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Hrtem studies of subsolidus alteration, weathering, and subsequent diagenetic and low-grade metamorphic reactions. (Volumes I and II).

Author: Jillian Fiona Banfield; Johns Hopkins University.
Dissertation: Ph. D. Johns Hopkins University 1990
Edition/Format:   Thesis/dissertation : Thesis/dissertation : Microfiche : English
Publication:Dissertation Abstracts International, 51-12B.
Summary:
The structure of a new TiO$\sb2$ mineral that forms submicroscopic lamellae in anatase was determined by high-resolution imaging, image simulation, electron diffraction, and distance-least-squares refinement. The new mineral has previously been reported as the synthetic polymorph TiO$\sb2$ (B). In situ heating experiments revealed that TiO$\sb2$ (B) converts to anatase by growth of subnanometer-high ledges parallel
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Material Type: Thesis/dissertation
Document Type: Book
All Authors / Contributors: Jillian Fiona Banfield; Johns Hopkins University.
OCLC Number: 224273093
Notes: (UnM)AAI9113640.
Source: Dissertation Abstracts International, Volume: 51-12, Section: B, page: 5784.
Adviser: David R. Veblen.
Reproduction Notes: Microfiche. Ann Arbor, Mich : University Microfilms International.
Description: 461 pages

Abstract:

The structure of a new TiO$\sb2$ mineral that forms submicroscopic lamellae in anatase was determined by high-resolution imaging, image simulation, electron diffraction, and distance-least-squares refinement. The new mineral has previously been reported as the synthetic polymorph TiO$\sb2$ (B). In situ heating experiments revealed that TiO$\sb2$ (B) converts to anatase by growth of subnanometer-high ledges parallel to their interface. TiO$\sb2$ (B) was also identified as a minor constituent of the anatase-dominated assemblage produced by perovskite weathering. Topotactic relationships and structural considerations suggest that Ca-leaching and framework collapse via closely related reaction mechanisms leads alternatively to anatase of TiO$\sb2$ (B).

A fundamental building block approach is useful to understand the relationships between these minerals and the other TiO$\sb2$ polymorphs. Subsolidus oxidation of olivine resulted in its replacement firstly by magnetite and amorphous silica and subsequently by regular and semi-regular polysomatic intergrowths of olivine and laihunite. Olivine weathering reactions producing topotactically oriented smectite and hematite are hindered by the presence of laihunite. Topotactic replacement, elemental redistribution, and micro-environmental controls are important in other weathering reactions in andesitic rocks from Lake Abert, Oregon. The characteristics of the source volcanics also influence the composition and mineralogy of weathering assemblages. Resulting lake sediments reflect detrital smectite compositions, the diagenetic uptake of Mg and redistribution of silica to form K-stevensite, dilution of the illitic component, and formation of authigenic K-feldspar.

The high coercivity of a lodestone is explained by a microstructure comprised of closely-spaced stacking faults parallel to the magnetic domain walls. Other microstructural characteristics resulted from intergrowth with augite and the low-temperature oxidation of magnetite to maghemite. Rutile from a variety of metapelites contains exsolved Fe-enriched platelet defects with the hematite structure. This interpretation offers an alternative explanation for features observed in synthetic non-stoichiometric Ti$\sp{\rm 3+}$- and Al-bearing rutile. Chloritoid, an orthosilicate, can intergrow coherently with a variety of sheet silicates including chlorite, stilpnomelane, berthierine, and paragonite. These complex microstructures apparently formed during retrograde metamorphism. Chloritoid crystals from metapelites that experienced a single prograde event are texturally simple, suggesting that a dissolution-reprecipitation reaction defines the chloritoid isograd.

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