Formation of coexisting 1M and 2M polytypes in illite from an active hydrothermal system

Journal Article · · American Mineralogist; (United States)

OSTI ID:5639839

Lonker, S W; Fitz Gerald, J D ^[1]

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1. Australian National Univ., Canberra (Australia)

Polytypes of illite from the Broadlands-Ohaaki geothermal system in New Zealand have been studied with transmission electron microscopy and electron diffraction. The illite is predominantly a one-layer polytype with 1.0-nm interlayer spacing. On the scale of a few millimeters, illite occurs as ordered crystals, disordered crystals, and crystals with regions of ordered and disordered stacking. Some crystals are composed entirely of either the one-layer or the two-layer polytype; others show regions of two-layer stacking in a one-layer host. Long period stacking sequences (3- or 4-layer) are less common and occur as lamellar intergrowths in 1M mica. No morphological differences were observed among the discrete crystals, of 1M{sub d}, 1M, and 2M. Textures indicate that regions with two-layer stacking can be produced from 1M{sub d} mica even though there is a general trend with increasing temperature of 1M{sub d} to 1M to 2M. Experimental data on the rate of transformation of 1M to 2M{sub 1} muscovite combined with estimates of the mole fraction of 2M mica and temperatures of illite crystallization were used to place constraints on the duration of hydrothermal activity in the Broadlands-Ohaaki system.

• OSTI ID: 5639839
• Journal Information: American Mineralogist; (United States), Journal Name: American Mineralogist; (United States) Vol. 75:11-12; ISSN AMMIA; ISSN 0003-004X
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
152002* -- Geothermal Data & Theory-- Properties of Minerals & Rocks
AGE ESTIMATION
AUSTRALASIA
CLAYS
COHERENT SCATTERING
CRYSTAL STRUCTURE
CRYSTAL-PHASE TRANSFORMATIONS
CRYSTALLIZATION
DEVELOPED COUNTRIES
DIFFRACTION
DIMENSIONS
ELECTRON DIFFRACTION
ELECTRON MICROSCOPY
GEOTHERMAL SYSTEMS
ILLITE
LAYERS
MICA
MICROSCOPY
MICROSTRUCTURE
MINERALOGY
MINERALS
MORPHOLOGY
MUSCOVITE
NEW ZEALAND
PHASE TRANSFORMATIONS
SCATTERING
SILICATE MINERALS
SYNTHESIS
TEXTURE
TRANSMISSION ELECTRON MICROSCOPY