Contributions to the Mineralogy of New Zealand.
[Read before the Otago Branch, September 10, 1946; received, by the Editor, September 16, 1946.]
An Occurrence of Diaspore [HAlO2]4.
During a recent mineralogical analysis of some concentrates obtained from the sands of a tributary of Mangakirikiri Creek in the Thames District, Hauraki Peninsula, crystals of diaspore were recognised to be one of the chief constituents; the following minerals placed in order of abundance were observed to be associated with the diaspore: ilmenite (in thick plates with well-defined rhombohedrons and poor development of basal plane), hypersthene, hornblende (both greenish-brown and oxy-types), quartz, kaolinised feldspars, pyrite, rhyolite glass (n = 1.497), apatite, epidote, and montmorillonite (α = 1.493, γ = 1.534, γ-α = 0.041.
The majority of the diaspore crystals were found to be sharply euhedral, but with a very marked flattening, the length of the crystals ranging from 0 12–0.25 mm. The ratio length (parallel to the c axis) to breadth (parallel to a axis) in the euhedra was found to range between 1:0.8 to 1:1.4, whereas the ratio length to breadth (in the direction normal to the flattening) is approximately 1:0.2. The crystals when viewed in oblique illumination beneath a binocular microscope are colourless, they exhibit a brilliant vitreous lustre, and show a series of vertical striations in the prism-pinacoid zone.
The morphological development of the mineral is relatively simple, and the forms m(110), b(010), e(011), and s(212) were seen with the form b(010) dominant; these observations were made by inspection of the grains beneath a binocular microscope, and illustrations of comparable crystals have been figured by Iddings (1906, p. 517). Two cleavages were seen, one parallel to the direction of flattening, the brachypinaeoid, and a second, presumably parallel to (210), which produces striations on b(010).
The refractive indices were determined in sodium light by immersion methods and found to be as follows:
α = 1.701 ± 0.003
β = 1.721 (average)
γ = 1.750
γ-α = 0.049
Owing to the very marked flattening of the crystals parallel to the braehypinacoid, the determination of the value for β of the refractive
index ellipsoid was difficult to obtain with exactness, so that the average of four determinations has been given; these determinations were: 1.720, 1.724, 1.722, 1.718. The optic axial plane is parallel to 010, with Bxo or X parallel to the c crystallographic axis. Optic axial angles determined in five crystals were found to be 84°, 86°, 85°, 86° with an extreme value of 90°; if this latter value is disregarded, then the average of 85° may be taken to be close to the true value. The mineral is optically positive.
Some of the crystals, though colourless under the microscope, appear to be brown when viewed with a hand-lens, but this effect appears to be due to semi-opaque dust-like inclusions; the diaspore crystals are very slightly paramagnetic, but the grains with inclusions did not appear to be any more so than those devoid of them. If some of the crystals with inclusions are concentrated by hand picking and brought into solution by fusion in sodium carbonate, a very much stronger reaction for iron is obtained than with a similar solution of diaspore crystals free from inclusions. It is believed that these inclusions are haematite or limonitic particles (ef. Nel, 1930, pp. 211–212).
Origin of Diaspore.
Since diaspore has not previously been recorded in New Zealand and as the present occurrence is not in situ, it is of interest to observe the paragenesis of this mineral in a number of occurrences, the following details having been obtained in the literature available:
As a constituent of bauxite and similar clays (Rao, 1929).
In highly aluminous flagstones and shales in association with zunyite (Nel, 1930).
In granite pegmatites (Sekanina, 1933).
In emery rock (Lapparent, 1934).
As a metamorphie mineral with andalusite in quartzmuscovite schists (Kerr, 1932).
Associated with alum and corundum in a pyrophyllite deposit (Yoshiki, 1933).
With alunite in hydrothermally altered andesites and allied rocks (Ransome, 1909, Astashenko, 1940).
In New Zealand the mineral was found in sands at the headwaters of a small stream draining country made up entirely of andesites, dacites, and tuffs and breccias of these rock-types (Fraser, 1910, p. 23); propylitization is widespread and every transition between fresh, unmetasomatised lavas to those showing complete alteration to secondary minerals is known. Although diaspore has not yet been recorded in the propylites from the Thames area, there seems little reason to doubt that these diaspore-bearing sands have their ultimate origin in the nearby altered lavas, particularly in view of the other minerals present in these sands. However, until the mineral has been found in situ it is not advisable to discuss the chemistry of its formation in propylitized andesites and dacites, but it is significant that in some mining areas, for example at Goldfield, Nevada (Ransome, 1909), diaspore occurs in intensely metasomatised andesites and rhyolites that are closely comparable in mode of origin to those of the Hauraki area.
An Occurence of Alunite K2Al6 (OH)12(SO4)4.
The occurrence of the hydrated potassium aluminium sulphate. alunite, has been observed in hydrothermally altered andesite, dacite, and perhaps latite lavas in the Thames area, and also as an associate of allophane and kaolinite in the Whangarei District.*
Both Skey (1890, p. 29) and Maclaurin (1903, p. 7) have reported, the occurrence of this mineral, but, as pointed out by Morgan (1927, p. 8), the analyses appear to have been made on very impure material so that these records are of little value.
In the Thames area alunite occurs in highly silicified andesitic or dacitic rocks half a mile N.N.E. from trigonometrical station D, Thames Survey District. The mineral forms radiating or plumose aggregates of basal plates averaging 0.12 mm. in length. There is a very good basal cleavage which allows the formation of thin platelets with an hexagonal outline when the crystal aggregates are crushed; in a few instances the outlines of some cleavage plates were similar to those characteristic of cross-sections of tourmaline crystals. Generally, however, the radiating crystals do not show complete crystal form.
In most cases the mineral is almost colourless, although rarely a pale yellow tint may signify the presence of jarosite, a mineral that forms an ionic substitution series with alunite. Alunite is usually heavily clouded with opaque inclusions, often pyrite.
The optical properties have been determined as follows:—
α = 1.570
γ = 1.591
These values were found for most of the grains employed, but a distinct range of values was apparent as follows:
α = 1.570–1.575
γ = 1.591–1.595
γ-α = 0.020–0.021
The mineral is uniaxial and positive and the elongation is parallel to the X vibration direction. The specific gravity was determined as 2.69 by centrifuging the powdered rock in bromoform-benzene mixtures.
Alunite has also been recognised as a minor constituent of a sample of allophane, associated with kaolinite, quartz, and feldspar. In this instance the refractive indices were found to be:
α = 1.569
γ = 1.590
α-γ = 0.021
Origin of Alunite.
It is pertinent to note at this point that alunite occurs in situ in metasomatised lavas of intermediate composition from which it is thought the diaspore-bearing sands described earlier were derived. Thus it is of special significance that Ransome (1909, p. 130) in his study of Goldfield, Nevada, has found alunite with a subordinate quantity of diaspore in rock petrographically and genetically similar
[Footnote] * Bote 216, Biggar's Property, Whangarei.
to those at Thames. In his opinion (Bansome, loc. cit.), the alunite and associated diaspore have formed as a result of a sequence of events as follows:
The first stage required the uprise of hot solutions containing hydrogen sulphide, some CO2, and probably some alkali in the form of sulphides.
Oxidation of the hydrogen sulphide in these solutions to sulphuric acid occurred at or near the surface, the acidic solutions percolating down through the warm rocks, reacting with them, and mingling with the uprising sulphide solutions.
The mineral paragenesis at Thames is so similar to that found at Goldfield that a comparable origin for the alunitized rocks is suggested. The abundance of pyrite and sulphates is indicative of the presence of sulphide solutions and of their oxidized derivatives, whereas the presence of allophane with the Whangarei material also suggests an acid environment.
Beyond these indications little more can be said without more detailed study of the entire association of metasomatised lavas and ore bodies.
A portion of the work involved herein was undertaken when the writer was an officer of the Geological Survey Branch of the Department of Scientific and Industrial Research. The courtesy of Mr. Owen Marshall for supplying some of the material studied is gratefully acknowledged.
Fraser, C., 1910. The Geology of the Thames Subdivision, Hauraki, Auckland. N.Z. Geol. Surv. Bull. No. 10.
Iddings, J. P., 1906. Rock Minerals, their Chemical and Physical Characters, and their Determination in Thin Sections, 1st ed., John Wiley and Sons. New York.
Kerr, P. L., 1932. The Occurrence of Andalusite and Related Minerals at White Mountain, California. Econ. Geol., Vol. 27, pp. 614–643.
Lapparent, J. De, 1934. Comportement, en leur Gîte, des Émeris de Samos. Compt. Rend. Acad. Sci. Paris, Vol. 198, pp. 760–761.
Maclaurin, J. S., 1903. 36th Colonial Lab. Ann. Rep., p. 7, No. 9381.
Morgan, P. G., 1927. Minerals and Mineral Substances of New Zealand. N. Z. Geol. Surv. Bull., No. 32.
Ransome, F. L., 1909. The Geology and Ore Deposits of Goldfield, Nevada. U.S.G.S. Prof. Paper 66.
Skey, William, 1890. 24th Colonnal Lab. Ann. Rep., p. 29, No. 5092.