. Bulletin of the British Museum (Natural History), Mineralogy. 234 PETROLOGY OF LORD HOWE ISLAND, PART I (2) Subidiomorphic and somewhat corroded grains, often in glomero-groups, containing abundant inclusions of pyroxene, ore and glass, giving them a " spongy " or " checkerboard " appearance (PI. 10, 4). These plagioclases also show some degree of preferred orientation. Many are normally zoned having bytownite cores (An 70- 77), proceeding outwards with some discontinuities, to exteriors of andesine (An 44- 48). A few grains of apparently uniform composition have very nar
. Bulletin of the British Museum (Natural History), Mineralogy. 234 PETROLOGY OF LORD HOWE ISLAND, PART I (2) Subidiomorphic and somewhat corroded grains, often in glomero-groups, containing abundant inclusions of pyroxene, ore and glass, giving them a " spongy " or " checkerboard " appearance (PI. 10, 4). These plagioclases also show some degree of preferred orientation. Many are normally zoned having bytownite cores (An 70- 77), proceeding outwards with some discontinuities, to exteriors of andesine (An 44- 48). A few grains of apparently uniform composition have very narrow sodic rims. Pericline and Ala twins are confined to these spongy plagioclases, being ab- sent from the remainder which have albite or Carlsbad twins. COMPOSITION PLANE(010)albite twin An 75. An 82 An 65 Fig. An54 at inner margin grading to An 40 at outer. Diagrammatic sketch of crystal from dyke at North beach, showing oscillatory zoning. (3) Clear idiomorphic, lath-shaped or prismatic crystals (presumably xenoliths) of bytownite (An 73-81) often occurring in glomero-groups. They are either un- zoned or have narrow exterior rims or terminations with an average composition of An 65. These crystals show, dominantly, Carlsbad twinning. (4) Crystals with oscillatory zoning ; these occur singly. Several idiomorphic examples occur, of which one is illustrated in Text-fig. 4. The composition of each shell is shown in the diagram. The extreme range in composition in this crystal is from An 40 to An 82. In PI. 11, i a discontinuously zoned plagioclase (two zones only) is shown. The example just quoted illustrates the variety and complexity of the so called phenocrysts from the dyke rocks. Twin laws. The distribution of twin laws in the phenocrysts of the lavas and the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original wor
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