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On the identity of Guarinite and Hiortdahlite

Published online by Cambridge University Press:  14 March 2018

F. Zambonini  and
G. T. Prior
F. Zambonini
Affiliation:
Of the University of Naples
G. T. Prior
Affiliation:
Mineral Department of the British Museum
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Extract

Although more than fifty years have passed since Guiscardi published the first description of guarinite, the true nature of this mineral has never yet been ascertained. As regards the crystalline form, Guiscardi's first idea was that the crystals of guarinite are orthorhombic (he had himself reckoned the ratio a : b = 0.9896 : 1 ; see literature, nos. 3 and 5); but, having observed crystals with a tetragonal habit, he finally decided to describe the mineral as tetragonal-hemihedral. V. v. Lang by his optical investigations appeared to place the orthorhombic symmetry of guarinite beyond doubt, and Des Cloizeaux later published a note on a combination observed by him.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 15 , Issue 70 , July 1909 , pp. 247 - 259
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1909

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References

Page 249 note 1 That this is so will be proved latch. Guiscardi took the terminal form observed by him as {0kl} without optical examination; and the crystal on which I measured {011} was not transparent, and was therefore unsuitable for optical determination. In the course of the new investigation of guarinite, I had sections cut from the crystals which still remained in my possession, and so was able to convince myself easily that the terminal form is to be regarded as {101}. In fig. 3 of my paper of 1902 the axes a and b should be interchanged.

Page 249 note 2 According to my measurements.

Page 249 note 3 According to Brögger, Zeits. Kryst. Min., 1890, vol. xvi, p. 369.

Page 251 note 1 Throughout this paper I refer always to the first mode of orientation of hiortdahlite adopted by Brögger.

Page 254 note 1 {1̅11} is not given by Brögger in the list of forms, but occurs in the table of angles.

Page 254 note 2 These optical characters of the guarinite crystals, which I have repeatedly observed, and of the correctness of which there is no doubt, vary from those given in my work of 1902. My determinations were made at that time on avery beautiful crystal which came from the greyish-violet trachyte with melanite, and unfortunately was afterwards lost. That crystal was, however, not guarinite, but rather danburite, or a mineral near to danburite ; this would explain the wonderful similarity which was found between the supposed guarinite and danburite. That the crystal which was examined was not guarinite is evident tram the fact that it showed uniform extinction, and had the plane of the optic axes exactly parallel to {001}, with exactly perpendicular emergence of the mean lines on the forms regarded as {100} and {010}. These characters do not at all agree with those of true guarinite.

Page 256 note 1 Prior's determination with the pycnometer gave 3.25 (see p. 9.57).

Page 257 note 1 Communicated by permission of the Trustees of the British Museum.

Page 259 note 1 The individual numbers for Nb2O5 and Ta2O5 are the result of the Marignae separation, and can have little significance considering the small amount of material dealt with.