ALBERT

Notes: Abstract Coarse-grained muscovite, biotite, and chlorite from the Lawler Peak granite, Yavapai County, Arizona, have been studied with electron microprobe analysis, powder X-ray diffraction, transmission electron microscopy (TEM), electron diffraction, and analytical TEM methods. All three sheet silicates exhibit multiple polytypes and an abundance of stacking faults and dislocations. Chlorite formed by subsolidus replacement of biotite. Muscovite also may have formed by replacement of biotite and other minerals, but there is no clear microstructural evidence to support either a primary or secondary origin for the coarse-grained muscovite. X-ray and electron diffraction reveal that the muscovite consists of two separate phases, which give rise to splitting of diffraction maxima. TEM experiments suggest that the two components resulted from exsolution, which produced two symmetry-related sets of irrationally oriented lamellae with a wavelength averaging about 10 nm. Exsolution occurred in both 1M and 2M1 muscovite. Although the lamellae are too small to analyze directly, the muscovite bulk composition and structural data are consistent with the separation of celadonitic and muscovite components.

Notes: Abstract  Transmission electron microscopy (TEM) results show there is a series of periodically and nonperiodically interstratified structures composed of berthierine and chlorite layers in low-temperature “chlorite” that is one of the alteration products of granulite-facies Archean ironstone from the eastern border of the Beartooth Mountains, Montana. An antiphase domain structure suggests that the interstratified structures are intermediate transformation products formed by reaction from berthierine (serpentine structure) to chlorite. Periodically interstratified structures consisting of chlorite (C) and serpentine-like (S) (or berthierine) layers include (CS), (CCS), (CCCS), (CCCCS), and (CCCCCCS). The layer sequences in interstratified chlorite-berthierine are indicative of the reaction mechanisms that produce the interstratified structures (e.g., crystallization from solution or solid-state transformation). The reaction from berthierine to chlorite is crystallographically much like a cell-preserved phase transformation, even though it is highly reconstructive. Berthierine can be considered a polymorph of the Fe-rich chlorite mineral chamosite, with berthierine as the los-temperature phase. Interstratification and integrowths in the chlorite-berthierine series may be common phenomena in low-temperature layer silicates resembling chlorite. Although such relations are difficult to recognize from chemical analyses or powder X-ray diffraction patterns, they can be observed readily with TEM method.

Notes: Abstract Olivine crystals in basaltic andesites which crop out in the Abert Rim, south-central Oregon have been studied by high-resolution and analytical transmission electron microscopy. The observations reveal three distinct assemblages of alteration products that seem to correspond to three episodes of olivine oxidation. The olivine crystals contain rare, dense arrays of coherently intergrown Ti-free magnetite and inclusions of a phase inferred to be amorphous silica. We interpret this first assemblage to be the product of an early subsolidus oxidation event in the lava. The second olivine alteration assemblage contains complex ordered intergrowths on (001) of forsterite-rich olivine and laihunite (distorted olivine structure with Fe3+ charge balanced by vacancies). Based on experimental results for laihunite synthesis (Kondoh et al. 1985), these intergrowths probably formed by olivine oxidation between 400 and 800°C. The third episode of alteration involves the destruction of olivine by low-temperature hydrothermal alteration and weathering. Elongate etch-pits and channels in the margins of fresh olivine crystals contain semi-oriented bands of smectite. Olivine weathers to smectite and hematite, and subsequently to arrays of oriented hematite crystals. The textures resemble those reported by Eggleton (1984) and Smith et al. (1987). We find no evidence for a metastable phase intermediate between olivine and smectite (“M” — Eggleton 1984). The presence of laihunite exerts a strong control on the geometry of olivine weathering. Single laihunite layers and laihunite-forsteritic olivine intergrowths increase the resistance of crystals to weathering. Preferential development of channels between laihunite layers occurs where growth of laihunite produced compositional variations in olivine, rather than where coherency-strain is associated with laihunite-olivine interfaces.

Notes: Abstract Transmission and analytical electron microscopy (TEM/AEM) of glaucophane from glaucophane + Ca-amphibole-bearing blueschist and eclogite from two Vermont localities (Ecologite Brook and Tillotson Peak) and one California locality (Cazadero) has revealed the first evidence from exsolution for the miscibility gap between sodie and calcic amphiboles. The Tillotson Peak samples and the Cazadero samples contain coarsegrained glaucophane—actinolite pairs, while the Eclogite Brook samples contain coexisting glaucophane and actinolitic hornblende. Ca-rich glaucophanes contain abundant fine-scale lamellae of Ca-rich amphibole. These lamellae are usually oriented near (100) and $$\overline 2 01$$ . High-resolution TEM (HRTEM) images show them to be coherent. The exsolution lamellae are so narrow, beyond the resolution of AEM, that their true Ca contents are obscured by analytical contributions from the surrounding host. The AEM data suggest that the lamellae are either winchite or actinolite, depending on the true Ca concentration. In most cases, the exsolution lamellae have very curved interfaces and show variable orientation. This is attributed to the close similarity of unit-cell parameters for the two amphiboles. Three-dimensional optimal phase-boundary calculations using EPLAG (Fleet 1982) show that the observed “100” and “ $$\overline 2 01$$ ” orientations are consistent with the minimization of area strain between the two lattices along the interface. Some samples show evidence for incipient exsolution in the form of homogeneously distributed, fine-scale precipitates. These results suggest that Ca-rich glaucophanes from other glaucophane—actinolite assemblages may be exsolved at the TEM scale. The coexisting amphiboles from Eclogite Brook also have been studied using the electron microprobe (EMP). The compositional gap defined by the Eclogite Brook pairs is consistent with previously reported results, but shows a wider break along the glaucophane—actinolite pseudobinary join, suggesting very limited solid solution up to temper-atures of about 500–550°C. The glaucophanes are relatively poor in Ca, except for one anomalous grain containing Ca-amphibole lamellae. Ferric iron, estimated by normalization to fixed cation numbers, is strongly partitioned into the actinolitic hornblende and the glaucophanes are very poor in the riebeckite component.

Notes: Abstract  Eleven samples from high-pressure/low-temperature (HP/LT) shales and shale-matrix melange from four areas in the Diablo Range were studied using electron microprobe (EMP), transmission electron microscopy (TEM), and analytical electron microscopy (AEM) to provide information about white K-mica evolution and about the controls on illite “crystallinity” (IC) in these areas. The data indicate that: (1) compositional gradients from phengitic to muscovitic compositions occur along the long axis, perpendicular to c*, of white K-mica crystals; (2) compositional gradients parallel to c* were not observed, and thus coherent scattering domains along c* are homogeneous; (3) white K-mica crystals with compositions close to muscovite generally contain fewer planar defects and have larger defect-free distances than those with more phengitic compositions; (4) muscovitic white K-mica is less common than phengitic white K-mica. In the literature, grain growth has often been described to occur by the process of Ostwald ripening. Ostwald ripening is defined as an isochemical process and involves the reduction of surface free energy due to simultaneous dissolution and growth by transferring material from smaller particles to larger ones. However, in the present case: (1) coherent scattering domain boundaries often are created by intragranular faults such as dislocations, intergrown smectite layers (and other polysomatic defects), or incoherent layer rotations, which interrupt the 1.0 nm periodicity; (2) recrystallization from phengite to muscovite involves chemical changes. This implies that crystal growth can not be described by the term Ostwald ripening. Therefore, grain growth of white K-mica from the Diablo Range is described as a function of several processes that result in (1) reduction of surface free energy; (2) reduction of strain energy; (3) minimization of the Gibbs free energy due to change in composition. During growth of muscovitic white K-mica, planar defects become less abundant, and the defect-free distance (=coherent scattering domain size parallel to c*) increases. Strain energy decreases. Log-normal frequency distributions of coherent white K-mica scattering domains were found for three samples. The mode of these data coincides with the number of unit cells parallel to c* as calculated from the Scherrer equation. Thus, IC values from shales and shale matrix melange from the Diablo Range are the result of a physical mixture of numerous small phengitic coherent scattering domains and smaller numbers of slightly larger muscovitic coherent scattering domains. This implies that IC from the Diablo Range does not directly reflect maximum temperatures achieved by these rocks. It follows that IC data from terranes with a metamorphic evolution similar to that of the Diablo Range must be interpreted with caution.

Notes: Abstract Partially turbid alkali feldspars from hydrothermally altered Tertiary granites on the Isle of Skye (the Red Hills granites) were studied using light microscopy, scanning and transmission electron microscopies, and energy-dispersive X-ray spectroscopy. Limpid cores and turbid rims of individual crystals were compared to determine the causes of the turbidity. The limpid cores were cryptoperthitic, with lamellar widths of 0.1–0.3 μm. In contrast, the turbid rims contained K-rich and Narich areas coarsened to 〉0.5 μm. Turbid regions contained abundant inclusions, whereas limpid regions did not. Two generations of turbidity were recognized. Feldspars from the Beinn an Dubhaich granite, a granite with near-normal values for 18O/16O possessed limpid cores surrounded by turbid rims that cast a reddish-brown hue in transmitted light. When viewed in darkfield light microscopy, the regions with the reddish-brown turbidity were blue. This is consistent with the hypothesis that the cloudy appearance of these turbid regions arises from the scattering of light by micrometerto submicrometer-sized inhomogeneities in refractive index caused by fluid-filled cavities. Feldspars from the Loch Ainort granite, a granite with low values for 18O/16O possessed limpid and reddish-brown-turbid cores surrounded by turbid rims that cast a blackish hue in transmitted light. Ion thinning of the turbid areas produced an abundance of small holes (≤1–2 μm) apparently the remains of fluid inclusions. Transmission electron microscopy revealed that some holes from regions of reddish-brown turbidity contained non-feldspar material, including halite and metal-rich phases of various compositions. In contrast, blackish turbid regions contained cavities filled with alteration products, such as kaolinite. Hence, the feldspars from granites on the Isle of Skye apparently record interactions with at least two fluids: a saline fluid (possibly a late-stage magmatic fluid) and a meteoric fluid.