A Study of the Potash Felspar Gneisses of Connemara and Felspar Comparison with the Galway Granite

Eighteen potash felspars from the potash felspar gneisses of the Connemara migmatite belt have been partially analysed and the associated plagioclase optically determined. Comparisons have been made with seventeen similar felspar pairs from the Galway Granite. Albite components are expressed graphically in an attempt to illustrate sodium distribution between associated felspar phases. On such a plot it is possible to contrast the “magmatic” felspars of the Galway Granite with those of the migmatitic gneisses; the separation of the two fields being a function of total rock chemistry. Crystallization temperatures have been determined by the two felspar method and the range 460 to 480° C. is thought to represent retrograde amphibolite conditions in the migmatitic gneisses, while petrographic evidence supports 550° C. as the temperature of amphibolite facies metamorphism in these rocks. Triclinicity measurements have also been made on the analysed potash felspars.

Amygdaloid Textures in the Lewisian Rocks of the Island of Tiree

Certain amygdaloid textural features have been observed in a diopside-potash felspar rock within the Lewisian of the Island of Tiree; so far as the writer is aware, such features have not previously been noted in Lewisian rocks.Evidence as to the genesis of these textures is sparse but they appear to have attained their present nature after the principal metamorphic events which have affected the enclosing rocks.

Marscoite and related rocks of the Western Red Hills complex, Isle of Skye

The rock from Skye which Harker called marscoite was recognized by him to be hybrid in origin because of the close association in the rock of basic plagioclase, quartz and orthoclase. The position of marscoite in the sequence of rocks forming the Western Red Hills Tertiary complex has now been defined, and evidence for its parentage and the mechanics of its formation obtained. The Western Red Hills intrusive centre, developed after the Cuillin and before the Broadford centres, as suggested by J. E. Richey, consists of five different, high-level granitic intrusions which were followed by a southern and northern series of late intrusions, including marscoites, ferrodiorites, and various additional granitic rocks. The high-level granitic rocks of Skye, which have so far been loosely termed granophyres, cannot all properly be described as such, and the term epigranite is proposed as a general name for them. The earliest rocks belonging to the southern late intrusions are porphyritic epigranites and felsites, having quartz and potash felspar phenocrysts resembling the xenocrysts of these minerals in the marscoite. Then came marscoite, somewhat chilled against the felsite but also back-veined by it. The marscoite in Harker’s Gully, and in other places on Marsco, passes gradually into ferrodiorite which sometimes contains basic andesine phenocrysts similar to the xenocrysts of the marscoite. The ferrodiorite has a composition suggesting that it was derived by extreme fractional crystallization of basic magma. The xenocrysts of the marscoite are highly characteristic and indicate that marscoite was formed by the mixing of a porphyritic acid magma, like that which produced the Southern Porphyritic Felsite, and a porphyritic basic magma, like that which produced the porphyritic ferrodiorite of Marsco. The chemical compositions of the presumed parent materials and marscoite support this view. Because of the even distribution of the xenocrysts in marscoite, the mixing must have been largely effected by the mechanical stirring together of two liquids, in both of which were suspended crystals. Diffusion within the liquid phase must also have contributed in some degree to the ultimate homogeneity of the liquid part of the mixture. The origin of the marscoite of the northern late intrusions is presumably similar to that of the southern, except that the basic parent is believed to be represented by the porphyritic hawaiite blocks in the northern marscoite. The northern marscoite cuts through, and is chilled against, the Glamaig, Eas Mor, and Maol na Gainmhich epigranites. Inwards from the contacts, the marscoite gives place, in a distance of 30 to 50 yd., to a rock here called glamaigite, which consists of rounded, dark patches, usually an inch or so across, and a less dark matrix, in approximately equal amounts. In both dark and light material there are xenocrysts of andesine, potash felspar and quartz, as in the marscoite. The difference in composition of the darker and lighter parts of the rock is not great, but is such as to suggest that the darker would have had a slightly higher temperature range of crystallization. The glamaigite is believed to have originated from a less well-homogenized mixture of basic and acid porphyritic magmas. From the mixture, the slightly more basic parts solidified first, and then flow movements of the magma resulted in rounding of the early semi-solid clots of hybrid material. The central parts of the composite, northern, late intrusions consist of a rock resembling glamaigite but tending to be more uniform and acid in composition. The greater homogeneity of this rock, distinguished as dioritic glamaigite, may be due to its central position within the intrusions, where slower cooling would allow more time for diffusion. It is suggested that the epigranites of the Hebridean igneous province originated by melting of crustal rocks of broadly granitic composition. The heat to produce the melting is believed to have been derived from basic magma intruded into the earth’s crust, the upward transfer of heat being aided by convection in the magma and bottom accumulation of early formed crystals. At some stage, a residual layer of ferrodiorite or hawaiite liquid, produced by fractionation, may have underlain a granitic liquid produced by melting. Two separate systems of convection currents are envisaged in the two liquids, because of the differing densities. At the junction of the two systems of currents, where they would be flowing in opposite directions, there would be an opportunity for mechanical mixing. Ultimately, a mass of hybrid magma may have developed, annular in form and with a forced circulation, which was the source of the marscoite and related rocks.

On the Age Relations of the Connemara Migmatites and the Galway Granite, West of Ireland

Contrary to a previously published view, evidence is given to show that the Galway Granite is entirely later than both the quartzandesine migmatization and the potash-felspar migmatization in Connemara, and that the Oughterard Granite is probably earlier than the Galway Granite, not later. There is also strong evidence against regarding the foliated marginal granite of the north and north-east border of the Galway Granite as being different in origin from the marginal granite often found in the southern, western, and north-western borders. These results agree with the radio-chemical date of 365 m.y. determined for part of the Galway Granite.

The orthoclase-microcline inversion

The association of orthoclase and microcline in the same rock has been commented on by many writers, and from these occurrences attempts have been made to deduce the relative stability of these two low-temperature forms of potash-felspar. Ahnost all investigators have concluded that nlicrocline is the lower-temperature modification, but difficulties in explaining the relationship of adutaria to orthoclase and microcline have been encountered.The identification of the monoclinie potash-felspar in association with microcline is almost always based on optical properties, viz. straight extinction in the zone [010] or the absence of visible multiple twinning.

An Unusual Pyroxene-rich Xenolith in the Diorite of the Glenelg-Ratagain Igneous Complex

A xenolith composed essentially of green pyroxene, microperthitic potash felspar, sphene, apatite, and iron ore, has been discovered in the diorite of the Glenelg-Ratagain igneous complex. it is believed to be an impregnated skarn rock associated with igneous activity earlier than the intrusion of the diorite. Introduction of P, Ti, and Na is considered to be important in the formation of the skarn.

Some Large Felspar Augen from near Ilorin Town, Nigeria

Large augen, composed principally of single crystals of potash felspar, are sporadically distributed in certain banded gneisses. It is inferred from structural and textural evidence that the augen have formed by metasomatic processes.

The Granitic Cotectic Curve

In a recent paper (Nockolds, 1946) it was shown that certain granite-aplites associated with acid plutonic rocks of Caledonian age in Western Scotland fell on, or were very close to, the plagioclase-potash felspar-quartz cotectic curve (the granitic cotectic curve), the approximate position of which could be determined from experimental data now available. It seemed of interest to determine whether other granite-aplites from other areas behaved in the same way and chemical analyses of granite-aplites and related rocks were collected from the literature with this end in view.

Myrmekite in graphic granite and in vein perthite

This paper describes the occurrence of myrmekite in a graphic quartz-microcline-rock from the mica-bearing pegmatites of Kodarma, Bihar, India. The myrmekite has grown out from the quartz-mierocline junctions into the potash-felspar with the appearance of having replaced the latter. The two are in optical and crystallographic continuity and, owing to the large ‘single crystal’ character of the microcline, definitely oriented sections can readily be prepared. A detailed study of these sections shows that the myrmekite has been formed by the segregation and coalescence of microperthitic albite formerly held in solid solution in the potash-felspar. Successive stages in this process of segregation can be observed.These sections also provide evidence of the paragenetic relationship between the quartz rods, the vein perthite, and the body microcline. This is supplemented by additional evidence from graphic granites and vein perthites obtained from other localities.

An X-ray examination of some potash-soda-felspars

The systematic classification of the potash-soda-felspars is of the greatest interest and importance to the mineralogist, but problems demanding physical investigation are raised in work carried out recently by Dr. Edmondson Spencer, who has described the results of exhaustive chemical, optical, and thermal investigations of a representative collection of these minerals. Dr. Spencer has very kindly allowed us to examine by X-ray methods a large number of his specimens, so that for the first time it has been possible to make a fairly complete survey of potash-soda-felspars of known chemical composition and with accurately measured optical properties.This paper presents the results of our general survey of potash-soda-felspars with compositions ranging from nearly pure potash-felspar to approximately 50 % soda-felspar. The natural minerals, and the same minerals after specified heat-treatment, have been examined. We do not discuss in detail the interpretation of our results in terms of the atomic structure of felspars; the subject is extremely complicated, and it is necessary to await the results of other investigations now in progress in this laboratory before attempting a complete structural interpretation.