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Title: Micropores in alkali feldspars
Author: Walker, Frank David Lathangie
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1991
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When observed optically, alkali feldspars often appear dusty or turbid. The cause of this turbidity is at or beyond the limits of optical resoltuion but by using electron microscopy it has been shown to be intimately related to the presence of micropores. Micropores are found in all pink or white alkali feldspars and are absent only from fresh black feldspars from rocks (such as fayalite or pyroxene bearing rapakivi granites, granulites and a lunar breccia) which petrographically show evidence of a very dry history. In general, micropores, averaging 0.5μm in length, make up around 1.5% of the alkali feldspars although porosities of up to 4.5% have been found. The shape of micropores is commonly controlled by Adularia habit overgrowths. Back-scattered electron imaging and transmission electron microscopy have shown that micropores are absent form strain controlled, coherent perthites and are found only in deuterically coarsened, incoherent, patch perthites. Exceptions to this rule are rare trains of micropores which cross strain controlled perthites and which are the remains of microtracks which healed in crystallographic continuity at temperatures above the alkali feldspars solvus. The character of the pristine areas varies with bulk composition. Ore-rich areas are normally 'tweed' orthoclase, with or without straight lamellar perthites. Intermediate compositions are lamellar or braid perthites of low albite and microcline. Turbid areas contain deuteric perthites and microcline in all cases. The structural integrity of the crystal is also lost in the turbid areas which are made up of numerous subgrains less than 1μm across with micropores in the gaps between the subgrains. Cleavage fragments from the Klokken layered syenite, South Greenland, were heated to 700oC at 0.1 GPa in 99% H218O for 75hr and then polished and imaged on an ion micropore for 18O. It was found that the 18O had penetrated into the microporous parts of the crystals and not into the pristine parts. At least some of the micropores and subgrain boundaries are therefore interconnected and have made the feldspar permeable as well as porous.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available