Go to National Library of New Zealand Te Puna Mātauranga o Aotearoa
Volume 66, 1937
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Structure of the Comendites.

The most solid and compact rocks are found near the base of the cliffs forming the southern wall of the crater. Here the wall rises with extreme steepness almost to its maximum height of 1200 feet. The rock has a rough prismatic jointing and no flow structure can be seen in it. In hand-specimens it is dark brown in colour with irregular faintly bluish patches. On the sea-cliffs along what is

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probably the peripheral area of material that has been emitted from the crater the structure of the rock is far less massive, though rough prismatic jointing is still perceptible. Here there is often a banded appearance, which, especially near the eastern bay, may be extremely delicate and striking. Even when the rock is not well banded irregular streaks of lighter and darker colour can be distinctly seen. In these cliffs even in a single deposit the colour of the rock often varies considerably; there may be bands of black obsidian which is particularly glossy, perhaps because of the relatively high percentage of iron-oxides the rock contains. In the analyses of obsidian made by Washington the percentage of iron-oxides is recorded at 3.22, while in the ordinary rocks of the island von Wolff (9) found 7.10, mainly Fe2O3, while Seelye found 6.13, 5.93, and 5.87 per cent. in different samples. Its specific gravity is 2.40.

Occasionally the rocks are vesicular in a very irregular manner, some of the vesicles, and in the eastern bay some of the joint-planes, having a deposit of opal. In places there are minute patches of opal with a fine play of colour with scarlet fire. The occurrence of fire-opal in the rhyolites of Mexico is well known, and precious opal has been mentioned as occurring at Tairua, in the Coromandel Peninsula, New Zealand. In addition, these cavities, especially at Opo Bay and on the east side of Omapu Bay, contain splendent though small crystals of two kinds. The two species of mineral that are represented are aegirine and riebeckite. The former has the typical form of ordinary augite which apparently has not previously been recorded for aegirine. The crystals may reach a length of two millimeters with the prisms and two pinacoids as vertical faces and the minus pyramids as terminal faces. The vertical faces vary in relative length to a considerable extent. No twinned crystals have been seen. The cavities in the lightest green coloured rocks often contain aegirine only. In dark-grey rock on the south side of Opo Bay the cavities may contain riebeckite exclusively, or some times with aegirine as well. The riebeckite occurs in long bladed crystals which are occasionally one centimeter in length, though the breadth does not exceed one millimeter. The face 010 is narrow, but the prisms are relatively wide. The face 010 also has vertical striations. The extinction angle is 2.5 degrees. The pleochroism is from deep-blue to brownish-yellow, X dark-blue, Z brownish-yellow, Y pale-blue. The following are the best measurements that could be obtained:—110–11¯0 57.3°, 11¯0–01¯0 62°, 01¯0–11¯0 62.2°, 11¯0°1¯10 55°, 1¯10–010 62.3°, 010–110 62.3°. The terminal faces are clinodomes and plus orthodome. An observer accustomed to the use of the microscope goniometer would of course obtain quite accurate measurements. A large number of the crystals in some of the cavities are of capillary form. Crystals of riebeckite have been recorded by Sollas in glacial pebbles on the east coast of Ireland (8). The occurrence of these well-formed crystals in vapour cavities makes it obvious that they have been deposited from solution in volatiles. The occurrence of opal in some of the cavities, and lining perhaps half of them, indicates that silica was also present with the volatiles. Mr E. T. Seelye has determined the presence of 0.06 per cent. of fluorine in

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the rock, and that is probably a small residue only of the amount which was present in the magma. This suggests that the opal was deposited from the fluoride of silicon in the later stages of solidification of the rock. This may have favoured the occurrence of riebeckite and aegirine molecules in the vapour phase, though no evidence was obtained in support of this.

It is possible that the small peninsula of Panui on the south side of Opo Bay is the site of a separate eruption subsidiary to that of the main crater. Beds of tuff 100 feet thick occur between it and the slopes of the main cone. The south coast of this Panui Peninsula is accessible and displays the rock-structure well. As seen without any accessory aids to investigation the colour of the material as well as its structure is surprisingly variable. There are wide belts of light-grey rock in which riebeckite occurs almost to the exclusion of aegirine; in others that are light-green there is aegirine almost without any riebeckite; and in addition there are belts of black obsidian. Though all of these were emitted by this little vent probably in the fragmentary though viscous condition and accumulated quite close to the point of emission, they developed a slight massive flow outward; for it can be distinctly seen that the rocks have a circular outcrop and slope steeply outward.

It is in the massive rocks at the base of the main crater-wall that the main nature of the mineral structure when unaffected by volatile matter is best seen. So far as the species of mineral are concerned there is little to add to the description of von Wolff (10) and more particularly that of Bartrum (1), except that in practically all samples riebeckite is present and sometimes tuhualite also. It is remarkable that riebeckite was absent in all of tide rocks that were examined by von Wolff and Bartrum (l.c.). The feldspar, quartz, aegirine and cossyrite in the rocks of the coastline have been so well described by Bartrum that there is little to add. The sections examined support his statement that quartz is very unusual among the phenocrysts and is almost restricted to the groundmass. The feldspar phenocrysts are as stated by him nearly all anorthoclase. In the groundmass there are frequent minute intergrowths of feldspar and quartz almost circular in cross-section, and, surrounded as they are by a zone of femic minerals, they give the rock a most distinct ocellar structure. In hand-specimens of the most compact type of rock, which is found at the base of the south inner crater-wall and as ejected blocks at Opo Bay, some of the feldspars have a deep-blue chatoyance. Aegirine is in the form of small idiomorphic crystals in such rocks and is intimately associated with aenigmatite, though this mineral is usually restricted to the groundmass as cossyrite. This form of it is very general and the litle crystals are almost equidimensional in cross-section. Crystals of aenigmatite occur also in the rocks on the south side of Opo Bay. In many sections the little grains of cossyrite have a red colour. The outline of some of the larger crystals of aenigmatite is a little wanting in sharpness. In the sections riebeckite is very general in rather mossy growths, but sharply bounded crystals are unusual in the rock. Aegirine and cossyrite also often have mossy growths in many of the rocks, which

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thus acquire a striking and unusual appearance, at once recalling some of the Central African rocks described by Prior and Campbell Smith. In the first specimens that the author described this mineral was in irregular grains only and was wrongly identified as arfvedsonite (Marshall, 5).

In the rocks at Opo Bay another hornblende in the form of small needles 0.1–0.001 mm. is often seen. It has brown to greenish-brown colour with moderate pleochroism. This has not been identified with certainty and it does not occur plentifully. A mineral with a brownish-yellow colour, which is undoubtedly a secondary product, often pervades much of the groundmass, and Col. Campbell Smith has been good enough to inform me that a substance similar to this is found in some of the rocks from South Africa.

The mineral tuhualite has a special interest because its occurrence in the rocks of Mayor Island appears at present to be unique. A description of this mineral is published in the present Part of these Transactions. Its bright violet colour allows it to be seen occasionally in the rocks, even in hand-specimens, when it is associated with transparent feldspar. Tuhualite occurs in situ in the rocks at or near the foot of the crater-wall on its south and south-east sides. It has, however, been found also in a number of ejected boulders on the slopes rising from Opo Bay.