A major goal of metamorphic petrology is to understand the interrelationships among the crystallization conditions (temperature, pressure, bulk composition, chemical activities of volatile constituents, etc.) during metamorphic processes. Although chlorite is a common rock-forming metamorphic mineral believed to have a composition and crystal structure that relates to these conditions of crystallization, no structural studies of chlorite have been made to identify how petrologic factors affect the structure.
In this project, single-crystal X-ray diffraction methods will be used to examine suites of well-characterized chlorite. For each suite except the Plumbago pluton suite, all samples contain graphite, so that the iron oxidation state is constrained. The metapelitic schists to be examined from Maine will have the following characteristics: (1) formation pressures at near 3.5 kbar, (2) Al saturated, (3) intermediate-to-high Mg2+-contents, and (4) varying formation temperatures. For comparison, a suite from the metamorphosed Plumbago mafic to ultramafic pluton in Maine, is selected because it is Mg-rich, but not Al-saturated. In addition, a more Fe-rich suite will be examined. After chemical analyses are made, structure refinements of chlorite crystals within each grade and between grades will be compared.
The results of the study will help determine how Al, Mg, Fe2+, and vacancies are distributed in the various crystallographic sites of chlorite as a function of the pressure and temperature of crystallization and chlorite composition. These results will help to explain why there are relatively sharp compositional boundaries for the permissible composition range for metamorphic chlorites. In addition, an understanding of the variables involved in cation ordering may provide additional understanding of the controls limiting Ti, Fe3+, Mn, Ni, Cr, etc. substitutions. A major objective is to understand how these factors affect or control hydrogen bonding between the octahedral sheet and the 2:1 layer in chlorite. Most importantly, these data will eliminate the ad hoc aspect to previous attempts to formulate thermodynamic solution models for chlorite that are directly relevant to natural rocks.
