Showing papers in "Mineralogical Magazine in 1975"

Fluorine and chlorine in peralkaline liquids and the need for magma generation in an open system

Abstract: Fluorine, chlorine, zinc, niobium, zirconium, yttrium, and rubidium have been deter-mined on fifteen obsidians from Eburru volcano (Kenya Rift Valley), spanning the range from pantel-leritic trachyte to pantellerite. All pairs of elements show positive correlation coefficients, ranging between 0·769 and 0·998, but with most values better than 0·900. In spite of some very high correlations, only two of the twenty-one best-fit lines pass near the origin of the Cartesian coordinates. Linear distributions are found within two separate groups of elements: F, Zr, Rb; and Cl, Nb, Yt. Zn behaves in general as a member of the second group but seems to be subject to an additional variation. When an element from the fluorine group is plotted against one from the chlorine group the resulting pattern is non-linear. Therefore, although the elements in both groups would generally be considered ‘residual’ (partition coefficients between crystals and liquid approaching zero) there are clearly detectable differences in their variation, and hence their behaviour. Major-element variations in the obsidians are such that a vapour (fluid) phase would be needed to account for any magma evolution. The trace-element patterns are also impossible by closed-system crystal fractionation and suggest that this fluid may have been rich in halogens, with the metallic elements forming preferred ‘complexes’ with either F or Cl. The F-Zr-Rb ‘complex’ also varies quite independently of the important major oxides (e.g. A12O3) in the rocks. In the case of Rb this is but one aspect of a more significant anomaly, in which there is no sign of any influence of alkali feldspar (which partitions Rb) in the variation. This is remarkable because trachytes and rhyolites have normative ab+or > 50 %, and any evolutionary process controlled by crystal ⇋ liquid interactions must be dominated by the melting or crystallization of alkali feldspar. The results on the Eburru obsidians show that if they are an evolutionary series then either, the process was not crystal ⇋ liquid controlled, or that any such process has been overriden (or buffered) by other processes that have superimposed the observed trace-element patterns. In the latter event, the buffering phase may have been a halogen-bearing vapour. The same considerations must apply to other pantellerite provinces where Rb appears to have behaved as a ‘residual’ element.

The Phosphate Mineralogy of Et-Tabun Cave, Mount Carmel, Israel

Abstract: Phosphates introduced into the et-Tabun cave as a result of human habitation, mainly as bone material, were partly dissolved by slightly acid solutions (organic acids derived from human or other animal activity). From these solutions, dahllite precipitated when the solutions came into contact with calcite. In the more weathered parts of the section, where little or no calcite occurs, the phosphate minerals formed are montgomeryite and crandallite.

A Mössbauer Study of Thermal Decomposition of Biotites

Abstract: The effect of heat treatment in air at temperatures from 200°C to 1000°C of two Cornish biotites has been studied using the M6ssbauer effect in 57Fe. One of the micas was also studied after heating in vacuo. Progressive changes in the Mossbauer spectra of the materials after heat treatment can be correlated with the following processes: Oxidation of Fe2+ ions in isolated octahedral sites to give Fe3+ in the co-ordination Fe3+(OsOH); oxidation of Fe2+ ions in adjacent a-octahedral (cis OH−) sites to give Fe3+(O6); and oxidation of Fe z+ in adjacent a and b (trans OH−) sites to give either Fe3+(O5OH) or Fe3+(O6) depending on whether a shared hydroxyl group remains intact or otherwise (these processes occur at temperatures below 500°C Also, at higher temperatures, dehydroxylation of Fe3+(O5OH) to give more Fe3+ in the Fe3+(O6) configuration. Finally, in the temperature range 900 to 1000°C structural breakdown yielding α-Fe2O3 as the iron-containing phase. The results and interpretation are in good agreement with thermogravimetric data and with a previous infra-red study. The mechanism of charge diffusion in the lattice during oxidation is discussed in the light of the results.

Mineralogy, chemistry, and origin of a concretionary siderite sheet (clay-ironstone band) in the Westphalian of Yorkshire

Abstract: SUMMARY. Concretionary siderite horizons are quite common in massive clay sequences. One such horizon, from the Westphalian of Yorkshire, has been studied in detail. Two iron-rich carbonate minerals occur together although they cannot be distinguished in thin section on account of very fine grain size. One is much richer in magnesium (pistomesite) than the other (siderite). The latter is relatively rich in manganese and the heavier stable carbon isotope laC whereas the former carbonate is richer in calcium and 1~C. The most important iron source is thought to have been hydrated iron oxides originating in soils. Much of the carbonate carbon started as organic molecules. The siderite appears to have formed earlier than the pistomesite. The stratiform character of these deposits appears to reflect siltier horizons in the mudstones, which presumably channelled pore water migration during compaction. This is probably why such carbonate horizons were formerly believed to be of simple sedimentary rather than diagenetic origin. SII~ERITE occurs very much more widely than is commonly realized. Most clay-rich sequences (which actually make up 7o to 8o % of the sedimentary column) contain dispersed as well as the more obvious concretionary siderite. The sediments studied here, from above and below the Alton Marine Band and Halifax Hard Bed Coal, include from 4 to 18 % iron carbonate (Pearson, 1973). Almost invariably, siderites are found to be extremely fine-grained and intimately mixed with clay, organic matter, and, not infrequently, phosphates. Physical separation of pure material is virtually impossible. In consequence most mineralogists have left these minerals severely alone and there is a noticeable lack of precise information. Most recent data are to be found in Soviet or East European literature (Zaritskiy, I964, for example). There has been a considerable revival of interest in other concretionary carbonate minerals, such as calcite (Saas and Kolodny, 1973) and dolomite (Murata, Friedman, and Cremer, I972). Both papers illustrate that much of the new interest lies in variation of stable carbon isotope ratios and implied fractionations. Such data give unique insight into the role played by microorganisms in diagenesis and are particularly relevant to problems of petroleum genesis. In an attempt to improve the situation with respect to compositional information, one of us developed methods of wet chemical analysis and made a study of covariation of composition with d-spacings (Pearson, I974a, b). An initial investigation of carbon isotope variation was also undertaken and reported (Curtis, Petrowski, and Oertel,

The Chemistry and Technology of Selenium and Tellurium

Abstract: It is an unenviable task to have to select, without significant omission, a number of important papers on this topic. The 43 papers that were chosen, however, cover the subject-matter well except that there is very little on the general crystal chemistry of germanium. Special attention has been given to papers on germanium in meteorites and on meteorite classification, especially those by J. T. Wasson, published in the usually accessible Geochimica et Cosmochimica Acta. The editor's comments are short and do not add much to the value of the book. The quality of production is generally good but the reproduction of photomicrographs is barely adequate and in the reviewer's copy the pages were bound in the wrong sequence. This book is unlikely to be a strong contender for the use of library funds especially when a library already has the Handbook of Geochemistry (Min. Mag. 38, 533-4). The chapter on germanium in the latter is undoubtedly of more use for a summary of the pre-I969 literature and for its good bibliography. Weber's book has the advantage that more than twenty of the selected papers were published between I966 and ~97o and some of these are from the less accessible Russian journals. P. HENDERSON

Nickel-Containing Montmorillonites and Chlorites from Brazil, with Remarks on Schuchardtite

Abstract: SUMMARY. Clay-like, hydrous nickel silicates from Niquelandia, Goias, Brazil, and from Jacupiranga, S~to Paulo, Brazil, are examined by X-ray diffraction, chemical analysis, and thermogravimetric methods. The nickel silicates from Niquelandia are ferric iron montmorillonites, nickel in the octahedral positions and, to a lesser extent, in interlayer positions. The chemical compositions of samples dried at 1 Io ~ average (Mg0.~sNi0.0eCa0.015Na0.005) (Fea0!~..,Fe~I.0o.sA10.55Mg0.aTNio.20 (Sia.ssA10ao.)O~0(OH)2(H20)0.gs. The layer charge originates largely in the octahedral sheet. The cation-exchange capacities average Io7 me/ioo g clay (rio ~ These minerals differ from nontronites, in which the layer charge originates mainly in the tetrahedral sheets. The nickel silicates from Jacupiranga resemble chlorites, with a non-expanding basal spacing of I4'2-14'3 A,. The better crystalline forms give X-ray data in good agreement with a Ia polytype structure, which is consistent with their formation by low-temperature processes. Chemical formulae derived on the basis of O10(OH)s give an average of 5'47 cations in octahedral positions. Thermogravimetric measurements show up to 4% HzO lost between 11o and 500 "C. These results suggest a partial vermiculitization of the interlayer sheets. A new method of calculating a structural formula is developed that gives an interlayer composition of the form R~.~5(OH)s.Ts(H.oO)..t, with (OH+ H20)/R ~ 2'8o, which approaches the ratio for a dioctahedral sheet structure. A comparison of these defect chlorites with available data on schuchardtites shows many similarities; however, schuchardtite is not yet sufficiently well defined for the Jacupiranga chlorites to be given this name. THE present study extends previous work on fine-grained hydrous nickel-containin g silicates by Brindley and Pham Thi Hang (I973) in which serpentine-like and talclike forms mainly were studied. In the present work, nickel-containin g chlorites from Jacupiranga, in the state of S~o Paulo, Brazil, and nickel-containin g montmorillonites from Niquelandia, in the state of Goias, Brazil, are examined. A comprehensive report by Pecora 0944) discussed the geology of the nickel silicate and associated oxide deposits in the region of Niquelandia (previously called S~.o Jos6 do Tocantins) and mentioned (p. 275) nickel-bearing vermiculites in the Jacuba deposits of that area. The present samples also came from the Jacuba deposits, but appear to be principally montmorillonites . We have found no similarly detailed report of the Jacupiranga area, but a short description by Felicissimo (I965) refers to the formation of hydrous nickel silicates by lateritic alteration of ultrabasic rocks and the occurrence of nickel vermiculite.

A Sapphirine-Cordierite-Bronzite-Phlogopite Paragenesis from Namaqualand, South Africa

Abstract: The rock composition and electron microprobe analyses of the five constituent minerals are presented for a sapphirine-cordierite-bronzite-phlogopite-spinel rock from Namaqualand, South Africa. These data are considered in relation to experimental data and the mineralogy of certain associated rock types, and it is suggested that the sapphirine-bearing rock represents a metamorphic residuum (restite) after extraction of anatectic granitic liquid from an original argillaceous sediment.

Kaersutite-bearing gabbroic inclusions and the late dike swarm of Kangerdlugssuaq, East Greenland

Abstract: The Kangerdlugssuaq late dike swarm, which strikes at a high angle to the well-known East Greenland coastal swarm, is described and chemical analyses presented. The basic members are characterized by a high potassium content. A variety of kaersutite-bearing gabbroic inclusions in one member of this swarm is described in detail and microprobe analyses of clinopyroxenes, amphiboles, plagioclases, sheet silicates, and spinel minerals are presented. On the basis of this evidence it is deduced that these inclusions derive from a cumulate sequence formed at depths of between 5 and 10 km beneath the Lower Tertiary land surface. It is likely that fractionation of such assemblages causes a transition, at relatively low pressure, from undersaturated to oversaturated compositions, but the products appear to be quantitatively minor.

Synthesis of K2SiSi3O9 with Silicon in 4- and 6-Coordination

Abstract: SUMMARY. TWO kinds of vitreous material with composition K2GeSi~Os and K2Si~O9 were treated under conditions of high temperature and pressure. The X-ray diffraction patterns of the products were similar to that of K~TiSinO~, which has the wadeite, K2ZrSi3Os, structure. A natural crystal of K-feldspar was also treated under the same conditions. The product was found to consist of three phases, coesite, kyanite, and the above-mentioned KsSi40~. Since Zr ions in wadeite are in a 6-coordinated site, one-quarter of Si ions in K2Si,Os are considered to be in 6-coordination. This is the first silicate in which Si ions occupy sites of 4- and 6-coordinations in one crystal at the same time.

Petrogenesis of the Tarr Albitite-Carbonatite Complex, Sinai Peninsula

Abstract: Large albitite bodies associated with explosive brecciation, fenite aureoles, intrusive carbonate rocks, olivine dolerites, and copper mineralization are described from the Dead Sea rift region in the south-eastern Sinai Peninsula. The main carbonate phases comprise breunnerite and dolomite with actinolite the main phase in the fenites. Field and isotopic evidence indicates that the intrusive carbonate rocks are of carbonatitic origin. The cogenetic, almost monomineralic main phases can be attributed to fractional crystallization and liquid immiscibility acting on a highly gascharged, slow-cooling gabbroic magma. The complex, which is probably of Cretaceous (or older) age, appears to be related to early continental separation along the Dead Sea branch of the African rift system. The porphyry-type copper deposits resemble others that are located along crustal accretion (Iceland) or subduction (Chile) zones.

Some Aluminous Clinopyroxenes from Vesuvius and Monte Somma, Italy

Abstract: Aluminous Ca-rich clinopyroxenes (5.95-7.63 wt % Al2O3) are next in abundance to leucite in the basic alkaline potassic lavas from Monte Somma and Vesuvius, Italy, and occur as phenocrysts (1 to 7 mm), micropbenocrysts (< 1 mm), and groundmass granules. Zoning of various types is a conspicuous feature in these pyroxenes. Optical and chemical data are presented and a comparison is made between these pyroxenes and similar ones from alkali basalts. It is known from published data that Al2O3 fluctuates strongly in the oscillatory zones of Vesuvian pyroxenes. This can be explained as due to temperature variation in the magma, to magma variation in silica content due to contamination, to oscillations in leucite precipitation, or to a combination of these factors. Of these three factors, oscillations in leucite precipitation, as it appears, would be more effective than the others since it would have a greater control over Si/A1 availability in the magma.

Wroewolfeite, a New Copper Sulphate Hydroxide Hydrate

Abstract: Wroewolfeite is a new mineral from the Loudville lead mine in Loudville, Massachusetts, U.S.A. The mineral occurs as blue, monoclinic crystals, twinned on {001}, very similar in appearance to posnjakite and langite. It is formed as small isolated pinacoidal crystals (up to 1.0 mm) implanted on covelline and chalcosine. There are three cleavages of equal facility of production. The cell dimensions are a 6.058 A, b 5.654 A, c 14.360 A, β 93° 28′, space group Pc or P2/c. The strongest diffraction lines (in A) are 7.152 (100), 3.581 (70), 2.628 (35), 2.004 (30), 2.431 (20), 2.379 (20), 2.278 (20). Electronmicroprobe analysis gives CuO 64.22 %, SO3 16.48 %, water by difference 19.30 %. Empirical cell contents are Cu7.88(SO4)2.00(OH)11.76.(H2O)4.22 or Cu4(SO4)(OH)6.2H2O with Z = 2. Wroewolfeite is strongly pleochroic with a light blue, β deep greenish blue, and γ, medium greenish blue. Absorption β > γ ≫ α. The mineral is biaxial with α 1.637, β 1.682, and γ 1.694, 2Vα = 53° The name is for C. Wroe Wolfe, American crystallographer, educator, and philosopher.

Some Observations on Assarsson's Z-phase and Its Structural Relations to Gyrolite, Truscottite, and Reyerite

Abstract: Z-phase was obtained hydrothermally at 120 °C by decomposition of Al-substituted tobermorite and by reaction of lime and colloidal silica. X-ray and electron diffraction show that the structural element is hexagonal, with a 9·65, c 15·3 A, and good {0001} cleavage. Reversible water loss and lattice shrinkage occur on heating, the layer thickness (c) decreasing to 12.1 A at 400 °C. For material in equilibrium with air of normal humidity, the composition is probably between CaO. 2SiO2. 1·7H2O and CaO. 2SiO2. 2H2O; Z = 8 for the structural element. New t.g. and infrared absorption data are presented; the infra-red spectrum closely resembles that of gyrolite, but OH ions attached only to Ca appear to be absent. Conditions of formation are discussed; if Z-phase has any stability field, it is below 120 °C. Crystal structures for Z-phase, gyrolite, and truscottite are suggested, based on the known structure of reyerite.

Geochemical Indicator of the Efficiency of Fractionation of the Skaergaard Intrusion, East Greenland

Abstract: SUMMARY. When magmatic fractionation involves the settling and removal of crystals from the body of magma, the efficiency of the fraetionation process may be defined as the degree of separation of the solid from the liquid phase. An expression is given that relates efficiency to the amount of mesostasis, or crystallized trapped liquid, in an igneous cumulate. The uranium contents of samples from a 349-m-long drill-core of part of the lower and hidden zones of the Skaergaard intrusion are used as a quantitative indicator of the amounts of mesostasis in the cumulates. There are marked changes in the amount of mesostasis over the length of the core and the average efficiency of fractionation was 85 ~. CRYSTAL settling is recognized as an important process in the formation of many layered and other igneous rocks (Wager and Brown, I968), and is often considered to operate as the crystallization analogue of Rayleigh distillation (Rayleigh, I896) in that the crystals are effectively removed from the melt as soon as they have formed. This concept has been used by a number of workers in predicting the variations of element concentrations in melt and solid fractions during fractionation, and has enabled conclusions to be reached on the petrogenesis of rock suites (Gast, 1968) or igneous complexes (Irvine and Smith, 1967). The unavailability of appropriate partition coefficient data for most elements has often necessitated the adoption of the assumption that element partition between the melt and a particular mineral phase remains constant during fractional crystallization. Information is often lacking also on the extent of the separation of the liquid and solid phases from each other, i.e. the efficiency of the fractionation process. Our inability to predict how much pore material (i.e. crystallized trapped liquid) a cumulate contains stems not so much from lack of knowledge about crystal packing but about the extent of other processes such as adcumulus growth (Wager, Brown, and Wadsworth, J96o) or postdepositional re-equilibration. An assessment of the amount of pore material or mesostasis in any particular rock can be made from textural and mineralogical evidence (e.g. Wager, 196o ) or, in certain cases, from appropriate geochemical data (e.g. Henderson, 197o). The present evidence (Wager and Brown, I968; Henderson, Mackinnon, and Gale, 1970 suggests that there may be considerable variations in the amount of mesostasis, and hence of fractionation efficiency, from one layer to the next and from one layered intrusion to another. This paper presents an attempt to quantify the efficiency of fractionation during the forma�9 Copyright the Mineralogical Society.

Lower Tertiary Tholeiitic Basalts from Southern New Caledonia

Abstract: SUMMARY. Eocene tholeiitic basalts occur throughout the length of New Caledonia where their emplacement preceded that of the peridotite massifs. In the south, two areas of basaltic rocks are intimately associated with Eocene sediments, which have been overridden by the peridotites. The aphyric, holocrystalline basalts consist of plagioclase, clinopyroxene, titanomagnetite, and minor quartz together with their abundant alteration products, which include epidote and uralite. The chemical compositions of these rocks fail to show agreement with oceanic tholeiitites, as suggested by earlier writers, although strong similarities exist with the basalts of the Papuan Ultramafic Belt. B AS ALTS of Lower Tertiary age are widespread in New Caledonia and constitute one of the major occurrences of this rock type in the south Pacific (fig. I). Routhier (I953) recorded basaltic flows, pillow lavas, pyroclastic deposits, volcanogenic argillites, cherts, and intercalated sediments as outcropping more or less continuously over 2oo km of the east coast of the island between Bourail and Koumac; to these rocks he ascribed a vague Palaeogene age. The structural and stratigraphic relationships of the basalts are important in that these rocks appear to underlie the large, perched, peridotite massifs of the eastern coast of New Caledonia (Routhier, I953; Brothers and Blake, I973; Rodgers, 1974). In fact, Brothers and Blake (I973) regard the basalts as a structural, rather than a stratigraphic unit, but Guillon and Routhier (I97I) point to difficulties in interpreting these rocks as allochthonous (i.e. a nappe). In the south of the island Guillon (I969) and Guillon and Routhier (I97I) have mapped less conspicuous related volcanics, which occupy a similar structural position beneath the peridotites of the vast Massif du Sud. The present account is largely concerned with these little-known basalts. A third, important group of related Tertiary basalts have been recognized by Brothers and Blake (1973) as being associated with blue-schist metamorphics of their 'melange zone' in central and western New Caledonia.

Volatility of Oxides from Silicate Melt and the Origin of Moldavites

Abstract: SUMMARY The volatilities of SiO~, Na~O, and Fe~Oa x increase with increasing thermal function within the range 1600 to 2100 ~ during the melting of a sandy-silty clay The remaining melt is to some extent enriched in Al~O3, K,O, CaO, and MgO Chemical analyses of the sandy-silty clay, of the glasses melted from the latter, of the silicate material evaporated and newly condensed and their correlation with increasing thermal function and bulk density of glasses, as well as with the hitherto known quantitative data on moldavites, testify to the fact that in moldavites evaporated and newly condensed silicate material substantially prevails over material that passed directly from a solid to a fluid state Evaporation of the silicate material from the rocks of the impact crater caused a significant change in relations between major oxides compared to the original relations in the source rocks Thus, the hypothesis that tektites are directly remelted rocks of the Earth's crust, based on over-all chemical similarities of tektites and some rocks in major oxides, appears incorrect

The crystal structure of schairerite and its relationship to sulphohalite

Abstract: The crystal structure of schairerite from Searles Lake, California, has been determined employing X-ray diffraction data collected on a single-crystal diffractometer. The crystal structure was refined by least-squares methods employing isotropic thermal parameters to a final R index of 0·07 for 2536 independent observed reflections. The cell content is 3[Na21(SO4)7F6Cl]. The space group is P31m with a 12·197 A and c 19·259 A. Schairerite exhibits a marked sub-cell (a 7·042 A, the same c axis and P3m1 symmetry), which may be related to the unit cell of sulphohalite when described in a hexagonal lattice. The crystal structure of schairerite may be considered as consisting of seven sheets of Na+ ions perpendicular to the c axis. These sheets are connected to each other .building up a three-dimensional framework. The Na+ ions in these sheets are arranged in an array built up of hexagons and triangles. Sulphur atoms lie in the sheets at the centres of each hexagon, the halogen atoms lying between the sheets midway between the centres of two triangles. A comparison with sulphohalite shows that the close lattice analogies may be related to a similar atomic arrangement. Apart from the differences in chemical formula (F:C1 ratio 1:1 in sulphohalite), the main difference in the structural framework consists of the unequal number of Na+ sheets (six in sulphohalite) and in the SO4 2− tetrahedra orientation.

Microstructural Interpretation of some Fibrolitic Sillimanite Aggregates

Abstract: Fibrolitic sillimanite in some high-grade gneisses from Broken Hill, Australia, shows microstructural features suggestive of compatibility with the main coarser-grained assemblage, namely: framework-silicate interfaces either meet (iio)-sillimanite/framework-silicate interfaces at approximately 9o ~ or are attached to the ends of 'fibrolite' rods; framework-silicate interfaces are deflected by 'fibrolite' grains; and interfaces of inclusions of one framework silicate in another are deflected by 'fibrolite' grains, producing irregular, rather than rounded, inclusion shapes. These criteria suggest that the 'fibrolite' in some rocks is a compatible member of the main metamorphic assemblage. Its persistence as small grains may be due to a high proportion of relatively low-energy prismatic boundaries in the fibrolitic aggregates.

Mercury in rocks and minerals of the Skaergaard intrusion, East Greenland

Abstract: SUMMARY. Mercury has been determined by radiochemical neutron-activation analysis in nineteen representative rocks from the Skaergaard intrusion and in the separated cumulus minerals of five. The chilled marginal gabbro contains o'23 ppm Hg, values in the Layered Series rocks ranging from o'o7 to o'34 ppm, in one exceptional case reaching I "23 ppm Hg. There is a tendency for mercury to be more abundant in leucocratic than in average or melanocratic rocks at a similar horizon. Somewhat higher levels of mercury are found in the granophyric rocks at the top of the intrusion. Apart from a weak and sporadic tendency to be preferentially enriched in plagioclase, the distribution of mercury between the various cumulus phases is fairly uniform and it is thought to occur as uncharged atoms mainly occupying spaces resulting from lattice defects and imperfections rather than in specific structural sites. The distribution pattern of mercury in the intrusion appears to be determined by its high volatility and chemical inertness; no real evidence of any chalcophile character is observed. MOSTLY due to the lack of reliable analytical data, little is known of the behaviour of mercury in igneous rocks; earlier spectrographic and colorimetric determinations could be regarded at best as semi-quantitative. In the present work, neutron-activation analysis has been used, the rocks and minerals of the Skaergaard intrusion providing a useful suite of samples in which to study the behaviour of mercury during the fractional crystallization of a basic magma. The composition of the initial Skaergaard magma (Wager and Brown, 1968) is considered to be that of a dry, highly reduced, tholeiitic basalt with a high alumina content. The chilled marginal gabbro, number E.G. 45o7 from the Southern Border Group of the intrusion, has been widely used as representing the best approximation available to the initial magma, but its composition is in some ways anomalous and its petrogenetic status must therefore be regarded with caution. Within the estimated 50o km 3 of tholeiitic magma, extreme crystal fractionation produced rocks ranging in composition from ultrabasic to acid (although extensive contamination by the gneisses Of the local metamorphic basement complex has probably played a significant part in the development of the latest acid fractions). Wager (see, for example, Wager and Brown, 1968) estimated on the basis of field observations, chemistry, and mineralogy of the rocks that some 7o % of the lower and earlier products of solidification of the magma remain unexposed at the present level of erosion. Chayes (197o) presented cogent arguments for raising the estimate of this Hidden Zone to 80 %; while more recent and as yet unpublished geophysical and geochemical work by McBirney, Goles, et aL (pers. comm.) casts doubt upon the �9 Copyright the Mineralogical Society,