A Vitreous Sedimentary Bomb from Pyroclastic Material at Auckland, New Zealand
[Read before Auckland Institute, October 19, 1949; received by the Editor, October 18, 1949]
The interesting rock described in the following brief note was found in 1948 by Mr H. R. Kenedy, of Auckland, in association with scoria and other pyroclastie materials being used for road surfacing; apart from two very much smaller fragments no other specimens of similar nature could be seen among the rocks by him. While the writer has been unable to trace the material to its parent quarry, there can be little doubt that it was derived from one of the local scoria cones.
The specimen is approximately four inches in diameter and in hand specimen examination exhibits two zones that contrast in their physical characters. The outer portion of the rock is black, dense glass, like rather dull obsidian, has a few small vesicles, and includes a number of patches having a much lighter, almost greyish-white colour, often sharply separated from the enwrapping darker glass by very thin white selvedges. The central material enclosed within this glassy mantle is two and a half inches in diameter and is composed of material more lithic in character and with more numerous and larger vesicles; it is blue-black in colour and resembles glassy varieties of the local basalts in both texture and appearance. The inner mass is separated from the enclosing outer glass by a thin layer of white glass. It was thought, at first, that the rock was a tachylite bomb, but examination of thin sections cut from both zones revealed a rather unexpected constitution that suggested a quite different nature.
The sections show that the outer material is a pale brownish glass in which are set scattered needles and nests of radiating needles with lengths of up to 1·2 mm. and thicknesses of up to 0·015 mm. (Fig. 1), together with much shorter and stouter rods, often arranged in strings and small aggregates. The rods and needles appear to be the same mineral with the following characteristics:
Mean index of refraction very close to 1·655; colourless; anisotropic and possessing straight extinction.
Interference colours up to a 1st order yellow-orange suggest birefringence of 0·012, if section is 0·03 mm. thick. Elongation is positive. Cross section is almost square and the optic plane is diagonal to it. Sections give positive biaxial interference figures that indicate a fairly small optic axial angle. Dispersion fairly strong, p > v. Cleavage is perfect and parallel to the diagonal of cross-section.
These properties agree very closely with those recorded by Winchell (1927) for the mineral mullite and the needles and rods in the specimen under description would appear to be that mineral.
The glass itself has a refractive index between 1·513 and 1·523 in value which according to the curve given by George (1924) would be consistent with a glass having a silica content of approximately 63 per cent, and a low iron and alkali content.
The white patches referred to above as included within the outer zone are more crystalline in nature and some appear to be almost wholly composed of small rod-like crystal and tiny needles of mullite; the long needles are usually lacking in most, and blebs of glass are present in others. In many cases the inclusion is separated from the dark enclosing glass by a narrow zone of almost colourless glass with fewer needles are the present in the general glass of the outer portion of the specimen.
The more lithic material of the core of the rock has a much smaller amount of glass in its constitution and appears as a felted mass of tiny needle-shaped crystals seldom longer than 0·16 mm. in length. It is more vesicular and lacks both the long needles and the tiny granular crystals of the encompassing glass. A few small grains of feldspar appear to be present.
It would seem that this rock represents an argillaceous sedimentary material which has been metamorphosed by contact with a basaltic magma and later ejected along with other pyroclastic ejectamenta during a cone-building phase of eruption from one of the many local vents. Brick-like sedimentary bombs are common in many of the local volcanic cones and xenoliths of sedimentary origin are frequently found in the basalts of the lava flows that emanate from them. Most of the bombs, like that at present under consideration, have no lava adherent to them; presumably any such adherent liquid would be wiped off them during their passage through the air. If the origin suggested for the present rock is correct, then it would be analogous to the sillimanite (mullite) buchite xenoliths described in the Mull Memoir (1924) and by Thomas (1922). The “characteristic aluminous xenoliths” in the Mull dykes are considered to have “started out as shale or mudstone poor in lime, magnesia and iron.” The first step in their metamorphism is imagined as the production of sillimanite (actually mullite) buchite, “a lilac-coloured rock consisting of glass crowded with minute felted and parallel needles of sillimanite one to two millimetres long” existing “partly as a residuum left during vitrification of the clay and partly as a precipitate during the cooling of the aluminous melt in which it lies embedded.” Thomas (loc. cit.) concluded that these xenoliths had suffered thermal metamorphism only and had not experienced any alteration as a result of chemical reaction with the magma, and that therefore the sedimentary rock had been metamorphosed before its inclusion in the dykes. The same conclusions apply to the mullite buchite bomb herein described which has probably been derived from the thermal metamorphism of the rock of a mudstone member of the Waitemata Formation by basaltic magma and ejected with other volcanic materials during an explosive eruption.
The writer wishes gratefully to acknowledge the assistance of Mr M. H. Battey, Geologist, Auckland Museum, in discussing this problem.
Fig. 1—Section of outer glassy zone, showing needles of mullite set in clear glass. An aggregate of granular crystals is shown in the top centre; the white patches are vesicles. X 34.
George, W. O., 1924. Relation of the Physical Properties of Natural Glasses to their Chemical Composition. Journ. Geol., vol. xxxii, pp. 353–372.
Mull Memoir, 1924. Tertiary and Post-Tertiary Geology of Mull. Mem. Geol. Survey of Scot., Edin., 1924.
Thomas, H. H., 1922. Certain Xenolithic Tertiary Minor Intrusions in the Island of Mull (Argyllshire). Quart. J. Geol. Soc., lxxviii, 1922, pp. 229–250.
Winchell, N. H. and A. N., 1927. Elements of Optical Mineralogy, N.Y., 1927.