Art. XLIV.—Notes on a Hypersthene Andesite from White Island.
[Read before the Auckland Institute, 15th August, 1898.]
This is a rock of a dark-grey colour, irregular fracture, and specific gravity 2.65. In hand specimens the feldspars are conspicuous, reflecting the light from their smooth faces, whilst the hypersthenes only appear as dark oblong patches. The following is a chemical analysis of this rock:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
The SiO2 in the above is some 4 to 7 per cent. higher than
in foreign andesites, whilst on the other hand the alumina, iron, lime, and magnesia are proportionally lower, suggesting that either secondary silica has been introduced into the base or that, the rock as a whole is slightly decomposed; more likely the latter, as I find it very hard in this district to obtain perfectly fresh specimens of andesitic rocks.
Microscopic Examination.—Hypersthene and feldspar form the phenocrysts (no augite being visible in the sections I determined), and are set in a base dusted with grains of magnetite.
The hypersthenes occur in small crystals up to about 1 mm. in length, generally in isolated well-defined crystals, though sometimes in bunches. They are fractured at right angles to the vertical axis, and exhibit also, in some sections, branching irregular cracks. The brachypinacoidal cleavage is noticeable, and. in cross-sections sometimes the prismatic. In ordinary light they appear light-brown. The pleochroism is strong, a = red-brown; b = red-yellow;. c = pale sea-green. The polarisation colours are fairly brilliant, and the extinction straight.
The feldspars generally occur in plates, accompanied by the lath-shaped type, and vary in size up to. about 3 mm. or 4 mm., being much larger than the hypersthenes: By ordinary light they are colourless, but by polarised light they often show polysynthetic twinning, and. the extinction angles prove them to be probably andesine, with a little oligoclase.
The base is largely developed in proportion to the phenocrysts. By ordinary light it is of a dirty-brown colour (in thin sections a light-drab), and is seen to be dusted with fine grains of magnetite, and by polarised light the base is seen to be micro-crystalline and feldspathic.
The chief accessory mineral is magnetite, occurring principally in the base, and it is not likely to be a decomposition product of hypersthene phenocrysts, which are fairly fresh and undecomposed. Vitreous inclusions, with gas-pores and negative crystals, occur in the feldspars. Traces of kaolin occur as a decomposition product of these, crystals.
The various constituents follow the normal order of consolidation, and the rock is a good sample of a hypersthene andesite, the hypersthenes especially being in beautiful crystals.
Appended (Plate XLVII.) are two diagrammatic water-colour drawings—No. 1, with polariser only; No. 2, with crossed nicols. h = hypersthene; f = feldspar.