Rhombic Pyroxene Research Articles

Peculiarities of recent sedimentogenesis in the eastern mediterranean sea

An investigation of Recent bottom sediments between Cyprus Island and the Syrian seacoast on the 27th cruise of the R/VVityaz (1993) gave comprehensive field data significantly complementing our understanding of the sedimentation process in this part of the Mediterranean. Mineralogical and geochemical indicators testify to different input into sedimentaion of the Syrian and Nile River sources. The Nile River plays the leading role in terrigenous sedimentation in the southeastern Mediterranean, especially in deep-sea areas. In contrast, the contribution of the weathering products of basalts and ophilites of the Syrian drainage area (hornblende, monoclinic and rhombic pyroxenes, olivine, spinel, palagonite, and epidote) are particularly detectable in sediments in the near-coastal zone. During the Late Quaternary, the terrigenous material input was irregular in time and distribution both from the Syrian and Nile sources.

Thermodynamic problems of distribution of cations in superlattices of minerals

Physical methods (infrared spectroscopy, X-ray, Mossbauer spectroscopy) afford exact ratings of ferruginousness of the subphases (sublattices) in the superstructures of minerals. This provides opportunity for studies of the distribution of Fe2+ between the subphases in a mineral or in several coexisting minerals, as influenced by P and T. The distribution analysis, in the case of rhombic pyroxenes and amphiboles of the cummingtonite-grunerite series, shows the fruitfulness of this type of approach. A substantial effect of T on the Fe2+ distribution in superstructures of these minerals appears indicated. Efficient geothermometers and geomanometers may be developed on that basis. On the other hand, a possibility presents itself for appraisal of sequences of the hydration reactions, as influenced by T, other things being equal. Experimental and theoretical studies will let us arrive at a closer appraisal of the effects of T and P on stability of chemical bonds in minerals, on the basis of endocrystalline ion...

Corrections [to “The iron, magnesium, and calcium distribution in coexisting olivines and rhombic pyroxenes of chondrites” by Klaus Keil and Kurt Fredriksson

Journal of Geophysical Research (1896-1977)Volume 70, Issue 2 p. 515-515 Free Access Corrections [to “The iron, magnesium, and calcium distribution in coexisting olivines and rhombic pyroxenes of chondrites” by Klaus Keil and Kurt Fredriksson] Klaus Keil, Klaus KeilSearch for more papers by this authorKurt Fredriksson, Kurt FredrikssonSearch for more papers by this author Klaus Keil, Klaus KeilSearch for more papers by this authorKurt Fredriksson, Kurt FredrikssonSearch for more papers by this author First published: 15 January 1965 https://doi.org/10.1029/JZ070i002p00515AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume70, Issue215 January 1965Pages 515-515 ReferencesRelatedInformation

On the retention of primordial noble gases in the Pesyanoe meteorite

A sample of the Pesyanoe meteorite has been examined by X-ray diffraction microtechnique, in an effort to find minerals of known gas-trapping tendency and thus explain the high helium content of the meteorite. Determinations of enstatite, feldspar, and troilite by earlier workers have been confirmed. The existence of many variant minor pyroxene phases, including clinopyroxene and rhombic pyroxene with stacking faults, shows that the history of the meteorite must have involved several complex heating and cooling cycles. In addition, there exists a variety of Sulfides, which includes not only both magnetic and non-magnetic troilites, daubréelite, and oldhamite, but also a new mineral, MgS, the composition of which has been checked by spectrographic analysis. All the sulfides present show a high degree of crystal imperfection. Vaterite (μ-CaCO 3) has been found, but it is not known whether it is of terrestrial or antecedent origin. The high concentration of dislocations in the sulfides indicates a comparable abundance of other lattice defects. It is suggested that lattice defects of various kinds are the most probable loci of the gas atoms. The gases apparently entered the sulfide crystals during their growth, either in a closed magma chamber or in a hot gas stream containing both noble gases and sulfur compounds. Some circumstantial evidence favours the latter alternative.

Phenocrystic pigeonite from Senganmori, Iino-machi, Fukushima Prefecture

In Senganmori, southeast of Fukushima City, is found volcanic neck which was intruded into the basement consisting of granites, during the period of sedimentation of the Ryozen formation (middle Miocene). Lithologically these volcanic rocks are defined as compact andesitic basalts having scarce phenocrysts. The phenocrysts are diopside, rhombic pyroxene, pigeonite, bytownite, and olivine replaced by cholorite. The groundmass consists of pigeonite, augite, bytownite to labradorite, and brown glass. The mode of occurrence of pigeonite phenocrysts shows the following four types: (I) occurrng as single crystals, (2) forming rims around diopside phenocrysts and intergrowing with the latter, (3) inverted from rhombic pyroxene and (4) surrounding augite and in the intergrowth relation with the latter. The chemical composition of pyroxenes, obtained through measurements of their optical properties, are plotted in Fig. 5. It is understood from the figure that the crystallization course of augite from Senganmori roughly coincides with that of augite in the Skaergaard intrusion and that the augite crystallized out in the stage generally corresponding to pigeonite.

CORONITES FROM INDIA AND THEIR BEARING ON THE ORIGIN OF CORONAS

Detailed petrographic and mineralogic studies of some coronites from India confirm observations of previous workers on similar rocks. The first rhombic pyroxene corona is formed by replacement of olivine, and the second corona of amphibole, amphibole-spinel, and/or garnet develops by replacement of plagioclase. Certain features such as the clouding of plagioclase, pyroxenes, and olivines and the recrystallization of primary minerals ( e.g., pyroxene) and the development of garnet are commonly noted in coronites. Their significance as metamorphic phenomena and their relationship with corona formation is stressed. From a crystallochemical examination of the reacting minerals and the resulting corona minerals, the writer concludes that the first corona of rhombic pyroxene is formed from olivine by rearrangement of the independent SiO 4 tetrahedra of the nesosilicate into chains in the inosilicate rhombic pyroxene. During metamorphism this reaction is favored along the olivine-plagioclase interface where disorder and chemical potential are high and is inhibited along the olivine-pyroxene interface where disorder and chemical potential are low. The reaction is initiated by an intergranular film of water which is effective in breaking bonds in olivine. For formation of the second corona minerals, disruption of the plagioclase structure is essential. Inosilicate amphibole is the simplest structural change from plagioclase and is presumably formed in the presence of lower concentrations of water. The formation of nesosilicate garnet requires complete disruption of plagioclase and a change in the co-ordination of Al +3 , which probably occurs as a result of greater concentrations of water. Biotite, amphibole, amphibole-spinel, and garnet coronas around iron ore and spinel are also metamorphic and are formed in part by replacement of plagioclase. Regional metamorphism at comparatively high temperatures, under water-deficient conditions and with little shearing stress, gives rise to coronas; the degree of development is partly a function of time. Data and observations are given in support of these conclusions.

On extinction angles cΛZ´ of monclinic and rhombic pyroxenes in random sections

In the orthoscopic observation, we usually observe pyroxenes at random orientation, and therefore, we get extinction angles measured on that random section. Calculation of an extinction angle on a given section is necessary, if we observe statistically arbitrary extinction angles of numerous pyroxenes, especialy in groundmass of volcanic rocks, in which both monoclinic and rhombic pyroxenes are present. General formulas for an extinction angle on a given section of pyroxenes are given in this paper. Some optical features commonly described in many textbooks can be explained as special cases of these formulas. Formulas (1), (2) and (3) are for monoclinic pyroxenes (b=Y), (1), (5) and (6) are for rhombic ones. The symbols used here are as follows : c∧Z=δ, c∧Z';=φφ, θx, θy, θz:, are the angles between a normal to a given section and X-, Y-, and Z-axes respectively. If the normal to a given section is plotted on the axial plane ZX, then the angle between this plotted line and the c-axis is ρ, which corresponds to longitude. Also if we plot two angles between c-axis and two optical axes on a given section, then plotted angles are represented by ζ and ζ ', respectively. The results of calculation of φ by given values of c∧Z and 2Ω in the formulas (2) and (6) are shown in Fig. 2 for monoclinic pyroxenes and Fig. 3 for rhombic pyro-xenes. The figures are indicated by stereographic projections of lines of equal extinction angle.

Contributions to the Petrology of Barnavave, Carlingford, I.F.S. 3: On some Hybrids from the E. and S.E. Slopes of Barnavave Mountain

The rocks about to be described are found in various parts of the eastern and south-eastern portions of the eucrite mass which forms the bulk of Barnavave Mountain. They are best displayed, however, between the 700 and 800 foot contours in the area just north of the large N.W. fault which crosses the region.1 The main characters of the normal eucrite are already well known from Sollas's now classical researches on the western side of the mountain.2 The normal eucrite of the eastern part of Barnavave is very similar to that occurring in the west. It is composed essentially of a diallagic pyroxene and a basic plagioclase, with or without a small but variable quantity of rhombic pyroxene (Text-fig. 1A). The plagioclase varies from Ab19An81 to Ab30An70, but the bulk centres round the more acid of these extremes. When subjected to metamorphism by the later acid intrusions, it shows very fine “clouding” and may appear black in hand specimen. The diallagic pyroxene is almost colourless in thin section but has a pale brown colour in thicker pieces. It has a % 1.680 ±.003, γ % 1.703 ±.003, γΛc % 41°, (+) 2V % 63°±4°, dispersion moderate p>v. Achemical analysis will be found on p. 477. In places it has a well defined schiller structure. The rhombic pyroxene (enstatite), when present, is colourless and non-pleochroic in thin section and also shows schiller structure. The accessory constituents include some titaniferous magnetite or ilmenite (normally enclosed in pyroxene). Apatite appears to be negligible in quantity.

An Association of Anthophyllite and Enstatite

Of the naturally occurring forms of (Mg, Fe) SiO3, only two are of any great petrological importance. Rhombic pyroxenes of the enstatite-hypersthene series are common in many igneous rocks and in the metamorphic derivatives of these and of sediments. Members of the anthophyllite group of rhombic amphiboles are of much more restricted occurrence, being practically confined to crystalline schists and metasomatic products. The stability relationships of these forms and of the rarer monoclinic clinoenstatite and kupfferite were originally investigated by Allen, Wright, and Clement. Enstatite was obtained by crystallizing a glass below 1100° C., and anthophyllite by very rapid cooling from a melt at about 1600° C. Some uncertainty still exists, but it is clear that at least under normal pressure clinoenstatite is the only stable form.

Notes on the extrusive igneous rocks of the Dundee District

I.—Introduction. The Lower Old Red Sandstone lavaform rocks on the north side of the River Tay, east of Dundee, have previously received a somewhat brief and passing reference. Sir A. Geikie merely mentions the fact that they form the north-easterly extension of the great lava-belt of the Ochils and north-eastern Fifeshire,1 while D. Balsillie and R. M. Craig noted that these and other Lower Old Red Sandstone lavas in the district commonly carry a rhombic pyroxene.2 II.—Field Relations. The lavas occupy a broad sinuous belt extending from the river in a north-easterly direction, when they thin out suddenly about one mile north-east of the Buddon Burn. This representation, as mapped by the Survey, is due to lack of field ex exposures, but it is probable that, from the continuation of the lavas towards Arbroath, there exists here a powerful dislocation approximately parallel in trend to that which has thrown down the lavas a few miles farther north-east. Along the north bank of the river the igneous rocks are exposed in a series of rather disconnected beach outcrops between Stannergate and Broughty Ferry, east of which they are concealed beneath a considerable stretch of marine alluvium and blown sand. On the Survey Map (Sheet 49) a band of volcanic tuff and conglomerate forms the base of the series, but unfortunately no exposures of this fragmental material are now available. What may probably have been its W.S.W. extension was revealed during the deepening of the Eastern Wharfs, the rock consisting of fragments

Contact-Metamorphic Assemblages in the System CaO-MgO-Al2O3-SiO2.

In previous papers1 the assemblages characteristic of the systems lime-magnesia-silica, and magnesia-alumina-silica, in thermal metamorphism have been described, special reference being made to the range of mineralogical associates of rhombic pyroxene.

Genesis of Rhombic Pyroxene in Thermal Metamorphism

Rhombic pyroxene as a constituent of thermally altered sediments has hitherto been recorded only from a few contact zones, and, as the conditions under which it is generated in metamorphosed sediments are of some interest, these may be examined.

V.—Note on the Igneous Rocks of Singapore, with special reference to the Granite and Associated Rocks carrying Rhombic Pyroxene

In a former paper the writer endeavoured to show the connection between the sedimentary rocks of Singapore and certain rocks in Pahang. On the occasions when opportunities for geological work in Singapore have presented themselves numerous specimens of igneous rocks have been collected, and it is the purpose of the present paper to describe briefly the observations recorded. It is hoped that this will give some further idea of the geology of the island of Singapore; but apart from a general description the pyroxene - bearing rocks associated with the granite appear to the writer to require a note as being of exceptional petrological interest.

Societies and Academies

Geological Society. Frbruary 5.—Sir Archibald Geikie, K.O.B., Sec.R.S., president, in the chair.—Antigorite and the Val Antigorio, with notes on other serpentines containing that mineral: Prof. T. G. Bonney. It is not certain that the first described specimen of antigorite was really found in the Val Antigorio. The rock probably does not occur there in situ, though it is found in pebbles, &c., from tributaries. Other specimens of antigorite-serpentine were described. The origin of the mineral is discussed. Pressure is essential; it can be formed from augite, and there is evidence of its coming from this mineral.—The St. David's Head “rock series” (Pembrokeshire): J. V. Elsden. These intrusions are of complex composition. There is a high magnesia percentage. There is no evidence of differentiation in situ, but the facts suggest a common origin from a differentiated magma basin. The rocks afford facilities for the study of both rhombic and monoclinic pyroxenes. Rhombic pyroxene generally crystallised earlier than the monoclinic pyroxene. There are two varieties of augite. The relation of these types lends support to the perthitic theory. The probable age of the intrusions is not greater than that of the earth movements which folded the Arenig shales in this district.

Communication from the Oxford Mineralogical Laboratory

The stones to which this paper refers were sent to Prof. Miers for determination by Mr. Francis Powell, and were stated to come " from the diamond washings." They are of a fine green colour, and quite transparent, and possess a highly perfect cleavage parallel to the faces of a prism having an angle of about 88°. This serves to distinguish the material from olivine, which it otherwise much resembles, and --coupled with the uniformly straight extinction through cleavage flakes, the specific gravity (3.199) and hardness (6-7)--suffices to identify the mineral as one of the rhombic pyroxenes.

IV.—On Various Crystalline Rocks

Crystalline rocks containing neither felspar nor olivine have received but little or no attention, except from one or two American geologists. Dr. G. H. Williams has recently drawn attention to these rocks, of which he proposes to constitute a family of Pyroxenites coördinate with Rosenbusch's family of Peridotites; and has described from Maryland and North Carolina a type consisting entirely of monoclinic and rhombic pyroxenes, to which he gives the name Websterite. The occurrence of a similar rock in Northern Italy seems worth recording. Our specimens were collected by Mr. E. J. Garwood.

VII.—On the Occurrence, as a Common Rock-forming Mineral, of a Remarkable Member of the Enstatite Group (Amblystegite, vom Rath)

During recent years, much attention has been directed to the importance of the Rhombic Pyroxenes (Enstatites) as rock-forming minerals. In 1879 Prof. Fouqué showed that hypersthene is a common mineral in the Santorin lavas, and in 1883 Mr. Whitman Cross demonstrated that there exists a large and important class of andesites, distinguished by the presence in them of the same mineral hypersthene. Mr. Teall, in the pages of this Magazine, has pointed out that similar lavas exist in this country, which are undoubtedly of pre-Tertiary age; and, now that the attention of geologists has been very generally drawn to the subject, the same rocks have been recognized by many observers in all parts of the globe.

The Java Eruption

I HAVE been greatly interested in your note on M. Renard's researches as to the composition of the volcanic material ejected during the recent eruption of Krakatoa. The ashes, as stated, are those of a magma that would have produced an andesite with rhombic pyroxene. Now such an andesite occurs at so many points, and in such immense masses, round the great Pacific “circle of fire,” that one is tempted to ask if it may not specially characterise this important volcanic region. I will, with your permission, briefly refer to some published, and one or two unpublished, facts with regard to the distribution of this andesite (called hypersthene-andesite by Whitman Cross and Iddings, and bronzite-andesite by F. Becke) round the Pacific circle.

II.—A Lava from Montserrat, West Indies

In the March and April Numbers of the Geological Magazine, Mr. J. J. Harris Teall describes some beautiful rocks from the Cheviots, containing a rhombic pyroxene with well-marked pleochroism, and as there seems little doubt that this mineral, which we must consider hypersthene, has been hitherto overlooked in a good many rocks, another note of its occurrence may perhaps possess some interest.