematite, ilmenite, sphene, and rutile has been studied in thin-sections of about 50 granitic rocks of Austria and Bavaria. From 19 granitic rock samples the magmatic biotites, their secondary chlorites, and muscovites were separated, purified by hand-picking, and investigated by optical, x-ray, and chemical methods. The silicatic products of biotite transformation were found to be iron-rich chlorites and iron-poor muscovites. The essentiall process of biotite alteration to chlorite is based in the emigration of 9,5–12, 2 Eq.wt.% Si and nearly the complete alcali. This leak is only compensated by the importation of 23,3–27,6 Eq.wt.% H2O+. The chemical changes of biotite alteration to muscovite are characterized by the emigration of 15–20 Eq.wt.% Fe, Mg, Ti and the importation of 9,3–14,2 Al, 4,6–7,1 Si, and 1,3–4,3 Eq.wt% H2O+. Owing to the transformation of biotite unit cell dimensions considerable changes. As a result of biotite replacement by chlorite c 0·sin β increases about 40%. a 0 and b 0 only extends for 0,8%. In comparison with these facts of chloritisation biotite transformation to muscovite was found to reduce a 0 and b 0 (−2,6%) much stronger than c 0·sin β (−0,7%). Modal analysis data of retrograde alterations of biotite and feldspar in the granites were connected with the observed and calculated values of main element changing. In the granitic samples about 50% of iron and titanium have been converted from silicatic into oxidic minerals like hematite, ilmenite, sphene, and rutile during the biotite transformation. 59–92% of H2O+ in these rocks fall to the share of secondary sheet silicate forming during the latest phases of granite crystallisation. The balance account of the various chemical reactions shows for the closed system a deficit of H2O and Al, and a surplus of Fe, Ti, and F. The deficit or surplus of Si, Mg, Mn, Ca, Na, K, and Li in the samples is very different and dependent on the values of biotite chloritisation, feldspar sericitisation, and so forth. At the present time quantitative control of the complete chemical exchanges during the automorphism of these granites is not possible, because the quartz-feldspar-reactions have not yet been investigated.

Sulfatreduzierende Bakterien in Schwefel- und Grulbenwässern Deutschlands und Österreichs

Abstract: The sulphate-reducing bacteria Desulfovibrio desulfuricans have been identified in 25 different samples from diverse German and Austrian sulphur springs as well as in 42 samples of connate water from the Carboniferous of the Ruhr basin. From 33 of these samples, the bacteria were cultivated and tests were conducted to establish their tolerances to the salt content, pH and temperature. Further studies demonstrated a considerable tolerance for certain metal ions. The ecological conditions of Desulfovibrio desulfuricans are described and their importance in the “diagenesis” of connate water and the genesis of the sulphur springs is pointed out.

Untersuchungen zur Diagenese an Kalkkonkretionen und umgebenden Tonschiefern

Abstract: Mit Hilfe der verschiedenen Untersuchungsmethoden. wurde versucht aus einem Vergleich von Kalkknollen und den sie umgebenden Tonschiefern Ruckschlusse auf die diagenetischen Vorgange zu ziehen.

Mineralbestand und Gefügebilder in Faseraggregaten von kohliger Substanz („Thucholith“) aus den Witwatersrand-Konglomeraten

Abstract: Es werden die typischen Faseraggregate von kohliger Substanz beschrieben, die oft an den Liegendkontakten wirtschaftlich wichtiger Gold-Uran-Konglomerate des sudafrikanischen Witwatersrand-Systems in dunnen Lagen auftreten. Der Mineralbestand dieser Aggregate wird erzmikroskopisch untersucht, wobei Gold und Uranpecherz besonders berucksichtigt werden. Die Faserung wird als Wachstum senkrecht zur Wandung schichtparalleler Ablosungsklufte erklart („parallelstengelige Kluftfullung“ im Sinne vonMugge, 1928).

Plagioclase albitization in the lamprophyric and lamproid dykes at the eastern border of the Bohemian Mass

Abstract: The petrographic study of more than 250 dykes of lamprophyric and lamproid rocks of the eastern periphery of the Bohemian Mass shows that the basic plagioclase has been widely albitized, primary albite occurring quite exceptionally only. There are treated and statistically evaluated all the signs proving or supporting the idea of plagioclase albitization, as well as the microscopic signs distinguishing primary albite from the secondary one.

Die Verteilung des Yttriums und der Lanthaniden in einem Olivin-Alkali-Basalt mit Peridotit-Einschlüssen

Abstract: In zwei Peridotit-Einschlussen und dem sie umgebenden Olivin-Alkali-Basalt des Hirzsteins (Nordhessen) wurde die Verteilung des Yttriums und der Lanthaniden quantitativ bestimmt. Die Summe der Lanthaniden (La-Lu, einschlieslich Y) betragt fur die Peridotit-Einschlusse ≈ 10–20 ppm, fur den Olivin-Alkali-Basalt 280–360 ppm. Die relative Verteilung der Lanthaniden in den Peridotit-Knollen, bezogen auf Chondrite, gibt keinen Hinweis auf grose Unterschiede zwischen den leichteren (La-Sm) und schwereren (Eu-Lu) Elementen. Dagegen sind in dem Olivin-Alkali-Basalt die Lanthaniden La-Sm deutlich starker angereichert gegenuber den Elementen Eu-Lu. Dieser Befund steht in Einklang mit der Vorstellung, das die chemisch starker „fraktionierten“ Olivin-Alkali-Basalte eventuell aus weniger stark differenzierten „primaren“ Gesteinen (Eklogit, Granat-Peridotit, tholeiitischer Basalt) entstanden sind. In Ubereinstimmung mit anderen Beobachtungen handelt es sich bei den Peridotit-Einschlussen des untersuchten Vorkommens wahrscheinlich um Gesteinsstucke, die in keiner unmittelbaren genetischen Beziehung zu dem umgebenden Basalt stehen.

Verteilung anorganischer Fremdionen bei der Kristallisation von Alkalichloriden

Abstract: Investigations were made on the distribution of the foreign cations Na+, K+, Rb+, Cs+, NH 4 + , Ca++, Sr++, Ba++ and Pb++ between NaCl-, KCl- and RbCl-single crystals and their aqueous solutions or melts. The growth conditions were kept as constant as possible: large single crystals grew slowly, that is from melts at constant undercooling and from solutions at low and constant supersaturation. The distribution coefficient D=C s /C l was calculated for the different systems (C s =concentration of the foreign substance in the crystal, C l =concentration of the foreign substance in the liquid phase, both in Mol-%). The different values for D can be explained qualitatively with the help of the ionic radii or by comparing the gain of energy with the expenditure of energy, which play an important part during the process of crystallization.

Der Einfluß verschiedener Substitutionen auf die Gitterkonstanten von koexistierenden Biotiten und Muskowiten

Abstract: Structural formulas were calculated for 8 analysed coexisting biotite-muscovitepairs from granitic rocks. Characteristic components of these structural formulas were plotted against the unit cell dimensions of these micas. In accordance with the results of Gower (1957) the substitution of OH− by F− in trioctahedral micas was found to reduce c 0 · β. However, in contrast to his statements, octahedral alumina was found to reduce c 0 · sin β also and to an even larger extent than fluorine. This observation is in agreement with the commonly encountered compacting of the mica structure through replacement of 3 large cations (Fe2+, Mg2+, Mn2+) by two smaller ones (Al3+, Fe3+). Concerning b 0 the combined amounts of octahedral Al and tetrahedral Si cause a reduction of this value, which means indirectly that tetrahedral Al alone would increase b 0. Reductions of b 0 and c 0 · sin β by Al in synthetic micas were also found by Crowley and Roy (1964) and Seifert (1966) for the substitution Mg + Si by two Al in trioctahedral micas. In an earlier paper the author (Muller, 1966) has already shown with the same mica samples that the substitutions of M2+ by Al3+ and of OH− by F− also have the strongest effects of all substitutions on the refractive indices, densities, and specific refractive energies of these micas. Therefore it is evident that the latter effects as well as the changes of lattice constants described in this paper are interrelated.

Experimentelle Magnesitbildung aus Dolomit+MgCl 2

Abstract: Es wurde die Arbeitshypothese entwickelt, das die Mg-Metasomatose, die zur Bildung von Spatmagnesitlagerstatten fuhrte, in chloridischer Losung erfolgte. Deshalb wurde die Reaktion CaMg(CO3)2 (Dolomit)+MgCl2 (gelost) ⇌MgCO3 (Magnesit)+CaCl2 (gelost) experimentell untersucht. Dieses Reaktionsgleichgewicht ist trivariant; die Gleichgewichtstemperatur wird bestimmt durch den Druck der fluiden Phase, durch die Gesamtkonzentration an gelostem Salz und durch das Ca-Mg-Verhaltnis der Losung.