The carbonaceous matrix of ureilite meteorites (C‐bearing olivine‐pigeonite achondrites) contain abundant minute phases that emit a multicolored fluorescence under electron bombardment. These phases include NaCl and KCl, found in all seven ureilites studied, high‐Si glass with pyroxene and chlorapatite quench crystals in North Haig, an unidentified high‐Ca‐Al‐Cl phase in Novo Urei, and possibly free SiO2 in Novo Urei and Dingo Pup Donga. The origin of these phases is uncertain but some, especially chlorides and glass, may represent residual postcumulus materials precipitated from a late‐stage interstitial liquid during the igneous phase of ureilite history.
Fracture surfaces of three different glass ceramics were examined to determine the critical flaw sizes and the corresponding microstructures of the materials directly from the fracture surfaces. Standard secondary electron (SE) images and classical topographical back‐scattered electron (BSE) images were used to measure the critical flaw sizes and to observe deviations of the critical flaw interfaces from smooth elliptical fronts. Compositional BSE images were used to characterize the microstructures in and around the critical flaws directly from the unetched fracture surfaces.
A direct correlation was found between the critical flaw size and microstructural units effectively acting as grains within the flaw and the fracture toughness. This relationship enabled the discrepancies between the calculated fracture toughness and the measured fracture toughness to be resolved.
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