Low Albite Research Articles

The crystal structures of two oligoclases : A comparison with low and high albite

The crystal structures of two pegmatitic oligoclases, Anl6 and An28, have been refined using intensities from the type a reflections which give average atomic parameters, bond lengths and anglcs and thus avcrage Al/Si distributions. These are: TIO0.74,0.63 AI; TIm 0.11, 0.21 AI; T20 0.16,0.22 AI and T2m 0.16, 0.22 AI, respectively. Structural and site-occupancy par-ameters are compared with low and high albite, using data from RIBBE, ~IEGAWand TAYLORand WAINWRIGHT and STARKEY. The Na/Ca anisotropy and (Na/Ca)-O coordination polyhedra of the oligoclascs arc remarkably similar to those of high albite. The (Na/Ca)-O bond lengths for each of the first five nearest oxygen atoms decrease linearly with the mean of the two T-O bond lengths to each oxygen atom. Those oxygen atoms in the oligoclases bonded to TIO show remarkable similarities to those in low albite, whereas 0,\(2) (bonded to 1'20 and T2m) is like that in high albite. Aluminum occupancy is, of course, the major factor governing individual 1'-0 bond lengths in feldspars, although the importance of linkage (i.e., Si-OSi or Si-O-Al) is clearly evidenced in plots of mean T-O distances 1)8. individual T-O bond lengths. The small differences in T-O-T angles for a given oxygen atorn amongst the four sodic plagioclases are inversely related to mean Al content of the two T sites. Although low albite (AI: Si = 1: 3) is ordered and high albite disordered with respect to Al/Si and the two low oligoclases have different AI: Si ratios (1.16: 2.84 and 1.28: 2.72) with partly disordered AljSi distributions, it appears that the lattice parameters b, c, iX, iX*, y, y* and 20131 20131 vary linearly with the diiierence in the Al contents of the '1\0 and TIm sites (where Ahlin = Ah,o = Ah,m).

The crystal structures of two oligoclases: A comparison with low and high albite*

The crystal structures of two pegmatitic oligoclases, Anl6 and An28, have been refined using intensities from the type a reflections which give average atomic parameters, bond lengths and anglcs and thus avcrage Al/Si distributions. These are: TIO0.74,0.63 AI; TIm 0.11, 0.21 AI; T20 0.16,0.22 AI and T2m 0.16, 0.22 AI, respectively. Structural and site-occupancy par-ameters are compared with low and high albite, using data from RIBBE, ~IEGAWand TAYLORand WAINWRIGHT and STARKEY. The Na/Ca anisotropy and (Na/Ca)-O coordination polyhedra of the oligoclascs arc remarkably similar to those of high albite. The (Na/Ca)-O bond lengths for each of the first five nearest oxygen atoms decrease linearly with the mean of the two T-O bond lengths to each oxygen atom. Those oxygen atoms in the oligoclases bonded to TIO show remarkable similarities to those in low albite, whereas 0,\(2) (bonded to 1'20 and T2m) is like that in high albite. Aluminum occupancy is, of course, the major factor governing individual 1'-0 bond lengths in feldspars, although the importance of linkage (i.e., Si-OSi or Si-O-Al) is clearly evidenced in plots of mean T-O distances 1)8. individual T-O bond lengths. The small differences in T-O-T angles for a given oxygen atorn amongst the four sodic plagioclases are inversely related to mean Al content of the two T sites. Although low albite (AI: Si = 1: 3) is ordered and high albite disordered with respect to Al/Si and the two low oligoclases have different AI: Si ratios (1.16: 2.84 and 1.28: 2.72) with partly disordered AljSi distributions, it appears that the lattice parameters b, c, iX, iX*, y, y* and 20131 20131 vary linearly with the diiierence in the Al contents of the '1\0 and TIm sites (where Ahlin = Ah,o = Ah,m).

Highly Undersaturated Rocks in Upper Mantle Conditions

OLIVINE nephelinite and the closely related magmas, basanite and olivine melilite nephelinite, are products of widely distributed but small volcanic eruptions. They commonly contain dense, coarse grained xenoliths produced at high pressures in the upper mantle, particularly lherzolites, pyroxenites and garnet pyroxenites. These magmas therefore derive directly from the mantle and cannot have been modified by shallow, crustal fractionation processes9. It is important to know the variation of the mineralogy of olivine nephelinite as a function of pressure and temperature. Exploratory measurements using a piston-cylinder high pressure apparatus to 36 kilobars, and using techniques which have given equilibrium mineral assemblages in basaltic composition1, have shown a variation in the mineralogy of a nepheline-rich basic composition at sub-solidus temperatures over the pressure range of the crust and upper mantle. The composition studied (Table 1) has very low albite and is classified as transitional between olivine nephelinite and nepheline-rich basanite. The results of the experiments are given in Table 2; phases were identified by X-ray powder diffraction and by optical examination. What follows is a discussion of the mineral reactions required by the data of Table 2.

Electrostatic charge distributions in the structure of low albite, NaAlSi3O8

Electrostatic charge distributions in the structure of low albite, NaAlSi3O8

The hydrothermal synthesis of low albite

Glasses on the join NaAlSi3O8-Na2Si2O5 were devitrified hydrothermally at pressures of 1 to 10 kb and at temperatures in the range 200 to 700° C to define more adequately the physical and chemical environments which favor crystallization of the fully ordered polymorph of albite. The presence of Na2Si2O5 allows the synthesis of low albite with an obliquity of 1.140° (Cu Kα radiation) in runs of relatively short duration. The effect of increasing total pressure and time, and of decreasing temperature and amount of water down to critical values, is to favor the synthesis of ordered albite. Excess sodium is the chemical constituent necessary for ordering to proceed at a relatively rapid rate; this rate seems to vary with the ratio aNa+/aH+, and hence with the peralkalinity of the aqueous fluid attending recrystallization. The chemical environment of recrystallization thus seems as important as temperature in determining the ultimate degree of Si-Al order attained in albite.

The structure of bytownite (`body-centred anorthite')

Bytownite (a plagioclase feldspar with composition aboul An 80 Ab 20 ) closely resembles anorthite in its lattice dimensions and its diffracted intensities, except for the systematic absence of those wiih h + k odd ('c' and 'y' type). The deduction of a body-centred lattice (whence the previous name, 'body-centred anorthite') is here shown to be incorrect. Three-dimensional electron-density maps and difference maps show conspicuously elongated or doubled Ca/Na peaks and 'half-atom splitting' for other atoms. The results are explained in detail if bytownite has a primitive lattice and a structure very close to (but significantly different from) that of anorthite, and if small anti-phase domains are present with origins related by the vector ½(a + b + c). The evidence for this conclusion, and against other possible explanations of the observed facts, is discussed fully. Bond lengths and angles (tabulated in full) resemble those in anorthite, with some significant differences. Inequalities of bond lengths and angles within the same tetrahedron are real; an analysis of their resemblances and differences in corresponding tetrahedra in different feldspars suggeests an empirical approach to the study of the structural stress systems involved, and emphasizes the difference between the groups comprising microcline, reedmergnerite, and low albite on the one hand, and anorthite, bytownite, and high albite on the other. Ordering of Si/Al is nearly complete, on the same set of sites as in anorthite. At a closer approximation, a few Al-rich sites show significant differences in thier Si content, with extreme value of 0% and 30% against the average value of 13 ± 5% for the rest.

The Paragenesis of Swedish Glacial Clays

Abstract The feldspars in Swedish glacial clays include K-feldspar, low albite and intermediate plagioclases, but not peristerites. The hydrolysis of K-feldspar leads to the formation of illite under glacial conditions of P, T and μmobile components; chlorite is the stable ferromagnesian mineral in the unweathered clays. Illite + albite + quartz represents a lower energy assemblage than K-feldspar + Na-montmorillonite + quartz.

Granite beneath Viséan sediments with mineralization at Rookhope, northern Pennines

The geological results of a borehole drilled to investigate the coincidence of centres of a zonal pattern of mineralization ( Dunham 1934 ) with areas of strong negative Bouguer anomaly are presented. The presence of a granite batholith with cupolas, postulated by Bott & Masson-Smith (1957) has been proved; the top surface of the granite was found at 1281ft depth and the boring was continued to 2650 ft in granite. The section begins in the Great Limestone at the base of the Namurian; a normal succession in the Middle Limestone Group (Lower Carboniferous) is revealed, the Smiddy Limestone with Girvanella Band being reached at 1056 ft 6in. The Lower Limestone Group differs from the section in Teesdale and the Pennine escarpment; in particular, beds equivalent to the Melmerby Scar Limestone, identified by their fauna of algae, corals, and brachiopods, have a rubbly lithology and are split by bands of seatearth, sandstone, and shale. Beneath them, clastic sediments 38 ft thick with marine fossils rest on the weathered surface of the granite. Two quartz-dolerite sheets were proved; the Little Whin sill, 6ft, is in the Three Yard Limestone as at Stanhope, while the Great Whin sill, 192ft 9in, lies beneath the Jew Limestone, stratigraphically lower than in Upper Weardale. The Weardale granite, carrying biotite and abundant muscovite, has a low-dipping foliation in the upper part, but this becomes less obvious below 2225ft depth. Preliminary X-ray studies indicate that low albite, orthoclase, maximum microcline, and forms intermediate between the last two are present. Analyses of representative rocks for major and trace elements are given. Xenoliths are conspicuously absent, but aplites and pegmatites are common. The mineralogical and chemical effects of weathering before Carboniferous sedimentation, and of the mineralization, are described. The borehole was sited near the crossing of the Boltsburn lead vein (ene-wsw) and the Red fluorite-iron vein (ese-wnw), both of which dip towards it. Strong metasomatic mineralization, including green fluorite, blende, and quartz occurs in the Tynebottom, Jew, and Lower Little limestones. Numerous small veins were also cut both in the sediments and the granite. Though chalcopyrite is present against the lower sediments, there is no evidence of a concentrated copper zone. Fluorite continues into the granite, and pyrrhotine, not present above, is found from 1355 ft depth. Mineralization continues to much greater depths than have previously been proved in the area; its source presumably lies beneath the granite cupolas, which may, however, have guided the rising fluids.

An electron-microscope investigation of a moonstone

Abstract Unmixing in a moonstone has been directly observed, using an electron microscope. The mineral consists of alternating, irregular sheets of orthoclase and low albite, approximately parallel to ( ) and of widely varying thicknesses. The albite sheets consist of finely twinned lamellae parallel to (010)—-the composition face of the albite twin law—-and exactly four albite unit cells wide. They often repeat regularly over a sufficiently large distance to constitute a superstructure which gives sharp subsidiary reflections in the electron diffraction patterns. The white schiller of the moonstone is discussed on the basis of these structural details.

On the occurrence of steenstrupine in the Ilímaussaq massif, Southwest Greenland

The rare mineral steenstrupine was first described from pegmatoid veins in the Ilímaussaq alkaline massif, South Greenland. It is now known to be of wide-spread occurrence in this massif, being present in agpaitic rocks (naujaite, sodalite foyaite and lujavrite) and in late veins of hydrothermal affinity. In the present paper three small areas within the Ilímaussaq massif have been selected for a rather detailed study of the mode of occurrence of the steenstrupine, namely the small island of Qeqertaussaq made up of naujaite which is cut by very thin veins of a black, lujavrite-like rock, felted ægirine and albite-analcime-natrolite rocks; the head of Kangerdluarssuk with thicker veins of black lujavrite and green ægirine felt cutting the naujaite; and the north coast of Tunugdliarfik which represents a deeper level in the intrusion, being composed of lujavrite with numerous inc1usions of naujaite. The naujaite of the last-named area is cut by thin veins of ægirine felt, acmite, albite and analcime-natrolite. The lujavrite here has apparently assimilated naujaitic material. Steenstrupine occurs in these three areas in lujavrite, late veins and in the naujaite adjacent to these rocks. It is also present in albite-analcime-natrolite· bearing replacement bodies in some zoned naujaite pegmatites. Some features favour a metasomatic origin of the lujavrite, but it is conc1uded that the combined observations are best explained by a magmatic interpretation. The lujavrite is clearly later than the naujaite, its intrusion being guided by the joints of the naujaite. Considerable tilting and rotation of the naujaite inclusions in lujavrite is seen in places. In some lujavrite veins it is seen that the mise en place of the lujavrite was preceded by the formation of ægirine felt along the naujaite joints. The intrusion of the lujavrite was accompanied and succeeded by the formation of thin veins containing one or more of the minerals acmite, arfvedsonite, albite, analcime and natrolite. In the lujavrite a very pure maximum microc1ine and a pure low albite have been formed in equilibrium indicating a low temperature of formation, probably of the order of 400 oC. The albite and in cases also the microc1ine, nepheline and sodalite of the lujavrite, may be replaced, to varying extent, by analcime, and, more rarely, by natrolite. This also favours a low temperature of formation of the lujavrite. At these low temperatures and in this very sodium-rich environment liquid immiscibility may have played a role as is mentioned in the discussion of peculiar spheroidal structures in the lujavrite. Some of the analcime of the lujavrite may be a primary precipitate of the lujavrite magma, but most analcime is probably secondary after the primary alkali· aluminium silicates. This analcitization is considered to be late magmatic. The sodium-water-rich rest liquid of the lujavrite may be squeezed out and has affected analcitization of the naujaite inclusions. These late fluids are also responsible for the formation of the late veins made up of albite, analcime, natrolite and a number of rare minerals containing rare earths, Nb, Th, Mn, Li, P, F, Zn and S. These elements are partly expelled from the lujavrite magma, partly leached out of the naujaite adjacent to the fractures by the percolating late fluids. The fact that the rare minerals (e.g. steenstrupine, britholite, monazite, lepidolite, schizolite, igdloite and sphalerite) of the late veins also occur in the analcitized lujavrite indicates that the formation of these minerals was closely connected with the later stages of crystallization of the lujavrite. The lujavrite and naujaite are rich in eudialyte; this mineral is rare in the steenstrupine-bearing rocks. It is therefore concluded that the eudialyte is stable at a higher temperature than the steenstrupine, monazite, etc. The steenstrupine-bearing replacement bodies of the naujaite pegmatites were formed simultaneously with the late veins and are thus considerably later than the naujaite and clearly formed from fluids of external origin, with respect to the primary pegmatite system. This feature is compared with the well-known replacement bodies in granite pegmatites. The chemical and physical properties of the late fluids and the mechanisms of transport and precipitation of the rare elements are discussed. New chemical analyses of ægirine, acmite, beryllium sodalite, chkalovite and igdloite and a number of partial chemical analyses of albite, analcime, microcline, monazite, natrolite, nepheline and sodalite are reported. Point countings have been made on the finer grained, more homogeneous rocks. The minerals of the rocks are briefly described, and a number of rare rock types are described and discussed.

Crystal Structure Refinement of Reedmergnerite, the Boron Analog of Albite

Ordering of boron in a feldspar crystallographic site T1(0) has been found in reedmergnerite, which has silicon-oxygen and sodium-oxygen distances comparable to those in isostructural low albite. If a simple ionic model is assumed, calculated bond strengths yield a considerable charge imbalance in reedmergnerite, an indication of the inadequacy of the model with respect to these complex structures and of the speculative nature of conclusions based on such a model.