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Sample ID:52-W-38
Station ID:52-W-38
Date Sampled:1952-01-01
Sample Type 1:Lava
Sample Type 2:Sill
Field Lithology:Dacite
Text Description:
Specimen 52-W-38 (Rittmann: labradorite dacite) is from a light-gray columnar sill 2.5 miles northwest of Cape Tusik on southern Kanaga. This sill is finer grained and megascopically less porphyritic than the sills on southwest Kanaga. Phenocrysts, mostly less than 1 millimeter in diameter, make up 25 percent of the rock as follows: abundant zoned labradorite-bytownite with thin rims of oligoclase-andesine and, commonly, oscillatory zoning; less abundant oligoclase-andesine crystals surrounded by a zone crowded with dark inclusions, then a rim of labradorite-oligoclase with strong oscillatory zoning (included here are many phenocrysts of labradorite with center parts showing patchy extinction); clinopyroxene and orthopyroxene, both rare (<0.5 mm); abundant subhedral opaque oxides (<0.2 mm); rare sphene; ghost hornblende (<2 mm) now composed of fine pyroxene, opaque oxide, and feldspar; rounded quartz relics (<0.6 mm) surrounded by rims of granular clinopyroxene in a brown, weakly anisotropic matrix. The strongly fluidal groundmass is composed of: abundant lath- and blocky-shaped crystals of zoned oligoclase-andesine (<0.2 mm); many stubby crystals of clinopyroxene (<0.15 mm); many blocky crystals and grains of opaque oxide (<0.07 mm); irregular grains and masses of carbonate, some within pyroxene crystals and some within plagioclase crystals; abundant interstitial isotropic, material having a low index of refraction and a fracture pattern suggestive of cristobalite or opal.

Geology of the Delarof and westernmost Andreanof Islands, Aleutian Islands, Alaska