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TEM Investigation of Lewiston, Idaho, Fibrolite: Microstructure and Grain

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TEM Investigation of Lewiston, Idaho, Fibrolite: Microstructure and Grain American Mineralogist, Volume 84, pages 152±159, 1999 TEM investigation of Lewiston, Idaho, ®brolite: Microstructure and grain boundary energetics R. LEE PENN,1,* JILLIAN F. BANFIELD,2 AND DERRILL M. KERRICK3 1Materials Science Program, University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A. 2Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A. 3Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A. ABSTRACT High-resolution transmission electron microscopy (HRTEM) revealed that a sample of ®ne-grained Lewiston, Idaho, ®brolite is predominantly ®brolite with trace amounts of poorly crystalline layer silicates. The ®brolite consists of aggregates of acicular grains where the c axis of each grain is parallel to the elongation direction. Widths of 195 grains were measured: The average is 0.41 mm, the mode is 0.29 mm, and the range is 0.05±1.57 mm. No stacking faults or other extended defects were observed in any of the grains. Grain boundary energies were calculated using the symmetrical dislocation tilt wall theory (SDTW) and measurements of misorientation between the c axes of neighboring ®brolite crystals. The angles of misorientation range from 18 to 118, yielding grain boundary en- ergies ranging from 310 to 967 ergs/cm2, respectively, with an average energy of 610 ergs/ cm2. Modeling the ®brolite grains as in®nitely long cylinders and using the experimentally measured average grain diameter, an average molar grain boundary energy of 320 J/mol was calculated. This excess grain boundary energy could correspond to a shift of as much as 1140 8C in the andalusite-sillimanite boundary and 130 8C in the kyanite-sillimanite boundary. Typical ®brolite grain boundaries adopt relatively high-energy con®gurations. We attribute this to ®brolite nucleation at pre-existing low-angle grain boundaries in layer silicates, preserving misorientations, and conferring a ®ne-grained texture. INTRODUCTION stood and are used to obtain pressure and temperature Phase relations in most coarse-grained rocks are well information for metapelitic rocks. Because micro-scale understood. However, when crystal size is small, a sig- features such as disorder, small grain size, defects, and ni®cant portion of the volume of material is comprised compositional variation can be indicators of higher en- of grain boundaries or surfaces. Atoms with unsatis®ed ergy (or non-equilibrium) conditions, microstructural and distorted coordination environments are a source of characterization of phases is essential for the determina- excess energy and thus can signi®cantly perturb mineral tion of thermobarometric histories. stability ®elds and phase transformation kinetics (Gold- Univariant reactions in the kyanite-andalusite-silliman- stein et al. 1992). The particle size dependence of stability ite system involve reconstructive phase transformations ®elds is most important where the structure energies of that change the coordination number of aluminum and the polymorphs are similar. In some systems (e.g., Fe-oxides, connectivity of the silicate tetrahedra. These reactions are Langmuir and Whittemore 1971; CdS, Goldstein et al. sluggish because they require rupture of strong Al-O and 1992; Ti-oxides, Gribb and Ban®eld 1997; Al-oxides, Si-O bonds and the driving forces are small. Consequent- McHale et al. 1997) ``metastable'' compounds can be- ly, small errors in experimental determinations of the uni- come stable as particle size is reduced, leading to particle- variant equilibria result in substantial uncertainty in the size dependent stability ®elds (Zhang and Ban®eld, un- phase boundaries and thus, the location of the triple point. published manuscript). Furthermore, ®ne grain size, small amounts of impurities, The aluminosilicate (Al2SiO5) polymorphs (kyanite, an- or compositional variation, in addition to any structural dalusite, and sillimanite) are common metamorphic min- and microstructural factors that modify the total energy erals. Fibrolite is the ®ne-grained acicular habit of silli- of the aluminosilicate phases, can signi®cantly modify the manite that is common in medium- to high-grade positions of the stability ®elds for kyanite, andalusite, and metapelitic rocks. Aluminosilicates are extremely impor- sillimanite. tant in petrologic studies because the aluminosilicate Over the past two decades, considerable effort has been phase relations are generally believed to be well under- expended to identify factors that affect the energetics of the aluminosilicate phases. Dislocation density, cation * E-mail: [email protected] substitution, aluminum-silicon disorder, non-stoichiome- 0003±004X/99/0102±0152$05.00 152 PENN ET AL.: TEM OF FIBROLITE 153 try, grain boundary area, and crystallographic mismatch mismatch between adjacent grains, no studies have con- between adjacent grains have been suggested as factors ducted the grain boundary characterization necessary to that modify phase stability (Helgeson 1978; Kerrick model the energy of ®brolite compared to coarse silli- 1986; Holdaway and Mukhopadhyay 1993). Comparisons manite. Kerrick (1990) described a preliminary study by of thermodynamic studies designed to determine the en- Wenk and Medrano where samples consisting of ®brolite ergetic differences between ®brolite and coarse-grained aggregates were examined using electron diffraction to sillimanite yield con¯icting results. Experimental hydro- determine that the angular mismatch between neighboring thermal studies of Richardson et al. (1968), which ex- grains of ®brolite was ,158. Based on that preliminary amined the extent of reaction between phases at various study, Kerrick (1990) used the symmetrical dislocation temperatures and pressures, used a sillimanite/®brolite tilt wall (SDTW) theory to estimate the grain boundary sample (Brandywine Springs, Delaware) that was about contribution to the total free energy of ®brolite. 30 vol% ®brolite, whereas those of Newton (1966, 1969) In this study, we measure the angular mismatch be- used ®brolite-free sillimanite (Licht®eld, Connecticut). tween adjacent ®brolite crystals and grain diameters, and Their results were in relatively good agreement, suggest- use this information to estimate the grain boundary con- ing that the presence of ®brolite does not shift the Al2SiO5 tribution to the total free energy using the SDTW theory equilibrium boundaries (Kerrick 1990). However, in an for a ®brolite sample from Lewiston, Idaho. We also eval- investigation of the andalusite-sillimanite phase bound- uate the importance of dislocation density and planar de- ary, Holdaway (1971) performed hydrothermal experi- fects to the thermodynamic analysis, the layer silicate ments using andalusite (Minas Gerais, Brazil) and silli- mineralogy, and the origin of the ®brolite grain boundary manite (Benson Mines, New York or Montville structure. Quadrangle, Connecticut) in one vessel and andalusite, sillimanite, and ®brolite (a mixture from Williamston, EXPERIMENTAL METHODS Australia or Jefferson City, Colorado) in another. He ob- Fibrolite from Lewiston, Idaho, has been extensively served early reaction of ®brolite to andalusite, suggesting characterized, e.g., for calorimetry (Hemingway et al., that ®brolite was less thermodynamically stable under the unpublished manuscript; Topor et al. 1989), non-stoichi- pressure and temperature conditions. ometry (Kerrick 1990; Hemingway et al. 1991), Al/Si Anderson and Kleppa (1969) found measurable differ- disorder (Hemingway et al. 1991), and thermal expansion ences in the heats-of-solution of ®brolite (Custer, South (Hemingway et al. 1991). Powders of ®brolite and coarse- Dakota) and sillimanite (Benson Mines, New York) using grained sillimanite from Benson Mines, New York were molten oxide calorimetry. Similarly, Salje (1986) found examined using a Scintag PadV X-ray Powder signi®cant differences in the heat capacities of sillimanite Diffractometer. (from Waldeck, Germany; TraÈskbole, Finland; and Sri Four 3 mm 3 3 mm pieces of ®brolite were cut from Lanka) and ®brolite (Garcujuela, Spain), although those two standard 30 mm thin sections, mechanically thinned, results are considered questionable due to insuf®cient argon ion-milled to electron transparency, and examined sample size for the procedure used (Hemingway et al. using a Phillips CM20 ultra-twin 200 kV high-resolution 1991). In contrast, Hemingway et al. (1991) reported heat transmission electron microscope (HRTEM) equipped capacities for sillimanite (Sri Lanka) and ®brolite (Lew- with a GE twin window energy dispersive X-ray (EDX) iston, Idaho) that were virtually identical at 298 K and and a NORAN voyager analyzer. differed by ,1% at 1000 K. Topor's et al. (1989) mean The degree of misorientation between adjacent crystals measurements of the heat-of-solution of sillimanite (Sri of ®brolite was determined using selected-area electron Lanka) and ®brolite (Lewiston, Idaho) suggest that ®- diffraction (SAED). Using the TEM sample holder tilting brolite has a slightly less negative heat-of-formation than mechanisms, the sample was tilted until one grain was sillimanite. However, the precision of the measurements oriented with respect to the electron beam, and the tilts preclude a de®nitive conclusion regarding the differences of the sample holder and the electron diffraction pattern in the enthalpies of formation. Thus, although some ex- recorded. The same procedure was repeated for the ad- perimental evidence suggests an energy
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