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Cherty iron-formations (= ferriferous cherts) have sedimentary textures analogous to the sedimentary textures of limestones; it is possible to define textural rock types analogous to the micrites, intramicrites, oosparites, etc. of limestone classification. The close analogies between limestone textures and the textures of cherty iron formation permit paleogeographic interpretation of Precambrian iron formation by comparison with the textural facies of Recent limestones. The Sokoman Iron Formation of the Central Labrador trough, Northeastern Quebec, Canada, is subdivided in five members on the basis of composition; these can be subdivided in sub-members distinguished by their textural properties. Comparison of the distribution of facies types of the Sokoman Iron Formation with Recent limestone environments suggests deposition in two basically different environments: (1) A lagoonal platform probably composed of rapidly shifting banks of oolitic and intraclastic sand, of mud banks, and of small, lagoonal basins. (2) A basinal environment where relatively monotonous fine grained intraclastic sands, and rocks analogous to micrites were deposited. A narrow domain of oolite shoals separates both environments. Traces of evaporites (length-slow chalcedony and pseudomorphs after gypsum) prove deposition of the Sokoman Iron Formation in a warm and comparatively dry climate.

Mining operations in the Permian Salado Formation for the Waste Isolation Pilot Plant (WIPP) reveal filled pits in halite beds which are interpreted as synsedimentary dissolution features. The pits range from a few centimeters to as much as 1 m wide and 1 m deep, and they clearly truncate thin, underlying beds. Halite hopper crystals and possible chevron halite structures occurring in the pits and continuous, undeformed bedding in overlying units demonstrate that the pits formed and filled before deposition of the overlying unit. Pseudomorphs of halite after gypsum swallow-tail crystals are part of the evidence that the dissolution pits formed in shallow water.

The origin of albitized grains in sandstones has been the topic of considerable discussion in recent years. Some workers believe that the albitization of feldspar can occur in situ during burial diagenesis, but others argue that albitization is a feature inherited from source clastic grains. Our petrological and chemical studies of subsurface samples of the Jurassic, Lower Cretaceous, and Tertiary reservoir sandstones from offshore Norway show that detrital grains of potassium feldspar have been albitized during burial diagenesis. Criteria suggesting that the albitization has occurred during burial diagenesis include 1) euhedral habit of albite crystals with sharp edges and comers and markedly smooth crystal faces; 2) generally untwinned albitized grains, mostly riddled with minute br wnish inclusions; 3) lack of cathodoluminescence in albite; 4) homogeneous and pure albite composition (> 99% Ab); 5) absence of albitized grains in carbonate-cemented zones; and 6) increase in the percentage and degree (partial to complete) of albitized K-feldspar with depth. Under SEM, albitized grains display two types of fabrics. Type I albitized grains are composed of numerous tiny albite crystals (1-30 µm) growing along cleavage planes in parent grains, and they have abundant dissolution porosity, while Type II albitized grains show blocky albite crystals (5-80 @m) forming pseudomorphs of detrital K-feldspar, and they lack any dissolution porosity. Type I albitized grains are found commonly at shallower depths (between 2.2 and 3 km and temperatures varying from 65° to 90°C), whereas Type II albitized grains dominate at greater depths (> 3.5 km and at temperatures > 90°C). Albitization proceeds by a dissolution-precipitation mechanism. At shallower depth, dissolution of the parent K-feldspar is more rapid than the precipitation of albite, and rates of albite precipitation increase with depth, favoring pseudomorphic replacement of K-feldspars. Other factors that influence albitization processes are aNa+/aK+, permeability reduction, and surface-dissolution kinetics of the parent grain.

Conversion of carbonate grains to fluorite gives a microporous pseudomorph in which the contained organic material may be dyed and observed. Photographs show the application of this technique to in-situ observation of perforating algae and other organisms in Bahama oolites. Other possible applications are to ancient carbonates and to carbonates trapped by blue-green algae.

In the Monac trachyte dome (Eastern Velay, French Massif Central), the destabilization of amphibole phenocrysts produces coronitic pseudomorphs. These structures exhibit an inner feldspathic corona (sanidine) at the edge of the relic amphibole, mantled by an outer corona which consists of a more complex association of opaque minerals (titanomagnetite) and clinopyroxene (salite) in contact with the groundmass. These minerals preserve strictly the initial amphibole outline and result from the following magmatic reaction : kaersutite + liquid 1 → salite + sanidine + titanomagnétite + liquid 2 Volumic proportions of minerals produced by this reaction were estimated and mass balances have been established. They show that the reaction is isovolumic and involves the following element exchanges with the magmatic liquid. — Increase of Na, Al, Mg, H in liquid 2. — Loss of Fe, Mn, Ti from liquid 2. Si, O, K and Ca are retained by the amphibole pseudomorph products., Dans le trachyte du Suc de Monac (Velay oriental) la déstabilisation des phénocristaux d'amphiboles (kaersutite) se traduit par des pseudomorphoses coronitiques. Ces structures comportent une première couronne de feldspath (sanidine) qui borde l'amphibole. En contact avec la mésostase, une seconde couronne, plus complexe, constituée de minéraux opaques (titanomagnétite) et de clinopyroxène (salite), lui succède. L'ensemble de ces minéraux préserve parfaitement l'ancien contour de l'amphibole et résulte de la réaction magmatique suivante : kaersutite + liquide 1 → salite + sanidine + titanomagnétite + liquide 2 Les proportions volumiques des différentes phases impliquées dans cette réaction ont été estimées et des bilans de matière ont été réalisés. Ils montrent que la réaction s'effectue à volume constant et traduit, pour le liquide magmatique, les échanges d'éléments suivants : — Apports de Na, Al, Mg, H. — Pertes de Fe, Mn, Ti. Les éléments Si, O, K, Ca sont conservés par les produits de pseudomorphose de l'amphibole., Cotonian Catherine, Potdevin Jean-Luc, Bertrand Hervé, Lombardo Nicole. Pseudomorphoses coronitiques d'amphibole dans le trachyte de Monac (Velay oriental). Bilan de matière et réaction magmatique. In: Bulletin de Minéralogie, volume 111, 1, 1988. pp. 89-95.

Petrographic study of crystal textures and fabrics, both original and replacement, has revealed the postdepositional history of carbonate and sulfate minerals in the Silurian Tonolway Limestone. Crystal forms, assumed to have been gypsum originally, are locally common in supratidal facies; they occur as coarse euhedral needles, randomly oriented to bedding, and commonly in clusters. When this gypsum precipitated interstitially in the lime sediment, micrite was often incorporated into the crystals; organic matter, concentrated on side faces, may have blocked nucleation, causing the fibrous crystallization texture. As a consequence of the changing chemistry of pore fluids, a series of mineral replacements began within these crystal forms. Gypsum was first replaced by celest te, as evidenced by its pseudomorphism. The enclosing aragonite sediment inverted to calcite, and strontium was thus freed for the making of celestite. Afterwards, calcitization of the sulfates took place, i.e., each crystal was replaced by a mosaic of very finely crystalline calcite. Calcitization began around inclusions of micrite within the crystal forms. Presumably, decomposition of algal material produced carbon dioxide that was used for the calcitization reaction. Sulfate ions released by this last replacement inhibited the growth of calcite, resulting in its very fine crystallization fabric. Lastly, these calcite "pseudomorphs" were themselves partly replaced by limpid dolomite. End_of_Article - Last_Page 787

Magadi-type cherts occur in lacustrine rocks of the Middle Devonian (Middle Old Red Sandstone) Orcadian Basin, Scotland. Chert nodules are replacive within organic-rich laminites, the deposits of a stratified lake. They exhibit soft-sediment deformation, including sediment injection along shrinkage fractures within the nodules. The thefts preserve stellate quartz pseudomorphs after an evaporite mineral, which may have been a sodium carbonate mineral (?trona). These are features common to other documented occurrences of Magadi-type cherts. Nodules were nucleated about vertical cracks in the host sediment and show polygonal or parallel-aligned patterns. Precipitation of the magadiite precursor occurred from alkaline groundwaters rather than in the lake water column.

Two superposed diamictites in the middle part of the Itarare Subgroup, cropping out along highway SP-280 in central-eastern Sao Paulo State, are separated by a boulder pavement associated with a disconformity at the top of the lower diamictite. Wedge-shaped bodies up to 3 meters in height made up of sandstone, conglomeratic sandstone, and admixed sandstone and diamictite, taper downwards from the disconformity into the lower diamictite. Other channel-like bodies of sandstone occur at the top of or enveloped by the lower diamictite. The former of these structures are interpreted as possible pseudomorphs or casts of ancient ice-wedges developed in frozen ground during the Gondwana glaciation. The latter probably include subglacial and surface channels eroded and filled by fluvio-glacial streams. These features suggest the occurrence of a rigorous periglacial climate in this part of the Parana Basin during the Late Paleozoic.

During low-grade regional metamorphism, pyrrhotite can form from gypsum by the reaction: CaSO4·2H2O+Fe sol. 2+ +2Corg.→FeS+Ca sol. 2+ +2H2O+2CO2. This reaction takes place in the anchizone, below ∼350°C and might be initiated by the thermal dehydration of gypsum (∼200°C) and aided by the generation of gaseous hydrocarbons. Evidence for the reaction is the occurrence in dolomitic layers in the Ballachulish Slate, East Larroch quarry, Argyll of lath and swallow-tail shaped quartz+dolomite+pyrrhotite pseudomorphs after gypsum. Quartz+pyrrhotite bodies in slate represent replaced and deformed (mainly flattened) crystals, concretions and possible veinlets of gypsum. Pyrite porphyroblast growth, after the peak of metamorphism and under relatively high fS2 conditions, failed to destroy some early pyrrhotite because it is encapsulated in quartz. Pyrite-silicate reactions and hydrothermal exhalations have been suggested previously to account for pyrrhotite-enriched horizons in regionally metamorphosed rocks. Replacement of gypsum by pyrrhotite is an additional explanation for pyrrhotite-enriched horizons, especially in dolomitic and graphitic lithologies.

The paper describes some sedimentary structures, found beautifully preserved in the rocks of the great Vindhyan System of India. These consist of discoidal ferruginous markings, algal structures, salt pseudomorphs, conical structures, stylolites, load-casts, flute-casts, gas-pits, and worm-tracks. The conditions for the deposition of the Vindhyan sediments are briefly described. This study is probably the first of its type of the sedimentary formations of India.

1.(Near Potosi, p. 586.) This rock has been much altered. The following constituents are present :—( a ) crystalline grains composed of earthy-looking granules and minute, rather fibrous microliths, showing light colour with the two Nicols, being one of the decomposition pseudomorphs after felspar, often called saussurite ; ( b ) augite and diallage, more or less converted into hornblende and a chloritic mineral; ( c )iron peroxide, probably ilmenite; ( d ) a little apatite. The general aspect of the saussuritic grains suggests that the felspar has been plagioclastic. Of the pyroxenic mineral the greater part is altered, being converted into hornblende with strong dichroism and very characteristic cleavage, associated with which are smaller more fibrous forms, some of which are probably varieties of hornblende, others rather more resemble chlorite. Portions of the diallage and augite remain unchanged in many of the grains; both minerals appear to be present. One or two serpentinous-looking aggregates may possibly replace olivine. We may venture to say that the rock has once been a gabbro, in which the pyroxenic constituent has been almost wholly converted into hornblende, as has not seldom happened*. 2.(Quacipati, p. 585.) The ground-mass of the rock consists chiefly of minute grains of quartz, looking as if fused together. In this are scattered many small grains of epidote and scales of mica. Of the latter there appear to be two kinds—one having its darkest tint an olive-green, rather strongly dichroic ; the other nearly colourless, and showing brilliant tints with the two Nieols

Individuals of the diatom Coscinodiscus have been treated hydrothermally in the presence of lime and carbon dioxide with the result that they have been pseudomorphed by calcite. The silica of the original diatoms has been replaced while the intricate reticulations of the alveoli have been preserved down to submicroscopic detail.

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Summary A short description is given of the lower part of the Oligocene sediments outcropping in the province of Navarra (Northern Spain) near the village of Liedena. One of the salient features is the presence of graded beds bearing flute casts in a continuous section which also contains layers revealing evidence of evaporitic conditions. The sediments in question form the basis of the huge molassic fill of the Ebro basin and should not be considered a flysch in spite of their rhythmic aspect and local grading.

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Fossil wood composed of tridymite is abundant in Patuxent (Lower Cretaceous) arkose on Hazel Run, Fredericksburg, Virginia. X-ray diffraction studies of the tridymite indicate that it has a disordered structure in which hexagonal close packing predominates. The specimens, which contain trace amounts of aluminum, iron, and other elements, are soft and fibrous, varying from white to shades of brown.

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Oriented cores for a paleomagnetic investigation were collected from ten sites in the sedimentary redbeds of the Salt Pseudomorph Beds of Middle Cambrian age m the Salt Range near Khcwra. All samples were subjected to progressive, thermal demagnetization procedures which revealed the characteristic direction of magnetization. The material of ten sites with a total number of 71 specimens was included in the analysis, giving an overall mean direction with D = 215.9 °,I = +42.4 °, C,9s = 13.7 '~. In the ultimate analysis the material of six sites was included, because four site~ shox~ fairly strong deviations from the mean direction. The virtual magnetic pole position is located at 26.6 ° S, 33.5 ° F, with A9s = 5.1 °. A Phanerozoic polar wander curve is given for lndiaq'akistan. The result presented here supports the view that in Gondwana reconstructions the west coast of-the Indian subcontinent is best fitted alongside the northeast coast of Africa.

Vat dyes, containing eight or nine aromatic rings can be regarded as model substances for “coal molecules”. Specially prepared in turbostratic arrangements they give diffraction patterns similar to some high rank coals. Grown as single crystals with known crystal structure they provide an exactly known starting point for carbonization studies. Under special conditions the chars formed are pseudomorphs of the original crystals. The orientation and the size of the layers formed during carbonization can thus be compared with those in the original dye. Further information can be obtained from the line broadening of the (002) and (hk) reflexions, from density determinations, and from the weight losses and chemical analyses after carbonization. The results described refer mainly to flavanthrone (C28H12O2N2) carbonized at 500°C. Because of the difficulty in obtaining large enough crystals, the information was supplemented by the carbonization under similar conditions of pelleted powders of the recrystallized dye.

The formation of ordered particle arrays plays an essential role in nanotechnology, biological systems, and inorganic photonic structures in the geosphere. Here, we show how ordered arrays of amorphous silica spheres form in deeply weathered lithologies of the Great Artesian Basin (central Australia). Our multi-method approach, using optical and scanning electron microscopy, X-ray microdiffraction, Raman spectroscopy, and electron probe microanalysis, reveals that particle morphologies trace the flow of opal-forming colloidal suspensions and document syn- and post-depositional deformation. The micromorphology of amorphous silica pseudomorphs suggests that the volume-preserving replacement of non-silicate minerals proceeds via an interface-coupled dissolution precipitation process. We conclude that colloid flow and post-depositional shearing create but also destroy natural photonic crystals. Contrary to previous studies, our results indicate that purely gravitational settling/ordering is the exception rather than the rule during the formation of three-dimensional periodic sphere arrays in the highly dynamic colloidal suspensions of chemically weathered clastic sediments.

Zirkelite, a cubic mineral of general formula (Ti,Ca,Zr)O2−x occurs as an accessory mineral in phoscorites, carbonatites and associated rocks in the Sebl'yavr complex. After pyrochlore, it is the main mineral concentrating niobium in these rocks. Zirkelite forms platy crystals comprising polysynthetic octahedral twins, often having a complex skeletal internal structure within a cuboctahedral external morphology. The mineral is largely metamict, but after heating to 800°C it produces a cubic crystalline structure with ao = 5.157 ±0.006 Å. It is black to brownish-black in colour, VHN = 760−780 kg.mm−2, density = 4.27 g.cm−3, and reflectance = 12.5%.Chemically, zirkelite is relatively enriched in Nb2O5 (up to 14.5 wt.%) and ThO2 (up to 7.7 wt.%), and it displays four compositionally-distinct zones which probably formed during primary crystallisation processes. It is patchily altered where it is associated with an unidentified Ba, Ti, Nb, Zr silicate phase which partly replaces and pseudomorphs it. Under the current IMA-approved nomenclature scheme, non-crystalline (metamict) minerals of the composition described here would normally be given the general species name zirconolite, with the name zirkelite confined to the cubic mineral. However, at Sebl'yavr, the name zirkelite is used because the mineral displays a well-defined cubic crystal morphology and has a cubic structure after heating.

The vein-type tungsten deposit at Nyakabingo in the central Tungsten belt of Rwanda is located in the eastern flank of the complex Bumbogo anticlinal structure. The host rock is composed of alternating sequences of sandstones, quartzites, and black pyritiferous metapelitic rocks. Two types of W-mineralized quartz veins have been observed: bedding-parallel and quartz veins that are at high angle to the bedding, which are termed crosscutting veins. Both vein types have been interpreted to have been formed in a late stage of a compressional deformation event. Both vein types are associated with small alteration zones, comprising silicification, tourmalinization, and muscovitization. Dating of muscovite crystals at the border of the veins resulted in a maximum age of 992.4 ± 1.5 Ma. This age is within error similar to the ages obtained for the specialized G4 granites (i.e., 986 ± 10 Ma). The W-bearing minerals formed during two different phases. The first phase is characterized by scheelite and massive wolframite, while the second phase is formed by ferberite pseudomorphs after scheelite. These minerals occur late in the evolution of the massive quartz veins, sometimes even in fractures that crosscut the veins. The ore minerals precipitated from a H2O–CO2–CH4–N2–NaCl–(KCl) fluid with low to moderate salinity (0.6–13.8 eq. wt% NaCl), and minimal trapping temperatures between 247 and 344 °C. The quartz veins have been crosscut by sulfide-rich veins. Based on the similar setting, mineralogy, stable isotope, and fluid composition, it is considered that both types of W-mineralized quartz veins formed during the same mineralizing event. Given the overlap in age between the G4 granites and the mineralized quartz veins, and the typical association of the W deposits in Rwanda, but also worldwide, with granite intrusions, W originated from the geochemically specialized G4 granites. Intense water–rock interaction and mixing with metamorphic fluids largely overprinted the original magmatic-hydrothermal signature.

Micrometeorites are cosmic dust particles recovered from the Earth’s surface that dominate the influx of extraterrestrial material accreting to our planet. This paper provides the first in-depth study of the weathering of micrometeorites within the Antarctic environment that will allow primary and secondary features to be distinguished. It is based on the analysis of 366 particles from Larkman Nunatak and 25 from the Transantarctic Mountain collection. Several important morphological categories of weathering effects were identified: (1) irregular and faceted cavities, (2) surface etch pits, (3) infilled cavities, (4) replaced silicate phases, and (5) hydrated and replaced metal. These features indicate that congruent dissolution of silicate phases, in particular olivine, is important in generating new pore space within particles. Comparison of the preservation of glass and olivine also indicates preferential dissolution of olivine by acidic solutions during low temperature aqueous alteration. Precipitation of new hydrous phases within cavities, in particular ferrihydrite and jarosite, results in pseudomorph textures within heavily altered particles. Glass, in contrast, is altered to palagonite gels and shows a sequential replacement indicative of varying water to rock ratios. Metal is variably replaced by Fe-oxyhydroxides and results in decreases in Ni/Fe ratio. In contrast, sulphides within metal are largely preserved. Magnetite, an essential component of micrometeorites formed during atmospheric entry, is least altered by interaction with the terrestrial environment. The extent of weathering in the studied micrometeorites is sensitive to differences in their primary mineralogy and varies significantly with particle type. Despite these differences, we propose a weathering scale for micrometeorites based on both their degree of terrestrial alteration and the level of encrustation by secondary phases. The compositions and textures of weathering products, however, suggest open system behaviour and variable water to rock ratios that imply climatic variation over the lifetime of the micrometeorite deposits.

Various pseudomorphs after lawsonite occur, together with stable lawsonite, in a decimetric fold sampled in the Corsican Schistes lustrés. Within a same lithology, the nature of replacement products depends on the microstructural location of the pseudomorphs in the fold. Lawsonite is preserved in the hinge zone whereas it is partially or totally pseudomorphosed into kaolinite in the limbs and into white micas farther from hinge. These different assemblages are thought to be coeval but related to distinct mineral-fluid equilibria and to local differences in the activity of dissolved species. Some of these equilibria have been calculated and the conditions of transformation are consistent with independent estimations for this tectonic unit (300 °C, 8 kbar). Differences in the activity of dissolved species between limbs and hinge zone have been evaluated ; they account for the chemical and mineralogical differentiation between the limbs and the hinge zone of the studied fold, by mass transfer through the metamorphic fluids., Des lawsonites et des pseudomorphoses variées coexistent dans un pli décimétrique des Schistes lustrés corses. Les pseudomorphoses partielles ou totales dépendent, pour une lithologie donnée, du site microstructural. Les lawsonites sont quasiment intactes en charnière. Dans les flancs, elles donnent naissance tout d'abord à des assemblages à kaolinite, puis plus loin à des associations à micas blancs. Ces divers assemblages sont interprétés comme le résultat de différents équilibres minéraux-solutions, décrits dans des diagrammes d'activité. Certains équilibres ont été calculés. Les conditions de transformation de l'échantillon sont ainsi compatibles avec celles que l'on a déterminées par ailleurs dans cette unité pour le même épisode tectonique (environ 300 °C, pour 8 kbar). Des différences d'activité des éléments en solution Ca2+, SiO2aq, Na+, Mg2+, H+ sont aussi mises en évidence entre les flancs et les charnières de ce pli mineur. Ces différences rendent compte de la différenciation chimique et minéralogique entre flancs et charnière du pli étudié, par transfert de matière en solution., Sicard Emmanuelle, Caron Jean-Michel, Potdevin Jean-Luc, Déchomets Roland. Transfert de matière et déformation synmétamorphique dans un pli. II. Pseudomorphoses de lawsonite et caractérisation des fluides interstitiels. In: Bulletin de Minéralogie, volume 109, 4, 1986. pp. 411-422.

Serpentinite replacement by carbonates in the seafloor is one of the main carbonation processes in nature providing insights into the mechanisms of CO2 sequestration; however, the onset of this process and the conditions for the reaction to occur are not yet fully understood. Preserved serpentine rim with pseudomorphs of carbonate after serpentine and lobate-shaped carbonate grains are key structural features for replacement of serpentinite by carbonates. Cathodoluminescence microscopy reveals that Ca-rich carbonate precipitation in serpentinite is associated with a sequential assimilation of Mn. Homogeneous δ18O values at the µm-scale within grains and host sample indicate low formation temperature (

SummaryRajite was found at Lone Pine, New Mexico, by R. A. Jenkins. It occurs as small (1.5 mm) crystals in rhyolite with mackayite, and may be a pseudomorph after teineite. Analysis gave CuO 18.4, 15.3, 18.0; TeO2 73.7, 79.8, 79.1; rem. (respectively) 7.9, 2.4, 2.3. Some CaO in samples 2 and 3: 2.5, 0.6%. This gives CuTe2O5, identical to the artificial compound prepared by Moret et al. (1969). Easily soluble in dilute acids; readily fusible.Monoclinic P21/c with a = 6.866 Å, b = 9.314, c = 7.598, β = 109.1°. Strongest lines 3.064(10), 4.654(8), 3.348(8), 3.111(7), 2.744(7), 3.793(6B), 2.844(5), 2.796(5). With Z = 4, ρcalc = 5.77 g/cm3.Crystals Duesbury green (RHS-131D), H = 4, ρmeas = 5.75 g/cm3. Pleochroic in greens γ > β > α; α = 2.115, β = 2.135, ‖ [010], γ = 2.26, α:[001] 22° in obtuse β, 2Vγ = 40°.

(1974). Chalcedony Pseudomorph after Coral or Agatized Coral. Rocks & Minerals: Vol. 49, No. 9, pp. 561-562.

(1977). Peter's Mountain: Hematite and Hematite Incrustation Pseudomorphs after Barite. Rocks & Minerals: Vol. 52, No. 5, pp. 279-280.

The discovery of previously unknown microfossils from the Sinian of western Hubei Province, South China (Fig. 1) is reported here. The Sinian there, up to 1,000 m thick, is naturally divisible into four formations (Fig. 2) from the base upwards: (1) the Liantuo Formation comprising feldspar–quartz–sandstones with basal conglomerates; (2) the Nantuo Formation, which is a tillite; (3) the Doushantuo Formation, which has micro-dolomites intercalated with black shales and cherty beds with some siliceous or phosphatic nodules; and (4) the Dengying Formation, which is mainly dolomites. The rock sample in which the microfossils were found was collected from the lower part of the Doushantuo Formation near the highway side ∼500 m east of Liantuo, Yichang County (Fig. 1). The Doushantuo Formation has been dated as 693±66 Myr (Rb–Sr isochron age)1. Pseudomorphs after gypsum sometimes occur in the lower part. The microfossils are preserved in siliceous nodules, particularly in the cherty portions. The dark colour of the nodules is due to the high organic content.

ITS discoverer, Gustav Tschermak, assigned the name “Maskelynite” to an isotropic substance with the chemical composition of plagioclase, which occurred in the Shergotty achondritic stony meteorite1. He first regarded this as a new cubic mineral, but later, finding it also in several chondrites and realizing that it formed pseudomorphs after plagioclase, he reinterpreted maskelynite as remelted or otherwise transformed plagioclase glass2. Because its amorphous character was not confirmed and no chemical analyses were made of the chondritic variety, however, Tschermak's reassessment of the nature of maskelynite was not universally accepted by meteorite petrographers.

The Owera Mine is opened on a line of lode or reef consisting of small auriferous quartz veins traversing decomposed hyperstheneaugite andesite. The quartz of the veins is highly ferruginous, and full of little patches of brown hæmatite indicating decomposed pre-existing pyrite, remnants of which are occasionally observed. It is in connection with the veins in the soft country rock and near the surface that the pseudomorphs under notice are found, and it seems probable that they owe their origin to infiltration of silica into cavities left by removal of calcite, though none has the actual form of a crystal of that mineral, which, in fact, is quite absent at the place Yet the specimen (No. 2) found with some of the pseudo-crystals tends to support this supposed origin, the hollows and, more clearly, wax impressions of them showing that the removed crystals were scalenohedra.

Cliftonite, a polycrystalline aggregate of graphite with cubic morphology, is known in ten meteorites. Some workers have considered it to be a pseudomorph after diamond, and have used the proposed diamond ancestry as evidence of a meteoritic parent body of at least lunar dimensions.We have synthesized cliftonite in Fe-Ni-C alloys in vacuumn, as a product of decomposition of cohenite [(Fe, Ni)(3)C]. We therefore suggest that a high pressure origin is unnecessary for meteorites which contain cliftonite, and that these meteorites were formed at low pressures. This concluision is in agreement with other recent evidence.

From the specimens of West Indian phosphates and coral I have the privilege of bringing under your notice, I desire to direct your attention to the fact that, under certain conditions, it is possible for coral-stone to be converted into phosphate of lime. Dana, in his work on Corals and Coral-islands, speaking about the formation of Howland9s Island, says: “Some interesting pseudomorphs occur buried in the guano of this island. Coral fragments of various species were found that had long been covered up under the deposit, and in some of which the carbonic acid had been almost entirely replaced by phosphoric acid. In such I have found 70 per cent. of phosphate of lime. In many others the change was only partial, and on breaking some of these, in the centre was usually found a nucleus or core of coral still retaining its original hardness and composition, while the external parts had been changed from carbonate to phosphate, which, though soft and friable, still preserved the structure and appearance of the coral.” My attention was first directed to this fact in the island of Barbuda, where I found a small vein, rich in phosphate of lime, starting from the bottom of a cave in the face of a coral cliff. The earth in this cave consisted of phosphatic guano formed from excrement of bats, and similar in composition to the earth that is generally found in tropical caves. The water draining from this guano-earth must have contained phosphoric acid in

The celestite occurs mainly as nodules or geodes in an outlier of Keuper (Triassic) dolomitic conglomerate, near Llantrisant (Wales). These nodules are pseudomorphs after dolomitic pebbles and were derived by metasomatism associated with descending waters.

SINCE the recovery of vascular flora in the Salt Pseudomorph bed (Cambrian) of the Punjab Salt Range1, we have continued our investigations with other Cambrian beds in the Salt Range, Kashmir, and Spiti.

Heavy mineral placer layers within channel sandstones of the Morrison Formation (western United States) have been shown to contain a very stable, accurate record of the Jurassic geomagnetic field. These laminae are composed almost entirely of hematite grains; in particular, they are largely the hematite pseudomorph of magnetite, martite. The laminae record the correct geomagnetic field direction during a period of rapid apparent polar wander, which suggests an early acquisition of the remanence. Several other remanence characteristics imply that the remanance is probably a detrital remanent magnetization (DRM): (1) there is large directional dispersion in one lamina which had been heavily burrowed by organisms (measurement of individual burrows shows that the burrowing disrupted and randomized the magnetization held in the martite grains), (2) directions of laminae that were deposited on sloping surfaces are significantly deflected from the directions of the flat–lying laminae, (3) more than 50% of the martite grains have high-temperature oxidation features, suggesting that they may have been martite at the time of deposition. These characteristics, in particular the burrowing disturbance, strongly suggest that the martite was deposited as martite and that the remanence is a DRM, as opposed to deposition of magnetite grains which were altered to hematite after deposition. Examination of recent weathering debris on igneous outcrops shows a substantial amount of martite on these outcrops. This observation provides further evidence in support of the idea that hematite, and in particular, martite, can be a deposited mineral in sedimentary rocks and thus can carry a DRM.

A layer of vesicular blue-green glass occurs in the 12th century–B.C. level of Tel Yin'am. The glass was formed by fusion of plant parts rich in opal phytoliths, and is 75% SiO2 with a high K2O/Na2O ratio. Surviving plant structures (now converted to tridymite pseudomorphs) indicate that the material fused was from a grain of some type, perhaps wheat beards. The glass layer overlies nests of probable rodent droppings, now converted mainly to aragonite by the heat of combustion. We believe the glass to be the product of accidental burning of a layer of straw or chaff from threshing, though there is still a possiblity of a deliberate attempt at glass manufacture.

Volcanic sedimentary rocks of the Veija Group of Trans-Pecos Texas contain zeolite, montmorillonite, and silica minerals that formed during diagenesis in an open hydrologic system. Volcanic glass shards dissolved in ground water to provide constituents for authigenic minerals. Diagenic mineral zones, from top to bottom, are (1) montmorillonite-opal-glass, (2a) montmorillonite-opal-clinoptilolite, (2b) montmorillonite-quartz-clinoptilolite, (3a) montmorillonite-quartz-analcime, and (3b) analcime-quartz. During diagenesis the original vitroclastic texture of volcanic sediment was preserved. Montmorillonite formed coatings on glass shards that preserved their outline during replacement by clinoptilolite. Clay or clinoptilolite cement filled much of the remaining inter-granular space. Analcime replaced both pseudomorphs of shards and interstitial cement that first formed as clinoptilolite. All authigenic minerals formed at low temperature and low pressure at burial depths no greater than a few hundred meters. Rocks in each zone were buried less deeply, and boundaries between zones are stratigraphically higher in the northeastern than in the southwestern part of the area. Distribution of clinoptilolite and analcime were controlled locally by permeability of the host rocks. Diagenesis converted sediment with an original composition similar to that of rhyolite into rock composed of Na2O, CaO, Al2O3, and SiO2, but relatively depleted in K2O. Field evidence shows that diagenesis occurred in early Oligocene time, but zeolites began to form only after a critical thickness of a few hundred meters of sediment had accumulated.

Translucent brown aggregates of calcium-carbonate crystals have been found in cores from the Zaire deep-sea fan (west equatorial Africa). The aggregates are well preserved but very friable. Upon storage they become yellowish white and cloudy and release water. Chemical, mineralogical (XRD), petrographical, crystal-morphological, and stable-isotope data demonstrate that the crystals have passed through three phases: (1) an authigenic carbonate phase, probably calcium carbonate, which is represented by the external habit of the present crystals; (2) a translucent brown ikaite phase (CaCO 3 ·6H 2 O), unstable at temperatures above 5 °C; and (3) a phase consisting of calcite microcrystals that are poorly cemented and form a porous mass within the crystal form of the morphologically unchanged first phase. The transformation from the first phase into ikaite was probably a kinetic replacement. The transformation from ikaite into the third phase occurred because of storage at room temperature. The presence of ikaite is indicative of a low-temperature, anaerobic, organic-carbon-rich marine environment. Ikaite is probably the precursor of a great number of porous calcite pseudomorphs, and possibly also of many marine authigenic microcrystalline carbonate nodules.

Saprolite formed by lateritic isovolumetric chemical weathering of granite contains pseudomorphs of primary mineral grains which can be separated and analysed. For most elements the chemical composition of rock and saprolite matches the sum of the contributions by the separated primary mineral grains in the rock and their pseudomorphs in saprolite. This agreement allows an element 'budget' to be calculated, which shows that the behaviour of some elements during alteration of granite is determined by the mineralogy of the primary mineral grains and their various alteration products. Most of the Si, Ba, Sr, Pb and Cu present in feldspars in granite was lost on weathering to halloysite, whereas Ga, Al and V were retained. Most of the Zn, Mn and Co that was present in biotite in granite was also lost during alteration, whereas the pseudomorphs retained Fe, Cr and Ni. Zn, Fe, Mn, Cr and V present in magnetite were wholely retained within the hematite pseudomorphs, whereas Ni was lost. Thus Zn and Mn exhibit a mobile behaviour when initially present in biotite and a residual behaviour when present in magnetite. Ni shows an opposite behaviour; it is mobile in magnetite but residual in biotite. These differences are a consequence of the formation of different alteration products from the primary minerals.

RODWELL and Elder1 have observed microscopically that when yellow orthorhombic crystals of mercuric iodide are touched, the change into the red variety continues through the whole mass of crystals, and the resulting pseudomorph consists of minute octahedrons of the red form.

(1968). Pseudomorphs—An Interesting Phase of Collecting. Rocks & Minerals: Vol. 43, No. 1, pp. 27-27.