Mineralogical Notes from the University of Otago
[Read before the Otago Institute, October 8, 1935; received by the Editor October 11, 1935; issued separately, June, 1936.]
Chlorite (No. 1358, Red Mountain, South Westland).
In his paper on the Metamorphic and Intrusive Rocks of Southerp Westland, Dr. F. J. Turner (1933, pp. 273–274) described an interesting chlorite “from a vein of almost pure, coarsely crystalline chlorite, 3 cm. to 5 cm. in width, cutting the peridotite of the Red Mountain.” Continuing, he states that “macroscopically the mineral is deep green in colour, and occurs in idiomorphic to subidiomorphic hexagonal tables from 1 mm. to 10 mm. in diameter. The section consists almost entirely of well crystallised very pale chlorite, with minor amounts of colourless to pale green serpentine, occupying the interstices between the chlorite crystals. The chlorite is uniaxial and positive, frequently twinned parallel to 001, and in vertical section shows brownish-yellow anomolous interference tints when viewed between crossed nicols, indicating weak birefringence and strong dispersion. In some crystals zones or irregular bands giving Prussian blue interference tints may also be developed. The pleochroism, though faint, is very distinct, and of a type most unusual in chlorites, in that the absorption for light vibrating parallel to the vertical axis (Z) is definitely stronger than that for vibrations parallel to the cleavage. The pleochroic scheme is
|X = Y = colourless|
|Z = pale brownish yellow|
|X = Y < Z|
Basal sections are thus colourless and non-pleochroic. Except for the unusual pleochroism, the properties agree perfectly with those of pennine.” In view of the abnormal absorption scheme of this mineral, Dr. Turner suggested to the writer that he might carry out a chemical analysis and determine the refractive indices.
The chlorite was obtained in a pure state by picking out the individual tabular crystals, which were then examined under a lens and freed by hand-picking from any adhering serpentine. The flakes are clear green in colour, flexible, but not elastic; the hardness is approximately 1.5–2, and the streak white. The refractive indices as determined by the oil-immersion method gave:—
|α = 1.588 ± 0.001|
|γ = 1.594 ± 0.001|
|γ–α = 0.006 ± 0.001|
The flakes, after careful grinding, were analysed by standard methods, giving the following results:—
|Molecular ratio.||Numerical ratio.|
|H2O ≥ 105°||12.60||0.6805||4.02|
|H2O ≤ 105°||0.35||—||—|
From the figures in the third column, the formula deduced is almost exactly 4H2O, 5(Mg,Fe,Mn) O, (Al,Fe)2O3, 3SiO2, corresponding to an isomorphous mixture of equal amounts of the amesite and serpentine molecules. Such chlorites are classed by Ford (1932, p. 669) in the 1932 edition of Dana's Textbook of Mineralogy as clinochlore. However, the presence of 0.14% of manganous oxide replacing a part of the magnesia is noteworthy, and probably accounts for the pale brownish-yellow tint for the Z vibration-direction. The optical properties of this mineral do not agree with those of the Indian manganchlorite described by Fermor (1909, p. 195), nor with those of a similar chlorite from the Harstig Mine, Sweden* (Ford, 1932, p. 671) containing 2.3% of manganous oxide, except that they are pleochroic with brown or bronze tints. As pointed out by Turner (loc. cit., p. 274), the New Zealand mineral closely resembles (even in its unusual absorption scheme) a chlorite described by Foslie (1931, p. 223) from Norway, but in absence of an analysis of the latter, a more complete comparison cannot be made.
Following the scheme given by Ford (loc. cit., p. 671), the chlorite from Red Mountain is classed as a manganiferous clinochlore.
Strontium-aragonite (No. 313, Alexandra, Central Otago, New Zealand).
A specimen of a heavy carbonate mineral recently sent to the Geology Department of the University of Otago by Mr Spain has proved on analysis to be a strontium-bearing variety of aragonite, such as was at one time called mossotite. The mineral was said to have been obtained in situ as a small vein cutting schists one to two miles west of Alexandra, Central Otago, New Zealand.
[Footnote] * This chlorite is wrongly stated by Fermor (1909, p. 194) to contain 32.3% of manganous oxide replacing part of the magnesia.
It is a creamy-white mass composed of radiating fibres, with vitreous lustre, averaging about 15 mm. in length. It has a hardness of 3–3.5 and a specific gravity of 2.90–2.91 at 13.5°C. It effervesces in cold dilute hydrochloric acid and gives the intense strontium-red colour to a Bunsen flame. Fragments boiled for ten minutes in cobalt nitrate solution assume a purple tint. In thin section, the mineral is seen to be composed of groups of slender, radiating fibres. It is biaxial, with a very small optic axial angle, the acute bisectrix being parallel to the length of the fibres, the optic sign negative. The refractive indices as determined by immersion of the powdered material in oils are as follows:—
|α = 1.527 ± 0.001|
|γ = 1.676 ± 0.001|
|γ–α = 0.149 ± 0.002|
The value for γ was the maximum that was obtained from numerous sections.
The following is the result of an analysis stated in terms of the compounds present in the mineral specimen:—
|SiO2 and insol||1.42|
On adding the finely crushed material to bromoform of density 2.90 no heavy grains drop out, the carbonate remaining suspended. After thirty minutes interval, the percentage of grains in the lower half of the separating funnel was greater than that in the upper half, indicating a density of slightly greater than 2.90. On dilution to about 2.80 all the grains, except the silica, sank. The mineral is therefore homogeneous and not a mechanical mixture of strontianite and aragonite.
The writer is much indebted to the late Dr J. K. H. Inglis, of the Otago University Chemistry Department, for allowing this work to be carried out in his laboratories and for his willing advice and assistance.
Fermor, L. L., 1909. The Manganese-ore Deposits of India, Mem. Geol. Surv. India, Pt. 1.
Ford, W. E., 1932. A Textbook of Mineralogy, 4th edition, New York, John Wiley and Sons.
Foslie, S., 1931. On Antigorite-serpentines from Ofoten with Fibrous and Columnar Vein Minerals, Norsk. geol. tids., B. xii, 1931, pp. 219–245.
Turner, F. J., 1933. The Metamorphic and Intrusive Rocks of Southern Westland, Trans. N.Z. Inst., vol. 63, pp. 178–284.