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In November of 2020, an exciting occurrence of epidote pseudomorphs after orthoclase was rediscovered in the Orogrande district in southern New Mexico (fig. 1). Considering that propylitic alterati...

We present the results of an optical microscopy, cathodoluminoscopy and isotopic study on nine glendonite concretions (calcite pseudomorphs replacing metastable hexahydrate ikaite) from Low...

Gold has been chemically mobilised by groundwater from host sulphide minerals in orogenic gold deposits of Otago. Mobilisation occurred near the Cenozoic Otago Schist erosional surface beneath a sedimentary cover. Initial Au mobilisation, on a scale of micrometres, occurred when solid solution and microparticulate gold in pyrite and arsenopyrite grains were liberated by sulphide oxidation to iron oxyhydroxide pseudomorphs. Larger-scale mobilisation involved leaching of gold from up to 100-m-thick zones which were the target of historic mining, with up to 10× enrichment of Au in reprecipitation zones. Gold in the supergene zones is commonly crystalline with octahedral shapes and nuggety forms which fill cavities and coat prismatic quartz crystals. This gold retains some or all of the Ag (typically 2–8 wt%) from the primary source gold. Oxidised groundwaters that have interacted with sulphides become enriched in dissolved sulphate, but retain high pH (7–8.5). Under these conditions, metastable thiosulphate ...

The presence of Mn-Fe nodules in the epipedons (surface horizons) of paleosols of presumed Upper Neogene age in the northwestern Venezuelan Andes have been interpreted as products of inorganic oxidation and reduction processes operating over the full range of glacial and interglacial cycles that affected paleosol morphogenesis. New microscopic/chemical data from combined SEM-EDS-FIB analyses of representative Mn-Fe nodules indicate microbes play an important role in Mn/Fe precipitation leading to their genesis in alpine Mollisols (Argiustolls). Although the prevailing new data are based mainly on fossil forms of filamentous bacteria and fungi and other biogenic pseudomorphs that may represent the former resident bacteria, the presence of extant microbes must await field experiments/collection, followed by a molecular microbiology approach to determine the biological drivers of metal precipitation. As in other terrestrial niche environments, microbes are seen here to play a role, perhaps a key one, in the ...

Calcite pseudomorphs after gypsum are observed in a patch reef in the upper Slite Group (Wenlock) on north-east (NE) Gotland (Sweden). The idiomorphic pseudomorphs occur (a) in silt-sized carbonate sediment in former moldic pores, (b) in the micritic matrix of bioclastic rudstones and (c) in the thrombolitic matrix of the reef. The dominance of stenohaline organisms in the reef indicates normal marine salinities during reef growth. The development of evaporitic conditions and the formation of evaporitic minerals therefore postdate reef growth. The authigenic gypsum crystals are interpreted to be formed by downward percolating hypersaline brines which existed on an extremely shallow carbonate platform during the early Homerian (latest Cyrtograptus lundgreni graptolite Zone) regression.

10.1080/00357529.2012.677374-F0001 Figure 1. Pair of quartz crystal rosettes (the largest is 2.5 cm across), from the Battle River area, northeast of Donalda. Note the progression from small drusy ...

Coesite-bearing eclogite samples from the Chinese Continental Scientific Drilling main hole (100-2000 m) were examined. Three major evolutionary stages are distinguished from mineral assemblages and textural relations. (1) The pre-peak stage is indicated by inclusion assemblages in rutile eclogite such as amphibole and paragonite, with pre-peak P-T conditions of ~13 kbar and ~910°C. (2) The peak ultrahigh-pressure (UHP) metamorphic stage is characterized by the mineral assemblage garnet-omphacite-phengite-rutile-apatite-coesite/quartz pseudomorph ± kyanite; P-T conditions reached ~910°C and ~37 kbar for rutile eclogite, and ~850°C and ~35 kbar for phengite eclogite. (3) The retrograde stage produced decompression textures via back reactions, such as thin coronas of amphibole or zoisite on garnet, fine-grained amphibole-plagioclase symplectites on omphacite and biotite-plagioclase, or K-feldspar-albite on phengite, ilmenite or titanite on rutile, at P-T conditions of 530-560°C and 7-9 kbar. The presence of...

(2006). Sakura Ishi (Cherry Blossom Stones): Mica Pseudomorphs of Complex Cordierite-Indialite Intergrowths from Kameoka, Kyoto Prefecture, Japan. Rocks & Minerals: Vol. 81, No. 4, pp. 284-292.

(2006). Encrustation Pseudomorphs of Prehnite after Laumontite from St. Louis and Lake Counties Minnesota. Rocks & Minerals: Vol. 81, No. 2, pp. 136-140.

A retrogressed eclogite from the Lotru Metamorphic Suite in the South Carpathians containing peraluminous sapphirine in the fine-grained breakdown assemblage replacing kyanite was subjected to detailed petrographic and analytical studies. Textural analysis reveals that sapphirine is not directly related to decomposing kyanite; instead it grew by digestion of former spinel-bearing symplectites, which, together with corundum + plagioclase intergrowths, replaced kyanite as a first step. Initial kyanite breakdown to plagioclase, spinel, and corundum occurred through metasomatic addition of Ca, Na, Mg, and Fe, while Al and Si behaved as immobile components at the scale of the aggregate. The mechanism responsible for the breakdown was fluid-mediated intergranular diffusion of the mobile elements. All kyanite pseudomorphs exhibit a domain structure containing the alternative corundum + plagioclase or spinel + plagioclase two-phase assemblages, bounded by monomineralic plagioclase seams, across which short-scale ...

Ultrahigh-pressure eclogites are metabasic rocks that have equilibrated at pressures within the coesite P-T stability field. This paper reviews their occurrences and petrographic characteristics in exhumed ultrahigh-pressure metamorphic terrenes within certain major Phanerozoic continental plate—collision belts. Unequivocal identification of ultrahigh-pressure eclogites depends on the presence of relict coesite or of polycrystalline quartz pseudomorphs after earlier coesite. However, preservation of such distinctive petrographic evidence is at best sporadic, leading to serious problems over whether or not particular eclogite samples have experienced “peak” pressures consistent with coesite stability. Accordingly, we also review here other petrographic, mineral-assemblage, and mineral-chemistry features that may aid in the identification of ultrahigh-pressure eclogites. In addition, we discuss the quantitative evaluation of P-T conditions for formation based on various reaction equilibria, including the do...

The Maksyutov Complex consists of two juxtaposed lithotectonic units—Unit #1 of probable Late Proterozoic formation age, and Unit #2, apparently generated in Cambro-Ordovician time. The eclogite-facies metamorphism of Unit #1 occurred prior to 370-380 Ma, when this unit was subjected to blueschist-facies overprinting. Unit #2 displays the effects of a somewhat similar blueschist- or high-pressure greenschist-facies recrystallization, indicating that it may have been metamorphosed contemporaneously with Unit #1. Our field work and geochemical studies have focused on the Sakmara River area. Preliminary conclusions are as follows: (1) Unit #1 was subjected to metamorphic temperatures of 620 ± 70° C and minimum pressures of 1.5 GPa, or 2.7 GPa if the previously reported interpretation of coesite pseudomorphs from similar rocks exposed near the village of Shubino, 75 km to the south (Chesnokov and Popov, 1965), is correct. Peak metamorphic pressures would have reached at least 3.2 GPa if blocky graphite descri...

The Ose Thrust of northern Norway represents a major late Caledonian out‐of‐sequence thrust which duplicates a previously assembled nappe stack with inverted metamorphic gradient. Fluid flow along this thrust gave rise to retrogression of peak metamorphic porphyroblastic phases in both footwall and hangingwall. Chlorite pseudomorphs after garnet are not smeared out suggesting that retrogression largely post‐dates thrust movement. Dominated by vein quartz, the 0se Thrust and adjacent footwall and hangingwall rocks record three stages of veining in a protracted history of fluid flow; schistosity‐parallel, prograde and peak metamorphic veins (VA‐VB), and retrogression‐related (post‐D3) veins which cross‐cut the regional S2 schistosity (Vc). Vc veins contain similar highly saline fluid inclusions to those associated with early ductile thrusts from the region. This indicates that various thrusts in the north Scandinavian Caledonides acted passively as important fluid conduits for late‐stage fluid flow and retr...

Hollow impressions with sharp boundaries reflecting the morphologies of a tree trunk, long sticks, and diverse small wood fragments appeared embedded in the 46–49,000-year-old travertine platform of Level I at the Romani Rockshelter. Biodegradation of the wood may have occurred after a rapid overlaying of the bark with calcite concretions, generated by biogeochemical agents influenced by the favorable environment of the site.

Coprolite‐like siderite masses from paludal deposits of the Miocene‐Pliocene(?) Wilkes Formation near Toledo, southwestern Washington, have been studied since 1939. Over the years, numerous fossil supply houses have also sold them as vertebrate coprolites. The origin of these curious objects has been attributed to formation as speleo‐thems, pseudomorphs of bryozoa, and as feces of vertebrates, including reptiles, fish, and mammals. No vertebrate fossils have been recovered from the Wilkes Formation, although plant fossils and other organic debris are common locally. Evidence cited in support of a fecal origin include the extruded shape, the variation in shape corresponding to viscosity variations, the limited length or quantity of material, and anal extrusion striations (Amstutz, 1958). A new hypothesis for the origin of these so‐called coprolites is proposed, relying on organic‐rich sediment, but inorganic processes. In this scenario, organic‐rich silt and clay was forcefully injected into hollow wood fr...

(1997). Connoisseur's Choice: Hemimorphite Pseudomorphs after Calcite: Joplin, Missouri. Rocks & Minerals: Vol. 72, No. 6, pp. 384-386.

The beryllium silicates bertrandite, euclase, and phenakite have been identified replacing primary beryl in a small alteration zone in the upper part of the Sels‐Vitberget pegmatite. Within a few square metres of outcrop, beryl crystals intergrown with microcline perthite are more or less completely altered to a coarsely crystalline and drusy mixture of Be silicates and quartz. Some seriate and minor apatite also occur in this association. The beryl pseudomorphs exhibit a diffuse zoning, with the core portion dominated by the most common Be silicate, bertrandite. Euclase occurs in lesser amounts, often intergrown with bertrandite. Phenakite is more rare and generally confined to finer‐grained drusy border rims of the former beryl crystals; the texture indicates that phenakite is not in equilibrium with bertrandite + euclase (+ quartz). The formation of Be silicates at Sels‐Vitberget is apparently related to the percolation of gradually evolving, late‐stage, acid to neutral fluids on a local scale...

Fluid inclusion homogenisation temperatures from the magnesites of the Celia Dolomite and the Coomalie Dolomite of the Rum Jungle area of the Pine Creek Geosyncline, N.T., indicate that temperature is a major control of the two distinctive morphological forms. The rhombohedral form recrystallised in association with fluids that homogenised mainly at temperatures less than 150°C, whereas the bladed (tabular) form developed by recrystallisation at a higher temperature. Previously the two forms have been interpreted as pseudomorphs after halite and gypsum, respectively.

(1989). Goethite Pseudomorphs after Marcasite. Rocks & Minerals: Vol. 64, No. 2, pp. 124-127.

Christensen, Leif: Faststaende og omlejrede Saale-moranelersaflejringer. Et eksempel fra Szdding-omradet, Vestjylland. Geografisk Tidsskrift 82: 91–94. Copenhagen, Dec. 1, 1982. In situ clayey till materials on the Skovbjerg Bakkea, western Jutland can be recognized by crop-marks revealing pseudomorphs of ice-wedge polygons in orthogonal, random orthogonal and hexagonal patterns. The tills inside these polygonal patterns have high shear-strength values as measured by vane tests. Remoulded fossil solifluction deposits of till origin reveal stripes and deformed polygonal patterns in the crops. High strength as measured by vane are encountered below fossil solifluction deposits 4 meters below ground.

The building of a water-power station with appendant tunnels of considerable length at Bergeforsen, about 6 km North-West of the Alno alkaline intrusion, has revealed a technically vexing continuation of the upper cone-sheets and radial dikes (Cfr 1, p. 85). Where the dikes taper out protracted fissures running for considerable distances through the gneiss-granite-country rock have been the conduits of peri-magmatic gases, mainly CO2 and H2O. These have altered the oligoclasc into a mixture of calcite and montmorillonite of very low water-content. When uncovered and shattered by blasting, the rock is exposed to the humidity of the air and even to water. By adsorption the montmorillonite of the oligoclase pseudomorphs expands, the ensucing strain leading to a total crumbling of the rock within the short time of 2–6 weeks. The expansive force of the totally altered gneiss, containing about 30 mol% of montmorillonite, was theoretically calculated at 1.1–1.6 kg/cm2, the pressure increasing synchronou...

Sungulite is variety of serpentine finely to coarsely flaky, white with a silky luster. The term should be applied to coarse tabular serpentine (antigorite) in pseudomorphs on vermiculite. It is formed out of vermiculite and is surficial in origini occurring as a gradual replacement of vermiculite in lower parts of ancient weathering crusts of magnesian rocks with ferro-magnesian micas. It appears that the genetic relationships between kaolinite and halloysite are quite similar to those between sungulite and chrysotile. — J. J. Finney.

(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.

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