Muscovite- and R1 illite/smectite-kaolinite intergrowths in the Eocene McAdams Sandstone: Non-existence of hydromuscovite and implications for illite metastability

Conference · Wed Jan 01 04:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5998894

Jiang, Weiteh; Peacor, D R; Essene, E J ^[1]

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1. Univ. of Michigan, Ann Arbor, MI (United States). Dept. of Geological Sciences

Illite-like grains in hydrothermally altered Eocene sandstones from Kettleman North Dome in the San Joaquin Valley, CA have been characterized by electron microscopy to investigate the cause of the low-K contents obtained in previous electron microprobe analyses that were inferred to represent true hydromuscovite-hydropyrophyllite solid solutions and to conclude that illite may be a stable phase. The illite-like grains are of two types: (1) curved, flaky grains that fill pore space in fractured K-feldspar and along grain boundaries between K-feldspar and quartz, and (2) aggregates of lath-like grains that fill cavities. EDS analyses show that the former has an illite-like composition of 0.7--0.8 K per O[sub 10](OH)[sub 2] and the latter is muscovite. Electron diffraction and lattice-fringe images show that the first type of grains is rectorite-like R1 illite/smectite (I/S). Both the R1 I/S and muscovite are interlayered with and/or irregularly embayed by kaolinite, implying that they may have been partially replaced by kaolinite or vice versa. Aggregates of randomly oriented, very fine-grained R1 I/S and kaolinite that formed as cements are abundant. It is not possible to define individual single-phase grains in any of these types of mixtures with optical microscopy. Analyses of such mixtures have low K contents and high Al/Si ratios. The data demonstrate that the previously-reported low-K, hydronium-rich phases that were implied to represent hydromuscovite-hydropyrophyllite solid solutions do not exist. There is no evidence for the presence of interlayer H[sub 2]O and H[sub 3]O[sup +] that were hypothesized to stabilize illite. Common illite is thus a K-deficient, Si-rich dioctahedral mica with composition that is approximately intermediate to those of pyrophyllite and muscovite and within the pyrophyllite-muscovite solvus. It is therefore metastable with respect to muscovite + pyrophyllite.

• OSTI ID: 5998894
• Report Number(s): CONF-921058--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 24:7
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ALUMINIUM COMPOUNDS
ALUMINIUM SILICATES
CALIFORNIA
CHEMISTRY
CLAYS
DEVELOPED COUNTRIES
GEOCHEMISTRY
GEOLOGIC STRUCTURES
GEOLOGY
HYDROTHERMAL ALTERATION
ILLITE
KAOLINITE
MICA
MINERALIZATION
MINERALS
MUSCOVITE
NORTH AMERICA
OXYGEN COMPOUNDS
ROCKS
SANDSTONES
SEDIMENTARY BASINS
SEDIMENTARY ROCKS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
SMECTITE
STABILIZATION
STRATIGRAPHY
USA