The island-arc volcanics situated in the eastern part of the Căpîlnaş-Techereu nappe (South Apuseni Mountains, Romania) were studied to evaluate the temperature, fluid properties, and mineral chemistry during low-temperature metamorphism. Detailed observations of metamorphic mineral assemblages were conducted using powder X-ray diffraction and electron microprobe. The metamorphism involved albitization of plagioclase feldspar and the formation of mafic phyllosilicates, zeolites, and other hydrous Ca-Al-silicate minerals. Mafic phyllosilicates consisted of transitional dioctahedral-trioctahedral smectites, mixed-layer chlorite-smectite (C/S, 6–96% chlorite), and discrete chlorite. The zeolites were analcime, stilbite ± stellerite, heulandite, laumontite, epistilbite, and mordenite. Also present, as secondary minerals filling amygdales and veins, are prehnite, pumpellyite, and secondary amphibole. Two mineral assemblages were identified which provide important information about metamorphic conditions (temperature, reaction progress, and fluid properties): (1) heulandite + analcime + quartz; and (2) laumontite + albite + quartz + prehnite + pumpellyite ± amphibole. The types of and relations between minerals in the first assemblage suggest the occurrence of low-temperature hydrothermal metamorphism in the zeolite facies at ~125°C, whereas the second assemblage was metamorphosed at 200°C. The composition and variability of the mineral assemblages in the study area suggest that, due to slow reaction rates, the low-temperature transformations and mineral assemblages were influenced not only by temperature but also by local rock composition, fluid-rock ratio, and fluid chemistry.

The enhanced spatial and spectral resolution provided by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the Mars Reconnaissance Orbiter (MRO) has led to the discovery of numerous hydrated silicate minerals on Mars, particularly in the ancient, cratered crust comprising the southern highlands. Phases recently identified using visible/near-infrared spectra include: smectite, chlorite, prehnite, high-charge phyllosilicates (illite or muscovite), the zeolite analcime, opaline silica, and serpentine. Some mineral assemblages represent the products of aqueous alteration at elevated temperatures. Geologic occurrences of these mineral assemblages are described using examples from west of the Isidis basin near the Nili Fossae and with reference to differences in implied temperature, fluid composition, and starting materials during alteration. The alteration minerals are not distributed homogeneously. Rather, certain craters host distinctive alteration assemblages: (1) prehnite-chloritesilica, (2) analcime-silica-Fe,Mg-smectite-chlorite, (3) chlorite-illite (muscovite), and (4) serpentine, which furthermore has been found in bedrock units. These assemblages contrast with the prevalence of solely Fe,Mg-smectites in most phyllosilicate-bearing terrains on Mars, and they represent materials altered at depth then exposed by cratering. Of the minerals found to date, prehnite provides the clearest evidence for subsurface, hydrothermal/metamorphic alteration, as it forms only under highly restricted conditions (T = 200–400°C). Multiple mechanisms exist for forming the other individual minerals; however, the most likely formation mechanisms for the characteristic mineralogic assemblages observed are, for (1) and (2), low-grade metamorphism or hydrothermal (<400°C) circulation of fluids in basalt; for (3), transformation of trioctahedral smectites to chlorite and dioctahedral smectites to illite during diagenesis; and for (4), low-grade metamorphism or hydrothermal (<400°C) circulation of fluids in ultramafic rocks. Evidence for high-grade metamorphism at elevated pressures or temperatures >400°C has not been found.

The structure of prehnite Ca2Al(AlSi3O10)(OH)2, including H positions, has been determined by a combination of single-crystal X-ray diffraction and neutron powder diffraction on four natural samples. The symmetry of the average structure with Al/Si disordered at the T2 siteis Pncm. However, for four of the crystals studied, numerous violations of the n- and c-glide reflection conditions indicate lower symmetry corresponding to space groups P2cm and P2/n and Al-Si ordered structures, possibly as domains of different symmetries and ordering within a single crystal. Time-of-flight neutron powder diffraction was carried out on a sample from Mali at 293 K and 2 K. The structure was refined in space group Pncm by Rietveld analysis. Although it was not possible to locate the missing H using the 293 K neutron data, these data were used to refine the H position located approximately by single-crystal XRD and to refine Uiso. For the 2 K neutron powder diffraction data, H was located directly by difference-Fourier synthesis and its refined position found to be in close agreement with that obtained by the combined XRD/neutron 293 K dataset.

The Cerro Prieto geothermal system provides a unique opportunity for the detailed study of calc-silicate mineral transitions between the diagenetic clay-carbonate and greenschist facies within the terrigenous sediments of the Colorado River delta. In this system, progressive devolatization reactions within carbonate-cemented, quartzofeldspathic sediment have produced a distinct hydrothermal mineral zonation at temperatures between 200–370°C and fluid pressures below 0.3 kbar. Descriptive and compositional data are presented for these minerals which include wairakite, epidote, prehnite, actinolite, clinopyroxene, garnet, sphene, biotite, microcline, and calcite. Partitioning of octahedral Fe, Mg, and Al between coexisting authigenic silicates is comparable with data from higher temperature metamorphic rocks and demonstrates an approach to local equilibrium within this system. Calculated fugacities of oxygen at temperatures above 300°C are (with rare exception) more reducing than that defined by the quartz-fayalite-magnetite buffer, a result consistent with the scarcity of hematite and grandite and the ubiquitous presence of organic material in Cerro Prieto sandstones.

Apophyllite, analcite, chabazite, prehnite and stilbite are described from pectolite-, chlorite-, calcite- and quartz-bearing veins and joint coatings in the late Carboniferous quartz dolerite of the Whin Sill of the northern Pennines. These veins represent the final hydrothermal stages in the sill's emplacement and are not related to the widespread lead-zinc-barium-fluorite mineralization of the northern Pennine orefield. Thermogravimetric studies of apophyllites from the two Whin Sill occurrences reveal that at one locality the minreal is fluorapophyllite, at the other hydroxyapophyllite.

Electron microprobe analyses of prehnites from the Way Linggo low-sulphidation epithermal Au-Ag deposit of southern Sumatra, show that the mole fraction of octahedral Fe3+, expressed as Fe3+/(Fe3++AlVI), ranges from 0.0 to ~0.6, the higher values being among the most iron-rich reported for prehnite in a hydrothermal environment. Prehnites from a diabase sill in the Heber geothermal field of California have mole fractions of octahedral Fe3+ ranging from 0.03 to 0.3. The Way Linggo prehnites formed below 220°C, some 20–30°C lower than those at the Heber field; the lower crystallization temperatures perhaps enhanced the opportunity for Fe3+ to substitute in octahedral sites. In both occurrences, prehnite predates late-stage calcite, consistent with the need for waters depositing prehnite to have aCO2 <0.01 moles. At higher CO2 activities the stability field of calcite would swamp the range of aCa2+/aH+ values appropriate for crystallizing prehnite. Consequently, the presence of prehnite in a hydrothermal environment primarily indicates that degassing of the hydrothermal fluid in CO2 occurred prior to deposition.

The space symmetry of prehnite, a mineral which occurs in cavities and veins within skarn from the Tieshan iron mineral deposit, Daye, Hubei province, central China, has been determined using selected area electron diffraction (SAED) and convergent-beam electron diffraction (CBED) on the submicrometer scale. Our results confirm that natural prehnite may belong to space group Pncm. The unit-cell parameters of the prehnite investigated ( a = 0.458 nm, b = 0.555 nm and c = 1.853 nm) have been calculated using the multicrystal diffraction rings of gold, the internal standard.

Babingtonite is a common constituent of calcareous volcanic detritus in the interstices of pillow lavas from an outcrop of the Takitimu Group, western Southland, New Zealand, Associated minerals include Fe-rich prehnite (Fe/(Fe+Al) = 0.12–0.33), Fe-rich epidote (Ps24–53), grandite garnet (av. Andradite70 Grossular30), quartz, calcite, chlorite, and pumpellyite. Babingtonite shows little departure from its ideal composition other than minor Mg, Mn, and Al and a trace of Na.

Textural evidence and microprobe data suggest that the babingtonite, Fe-rich epidote, calcite, and quartz formed from hydrothermal solutions by direct precipitation in open cavities and by replacement of higher temperature silicate phases (in particular plagioclase and basaltic glass) in a shallow marine volcanic environment. During subsequent burial and low-grade metamorphism babingtonite was partially replaced by prehnite, less Fe-rich epidote, grandite, and chlorite.

The system Na2O-CaO-MgO Al2O3-SiO2-H2O is proposed to model phase equilibria and mineral parageneses for low-temperature metamorphism of basaltic rocks. Univariant reactions marking the transitions between various sub-greenschist facies are identified and some have been experimentally determined. The introduction of Fe2O3 into the model system at fixed FeO/MgO ratio creates continuous reactions for facies boundaries and discontinuous reactions for invariant points of the model system. Both qualitative and quantitative effects on P-T displacement and phase compositions are discussed. The XFe3+ isopleths for epidote were plotted to exemplify the transition from the zeolite through prehnite-pumpellyite to prehnite-actinolite facies. T-XFe3+ relations were established for continuous and discontinuous reactions relating such facies transitions. Because of the common occurrence of two or three Ca-Al hydrosilicates in low-grade metabasites, an isobaric Al-Ca-Fe3+ projection from chlorite may be used to illustrate mineral assemblages and compositions of the coexisting Ca-Al silicates in the presence of quartz, albite, and chlorite. Reported occurrences in several classic burial metamorphic terrains and ocean-floor metabasites in ophiolites are described. Only the composition of a mineral from a buffered assemblage can constrain the intensive properties for metamorphism; previously reported compositional trends for pumpellyite and epidote with increasing metamorphic grade are oversimplified.