Transmission Electron Microscopic Study of Coexisting Pyrophyllite and Muscovite: Direct Evidence for the Metastability of Illite

Transmission electron microscopy has been used to characterize coexisting pyrophyllite and muscovite in low-grade metamorphosed pelites from Witwatersrand and northeastern Pennsylvania. The Witwatersrand sample consisted largely of porphyroblasts of interlayered muscovite and pyrophyllite in a fine-grained matrix of the same phases. In both textures, muscovite and pyrophyllite occurred as interlayered packets (with apparently coherent interfaces) from about 300 Å to a few micrometers in thickness, with no mixed layering. Their compositions were determined with a scanning transmission electron microscope to be $$({_{{0}{.11}}}{ }{{K}_{{1}{.72}}}{Na_{{0}{.17}})(A}{{l}_{{3}{.91}}}{ F}{{e}_{{0}{.03}}}{M}{{g}_{{0}{.05}}}{T}{{i}_{{0}{.01}}}{)(S}{{i}_{{6}{.11}}}{ Al}{{l}_{{1}{.89}}}{)}{{O}_{{20}}}{{(OH)}₄}$$ and $$(_{{1}{.90}}{N}{{a}_{{0.06}}}{{K}_{{0}{.04}}}{)(A}{{l}_{{3}{.94}}}{F}{{e}_{{0}{.01}}}{M}{{g}_{{0}{.05}}}{)(S}{{i}_{{7}{.94}}}{A}{{l}_{{0}{.06}}}{)}{{O}_{{20}}}{{(OH)}₄},$$ respectively. The pyrophyllite and muscovite in the Pennsylvania shale likewise occurred only as coexisting coherent to sub-parallel packets as thin as 200 Å, with compositions of $$(_{{1}{.89}}{N}{{a}_{{0.04}}}{C}{{a}_{{0}{.02}}}{{K}_{{0}{.05}}}{)(A}{{l}_{{3}{.93}}}{F}{{e}_{{0}{.04}}}{M}{{g}_{{0}{.02}}}{T}{{i}_{{0}{.01}}}{)(S}{{i}_{{7}{.92}}}{A}{{l}_{{0}{.08}}}{)}{{O}_{{20}}}{{(OH)}₄}$$ and $$({N}{{a}_{{0}{.04}}}{C}{{a}_{{0}{.02}}}{{K}_{{2}{.03}}}{)(A}{{l}_{{3}{.54}}}{F}{{e}_{{0}{.24}}}{M}{{g}_{{0}{.16}}}{T}{{i}_{{0}{.06}}}{)(S}{{i}_{{6}{.09}}}{A}{{l}_{{1}{.91}}}{)}{{O}_{{20}}}{{(OH)}₄}.$$ The textures of both samples were consistent with an equilibrium relationship between pyrophyllite and muscovite. The Pennsylvania sample also contained NH4-rich illite, kaolinite, and an illite-like phase having intermediate Na/K, which collectively imply non-equilibrated low-grade conditions. The compositions of these coexisting pyrophyllite and muscovite define a solvus with steep limbs and extremely limited solid solution. Illite is a white mica, intermediate in composition between pyrophyllite and muscovite, formed at much lower temperatures than muscovite. These relations show that illite is metastable relative to pyrophyllite + muscovite in all of its diagenetic and low-grade metamorphic occurrences. This further implies that illite precursor phases, such as smectite, are also metastable. The prograde reactions involving smectite, illite, and muscovite are therefore inferred to represent Ostwald-step-rule-like advances through a series of metastable phases toward the equilibrium states attained in the greenschist facies. “Illite crystallinity” can therefore be a measure of reaction progress, for which temperature is only one of several determining factors.