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In heavy concentrates obtained from the Snowy River dredge and stream gravels of that south-western Nelson river, the variety of zircon known as hyacinth is one of the more important constituents along with ilmenite, monazite, xenotime, and garnet. Attention has been drawn to the morphology and physical properties displayed by this mineral in an earlier paper (Hutton, 1950, pp. 689–692), but additional data are now reported.
Single crystal study of the least abraded particles of the simplest form, 0.062–0.125 mm in length, shows that in seven crystals, the forms m {110} and p {111} are dominant, with a {100} apparently absent. On the other hand, the remaining three crystals exhibit a {100} and e {101} as the dominant forms with m {110} and p {111} apparently not developed. The development in zircon of second order prisms as the sole form in the zone [001] would appear to be unusual.
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Films yielded by rotations about the a- and c- crystallographic axes of carefully oriented crystals1 exhibit reasonably precise reflections at low angles on the zero-and first-layers, but at angles greater than 80° 2θ, the reflections on the zero-layer become diffuse and there is no resolution into α1 and α2 lines. Third-layer lines in films for both a- and c- axis rotations are barely visible, and the reflections therein are merely diffuse streaks 3–4 mm in length (compare Fig. 1A, Plate 43, with Pabst, 1952, p. 155, top illustration in Fig. 6).
From zero-layer Weissenberg films that have been calibrated with quartz, unit cell dimensions of hyacinth have been determined to be as follows: ao = 6.625 Å, co = 6.010 Å (cell volume = 263.782 Å3), and both values are considered to be subject to error not in excess of 0.002Å.
Owing to the need to preserve the measured crystals undamaged for subsequent heat-treatment, powder photographs of hyacinth were obtained from crystals that exhibited similar hue and tone to those employed in single crystal work. A typical powder pattern, set out in Table I, column A, has been indexed as fully as possible in spite of the diffusitivity of reflections at higher angles2, and the lines therein do not exhibit any degree of asymmetry comparable to the skewing of peaks observed by Hurley and Fairbairn (1953, p. 665).
Crystals for which cell dimensions had been obtained, were then heated in silica capillaries in vacuo, and also in air, at 760° C. for 15 minutes.3 From a superficial point of view, the only effect of this treatment was decolouration of the crystals, but x-ray films secured for a- and c- axes rotations are distinct from those obtained before such treatment on account of the degree of ordering that had then taken place. The reflections are precisely defined, resolution into α1 and α2 reflections is evident, and the third-layer in films of both orientations is clearly developed (for copper radiation). No powder arcs are present except in rare cases to be described later in this paper, and accordingly, the situation for hyacinth employed herein is distinct from that found by Pabst (1952, p. 154) for Indiahoma, Oklahoma, zircon with a much greater degree of metamictization, but comparable to that found by von Stackelberg and Chuboda (1937), and von Stackelberg and Rottenbach (1940) for only partially altered zircon.
Careful measurements of a- and c- axes zero-layer Weissenberg films, standardized by quartz, gave cell dimensions as follows: a0 = 6.605 Å, c0 = 5.978 Å (cell volume = 260.796 Å3); the cell dimensions were found to vary by no more than 0. 002 Å in measurements made with several films.
As in the case of the unheated hyacinth crystals, the heat-treated material has been retained, and powder patterns prepared from other, but strictly comparable, crystals that were subjected to similar heat-treatment.
A typical powder pattern of the ordered form is set out in Table I, column B; this has been fully indexed, and at the same time, calculated d-spacings that correspond to all reflections observed on Weissenberg films have been listed. A few reflections such as (444), present in the powder pattern, but lying in blind areas between the a- and c-rotation axes, have been found on films secured by rotation about [110].
This pattern compares satisfactorily with that recorded by Swanson, Fuyat, and Ugrinic (1955, p. 71), although several additional faint lines are recorded by the present writer; one line observed by Swanson et al., viz. d = 0.853, was not found on the powder film, but the corresponding reflection was observed on the
[Footnote] 1 Rotation and crystallographic axes do not depart from one another by more than 5 minutes.
[Footnote] 2 The corrected diffraction angle (2θ) for the line due to (112) is 35.47, and according to Hurley and Fairbairn (1953, p. 666, Fig. 2), the hyacinth studied here would appear to have an activity equivalent to about 160 alphas/mg/hr.
[Footnote] 3 Heating, either in air or in vacuo, produces similar results, and no differences in cell dimensions were detected in hyacinth treated by the two methods.
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a-axis zero-layer Weissenberg film. Two lines at d = 1.218 and d = 0.902 Å that result from reflections from (314) and (534) respectively have not been recorded by Swanson et al., although they are both permitted by the space group for zircon, and have been observed on the writer's Weissenberg and powder films.
In one of the powder patterns listed for comparative purposes by Swanson et al., viz. the British Museum pattern, there is a diffuse line at 1.217 Å with an intensity of 20. This has been dismissed by these writers as alone due to monoclinic form of ZrO2, or baddeleyite. They are justified, in the main, since the British Museum pattern also exhibits a strong line at 2.76 Å1 that is undoubtedly due to baddeleyite, but the writer believes that they are incorrect in considering the line at 1.217 Å as solely due to baddeleyite.
Again it should be noted that, although Swanson et al. do not record a line at 0.902 Å ca, the United Steel Companies pattern listed by them does contain this line; the intensity, however, is given as 50 on their scale, although the corresponding reflection due to (534) is faint in both the present writer's powder pattern and a-axis 3rd-layer Weissenberg film. Inspection of the scale of intensities listed with d-spacings by the United Steel Companies, suggests that their films have been heavily over-exposed; this would permit fainter lines to assume greater densities.2.
[Footnote] 1 The low-angle lines in the British Museum pattern all appear to have d-spacings that are too low; accordingly, the 2.76 Å line almost certainly corresponds to that at 2.84 Å in baddeleyite.
[Footnote] 2 In this connection note that the (312) line at 1.71 Å in the United Steel Companies pattern is given an intensity of 100, whereas in carefully exposed Weissenberg films, it is less than one half the density of the reflection due to (200).