Transmission Electron Microscopic Study of Coexisting Pyrophyllite and Muscovite: Direct Evidence for the Metastability of Illite1

Wei-Teh Jiang, Eric J. Essene and Donald R. Peacor
Department of Geological Sciences, The University of Michigan Ann Arbor, Michigan 48109
1 Contribution 467 from the Mineralogical Laboratory, Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109.

Abstract: 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

(□0.11K1.72Na0.17)(Al3.91Fe0.03Mg0.05Ti0.01)(Si6.11Al1.89)O20(OH)4

and

(□1.90Na0.06K0.04)(Al3.94Fe0.01Mg0.05)(Si7.94Al0.06)O20(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

(□1.89Na0.04Ca0.02K0.05)(Al3.93Fe0.04Mg0.02Ti0.01)(Si7.92Al0.08)O20(OH)4

and

(Na0.04Ca0.02K2.03)(Al3.54Fe0.24Mg0.16Ti0.06)(Si6.09Al1.91)O20(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.

Key Words: Analytical electron microscopy • Illite • Muscovite • Pyrophyllite • Solid solution • Thermodynamic stability • Transmission electron microscopy

Clays and Clay Minerals; June 1990 v. 38; no. 3; p. 225-240; DOI: 10.1346/CCMN.1990.0380301
© 1990, The Clay Minerals Society
Clay Minerals Society (www.clays.org)