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Volume 58, 1928
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A Natrolite Tinguaite from Dunedin.

By P. Marshall, M.A., D.Sc., F.R.G.S., F.N.Z. Inst., Hector and Hutton Medallist, N.Z. Institute.

[Read before the Wellington Philosophical Institute, 7th September, 1927; received by Editor, 21st December, 1927; issued separately, 19th March, 1928.]

Plate 42.

In 1906, the author published a general geological description of Otago Peninsula.* Among the many types of rock that were mentioned a tinguaite was described as occurring at Hooper Inlet and the Fish Hatchery. In the description of this rock it was stated that “a peculiar spherulitic structure appears in the ground-mass. The mineral of which the spherulite is composed is highly birefrin-gent — as highly, as quartz — and is quite transparent. With ordinary light it is impossible to distinguish which part of the section is spherulitic. The spherulites between crossed nicols show an irregular black cross and the mineral of which they are composed is platy rather than fibrous.”

A microscopic preparation of this rock was sent to Professor H. Rosenbush, who was good enough to make the following comment in manuscript. “Ein typische Tinguait. Die gemenge Saridin, Nephelin, Grundmasse ist in einer mit nach wie vorgekommen weise zeolitisirt und zwar (a) in Natrolith Spherulithe von grossen Dimensionen so dass zweisehen gekreuzten Nicols bei schwacher Ver-grosserungen Interferenzkreuze erscheinen deren Arme uber der ganze Geseitsfeld reichen. Der optische Character der Fasern ist positiv. (b) an Andestellen des schliffs ist Nephelin und Sanidin in einen isotropen Zeolith umgewandelt, wohl Analcim.”

A more complete examination of this rock has now been made in the field and in the laboratory. Three dykes actually occur. One of these is at the outer end of the Portobello Peninsula, about four feet in width, and is much weathered; but sound rock can be obtained. In this example the spherulites are 10mm. in diameter. A second dyke is on the north-east side of Hooper Inlet on the south headland of its most westerly bay. This is very decomposed, and sound rock cannot be obtained. The third dyke from which an abundance of sound rock can be obtained is also on the north-east side of Hooper Inlet half a mile further to the eastward.

In hand specimens the rock is dense with the dull-green colour that is so characteristic of tinguaites. Where the rock has been weathered greyish round spaces of the size of a pea become noticeable,

[Footnote] *P. Marshall, The Geology of Dunedin, Q.J.G.S., 62, 1906, p. 394.

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and if it has been subject to the action of salt water the round bodies are weathered out and a pitted surface is developed.

A microscopic section viewed in ordinary light seems a typical tinguaite with a few long slender sanidine crystals which are crowded with inclusions. There are a few nepheline crystals with sharp boundaries and abundant slender microlites of aegerine augite widely separated by a transparent colourless mineral. In this, there are numerous extremely minute perfectly clear, but curved sanidine microlites. When viewed between crossed nicols it is at once seen that much of the colourless mineral is arranged in large radiating groups. It has a moderate birefringence measured as .012 while the index of refraction is lower than that of feldspar. The mineral agrees in all respects in its optical properties with natrolite. The transparent mineral which lies between the feldspar microlites is isotropic, and has a low refractive index, and is thought to be analcite. When a section is treated with silver-nitrate and afterwards with potassium-chromate the isotropic mineral becomes stained with silver-chromate.

As will be seen from the letter that the late Professor Rosen-bush kindly sent me, that eminent authority thought that the zeolites were formed from a ground mass of nepheline and sanidine. This does not seem likely in view of the fact that perfectly fresh sanidine microlites penetrate into the ground mass everywhere. Feldspar laths also extend into the spherulites as well as perfectly fresh squares of nepheline. In addition it is noticeable that the finest needles of aegerine are quite unaltered and retain their fresh green transparency throughout. The inclusions in the crystals of feldspar are partly a colourless isotropic substance which is considered to be analcite, and also very fine needles of aegerine. All the crystals of the different minerals appear to be so fresh and clear that any mineral alteration since their original formation is most improbable.

The chemical composition of the rock is as follows:—

A B C D
Si O2 52.60 56.16 54.46 52.40
Ti O2 0.42 — — —
Al2Oa 19.12 19.25 19.96 19.92
Fe2Oa 4.32 4.77 2.34 3.83
Fe O 1.73 2.72 3.33 1.51
Ca O 1.50 1.26 2.12 1.34
Mg O 0.08 0.21 0.61 0.32
Na2O 10.80 6.08 8.68 11.71
K2O 4.05 4.66 2.76 4.10
P2Oa 0.16 0.21 — —
H2O 5.80 4.09 5.20 3.94
100.58 99.41 99.46 99.07
A.

Natrolite tinguaite, Hooper Inlet, Dunedin, New Zealand, Anal. P. Marshall.
Picture icon

Fig. 1.—Thin section of natrolite tinguaite, Hooper Inlet, Dunedin, X 30. Two small crystals of nepheline can be distinguished.
Fig. 2.—The same X Nichols. The natrolite is distinctly seen to have a spherulitic development.

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B.

Dyke of tinguaite, Acheron Point, Otago Harbour, New Zealand, Q.J.G.S., Vol. 62, 1906, p. 395, Anal., P. Marshall.
C.

Tinguaite, Umtek, Kola, Rosenbush, Elem. der Gest., 2nd edit. 1901. p. 233, No. 12.
D.

Tinguaite, Mt. Kosciusko, N.S.W. Anal., F. B. Guthrie, Journ. and Proc. Roy. Soc, N.S.W., Vol. 35, 1901, p. 366.

Comparison with the analyses of tinguaites from Umptek and Mt. Kosciusko, N.S.W., shows that the rock is quite a typical tinguaite though the percentage of soda and of water are both high. It appears to be the case that in a rock of this nature the proportions of the silica, alumina, and alkali are such that a crystallization of feldspar and nepheline in some ratio accounts exactly for their relative amounts. Here, however, the relatively small amount of alumina compared with the high amount of soda and the high percentage of water has determined the crystallization of natrolite and analcite.

Though the percentage of water is high, it is not exceptionally so when the composition of this rock is compared with those of other tinguaites, as is shown above. Since the evidence appears to point conclusively to the opinion that the zeolites are original in this rock, the question arises whether original zeolites are present in other tinguaites. Rosenbush specially mentions that the nepheline changes to zeolites easily, usually to analcite, but he also makes the statement* that analcite is never an original constituent of rocks.

[Footnote] *H. Rosenbush, Gesteinlehre Edit. 2, 1901, p. 224, and Mikroscop Phys., 1905, Vol. 1, Hf. 2.

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