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.

Quantitative measurement of relative flexibility have been made on amphibole fibres based on calculation of bending strain of maximally bent fibre segments. Enhanced flexibility was found to be an inverse function of fibre diameter. Asbestos is therefore a morphological entity. These conclusions are supported by consistent observations of fibre diameter/flexibility relationships in amphiboles, serpentine, and rutile. Flexibility may arise from growth of thin crystals (primary flexibility), or from crystal diameter reduction (secondary flexibility). Mechanisms for secondary flexibility in amphiboles include splitting and solution along cleavage planes. Gross morphological characteristics are a poor indicator of microscopic habit, since coarsely fibrous minerals such as picrolite antigorite may not be fibrous after particle width reduction, and secondary asbestiform fibres are often derived from minerals with a non-fibrous morphology on gross examination.

Amphiboles of the tremolite-ferro-actinolite series from the Bergslagen district in central Sweden have between 0 and 46% of their M1-M3 sites occupied by Mn. These amphiboles are divided into two groups; (1) a group characterised by Mn (M1-M3) > (Mg or Fe) (M1-M3), and (2) a group with much lower contents of Mn (M1-M3), showing the following chemical characteristics: (a) Mn (M1-M3) never exceeds Fe (M1-M3); (b) in Mg-rich members of the group [Mg (M1-M3) > (Mn or Fe) (M1- M3)], the highest amounts of Mn (M1-M3) occur in amphiboles with about 2.20 Mg (M1-M3) per formula unit (pfu); in Fe-rich members of the group Fe [(M1-M3) > (Mg or Mn) (M1-M3)] high amounts of Mn (M1-M3) [> 0.4 Mn (MI-M3)] pfu only occur in amphiboles with >3.40 Fe (M1-M3) pfu; (d) the lowest amounts of Mn (M1-M3) occur in amphiboles with Mg (M1-M3) = Fe (M1-M3).

Mn (M4) increases with increasing amounts of Mn (M1-M3), indicating that solid solution towards the tirodite-dannemorite series is enhanced by incorporation of Mn (M1-M3).

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.