Transactions of the Geological Society, 1st series, vol. 3/On the Oxyd of Uranium

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II. On the Oxyd of Uranium, the production of Cornwall, together with a description and series of its Crystalline forms.

By William Phillips, Member of the Geological Society.

Read February 17th, 1815.

The only mine in Cornwall which until within the last few years was known to have yielded the oxyd of uranium, was that called Carharack, which was situated about two miles nearly south of St. Die. The crystals on a specimen from that mine in my possession are tabular, of a green colour and transparent, except such of them as are partially or wholly coated by a deposition of an ochreous substance, similar to that termed gossan by the miner. This substance also is interposed between an aggregated quartz tinged with iron, and the crystals; some of which are imbedded in it. It may therefore be termed, in regard to this specimen, the matrix of the crystals. On many of them have been deposited numerous minute cubes of a light green colour; which, as there is a considerable deposition of cubic arseniated iron in a cavity of the same specimen, I consider to be that substance rather than the oxyd of uranium: for though the latter sometimes takes a form so nearly approaching to the cube as that the eye cannot perceive any difference, yet such instances are certainly not very common.

In 1805, I noticed some crystals of the oxyd of uranium on the refuse heaps of Tin Croft mine, which is at the foot of a granitic hill called Carnbrae near Redruth; the veins of that mine run partly through granite, partly through schist.[1] A further search enabled me to obtain several beautiful specimens containing many varieties in the form of the crystal as well as in colour. The crystals are for the most part well-defined, but the largest scarcely exceed half a line in length or breadth, and some of them are accompanied by black pecherz (Uran oxydulé Haüy); they were found on, or in the cavities of, considerable blocks of a hard substance enclosing portions of decomposed felspar; but quartz formed the principal part of the mass, which had decidedly the character of being an aggregation resulting from the decomposition of granite. The specimens in my possession, are for the most part thinly coated with a black substance which I consider to be the pecherz in a pulverulent form; internally they are of a redish hue, arising probably from iron, with which the mine abounds, and occasionally some minute rounded portions of iron hæmatites may be observed. The depth at which these blocks were found, according to the best information I could obtain, was about 90 fathoms from the surface, in a copper vein.

I also obtained many specimens containing beautiful and well defined crystals of the oxyd of uranium from Tol Carn mine, which is about two miles north of Carharack, and near St. Die. The veins of Tol Carn mine pass through a decomposed granite, of which the prevailing substance seems to be felspar, enclosing portions of quartz and silvery mica. With this substance the vein seems to have been filled in that part in which the oxyd of uranium was found, but it was of a dark colour, and had attained, though considerably friable, a texture much more firm than that of the neighbouring country; and as, on almost every specimen, the crystals of the oxyd of uranium were accompanied by pecherz, it seems probable that both its colour and hardness may be ascribed to the dissemination of that substance throughout the mass, which leaves a black streak on paper. Some specimens also abounded with pyrites both arsenical and martial, and some of them have decomposed since they came into my possession. The veins of Tol Carn mine afforded little or no copper; the uranium was found in one of them at about 30 fathoms from the surface.

The colours of the crystals both from Tol Carn and Tin Croft mines are nearly the same. They vary from almost opake white to yellowish, and pass into the most brilliant yellow; some being transparent, others opake. Of some tabular crystals, the center is transparent and nearly colourless, and the edges are yellow. On other specimens the crystals are transparent and of a greenish hue, whence they pass through almost every shade, into deep grass green; while in others, the center of the crystal is yellow and the edges only are green. Again, from a brownish tinge they pass into a rich brown, but as the surfaces of these crystals glisten, the colour seems to be original; while on many specimens the crystals, which are of a light green colour, are hollow at their centers and of an ochreous brown, a circumstance arising doubtless from decomposition, and which in other specimens has proceeded so far as that the forms of the crystals can no longer be defined.

The crystals on some specimens from Tin Croft mine are accompanied by spiculæ of blue carbonated copper; in others, by green carbonated, and red and black oxides of copper, and on one specimen, they are deposited on minute spiculæ of oxide of iron. I have some specimens of the red oxyd of copper from the mine called Huel Jewel, on which there are very numerous and minute tabular crystals of the oxyd of uranium of a light green colour, and one specimen of wavellite from Stenna Gwyn near St. Austin on which some crystals of a light yellow colour are deposited.

But by far the most brilliant specimens of the oxyd of uranium that have been found in this, or perhaps in any other country, were discovered within the last three or four years in Gunnis Lake copper mine near Callington in Cornwall. The gangue of two specimens in my possession is of quartz, bearing the characteristic marks of being the result of decomposed granite, and which is rather confirmed by the circumstance of its cavities being filled with grouan, or decomposed felspar, of a flesh colour; of another specimen, the gangue is wholly a hard gossan. All the crystals from this mine that I have seen are described by fig. 15; they are extremely thin, but on some specimens they are more than half an inch in diameter. They are for the most part lying flat together, forming fasciculi which interrupt each other at various angles, and give an extremely beautiful appearance to the group. In Gunnis Lake mine the oxyd of uranium was found at about 90 fathoms from the surface, and in a part of the vein in which gossan abounded. On the few specimens in my possession from that mine, I have not noticed any trace of pecherz, but on those from Tol Carn and Tin Croft mines, particularly the former, it prevails very much. It is sometimes of a resinous transparency, but is more generally of a dark brown or black, either amorphous or in globules: on several specimens from Tol Cam mine it is quite friable.

It has already been said that the crystals of this substance from Tol Cara and Tin Croft mines, from which the drawings of the accompanying series were made, are for the most part very small; all the larger ones are so deeply striated in a horizontal direction, as to present the appearance of distinct tabular depositions progressively altering in size; and these are easily separated in the direction of the striæ, by that means discovering occasionally a thin ferruginous deposit between them, but not extending to the edges of the tables. Those crystals which are so small as to merit the term minute, though of a thickness nearly equal to their breadth, particularly those of a deep green colour, rarely exhibit the horizontal striæ on their lateral planes, which are perfectly brilliant: yet I have not been able to find one that could satisfactorily be submitted to the reflecting goniometer. On the measurement of the angles of the oxyd of uranium, it is therefore impossible for me to offer any thing.

The ease with which the crystals of this substance are separated parallel with their terminal faces, was long since noticed by the Abbé Haüy, who adds that its other cleavages can only be perceived by the assistance of a vivid light. The Comte de Bournon who acknowledges much attention to this substance, says (Cat. p. 340) that by the assistance of a strong light, he could perceive indications of cleavages in the directions of both the diagonals of a tetrahedral prism (Fig. 2 or 3), which by both these scientific mineralogists is adopted as the primitive form of the oxyd of uranium. I have very satisfactorily obtained cleavages parallel with the lateral planes of that solid.

The authors above cited do not however agree in regard to the height of the prism which both have assigned as the primitive form of this substance. The former of them has supposed that the height is to the breadth, as 16 to 5, while the Comte de Bournon considers that there is some, though not conclusive evidence for adopting a tetrahedral prism with square bases much less in height than that assigned to it by the Abbé Haüy. It is not in my power to offer any thing decisive of this question, for as it depends, in the first place on the precise admeasurements of certain angles, which no crystal I have hitherto seen has enabled me to make, and in the second place, on calculations founded thereon, for which I possess not the requisite qualifications, I am compelled simply to state the opinions of these crystallographers, and shall be amply gratified if, in their estimation, what is now offered in regard to cleavages not before obtained, and modifications not before noticed, shall tend to throw any light on this important part of the subject.

The occasional notice of what appear to be perfect cubes, added to the certainty that cleavages are practicable parallel with every plane of that solid, at first tempted me to presume that it ought to be considered as the primitive form; but the consideration of some circumstances connected with the crystallization of this substance, induced me wholly to abandon the idea. The inspection of the annexed drawings will evince that, of the five modifications described, the first alone is compatible with the cube.[2]

All the crystals represented in the accompanying series which are so remarkably flat as to have the appearance of mere laminæ, are from Tol Carn mine, which yielded only such as are of that description, while on the other hand, the greater part of those from Tin Croft mine nearly approach in height the dimension of the cube, or exceed it. A considerable proportion of such as are rather less than that dimension exhibit no striæ on the lateral planes, which are very splendent: many of those which are deeply striated on these planes are iridescent on the surface. The crystals from Tol Cam mine are generally of great length and remarkable transparency, those from Tin Croft are more generally opake; but the long and slender crystals from the former mine rarely suffer interruption from each other, being generally deposited at right angles, and thereby shewing a constant tendency to assume a quadrilateral figure.

There are in my possession about 55 specimens of the oxyd of uranium from the various mines in Cornwall above cited, and upwards of 200 detached portions, each having one or more well defined crystals on them and placed on pieces of wax; from these the accompanying drawings were made. Pl. 5, 6, and 7.

Varieties of the Primitive Crystal.

It has been already noticed that the primitive crystal of the oxyd of uranium is considered to be a tetrahedral prism, with square bases. I have never observed any crystal, exhibiting the primitive planes only, of a greater elevation than that described by fig. 1. Fig. 4 represents an elongated crystal: this elongation is so considerable on several crystals from Tol Carn mine that he length exceeds the breadth many times.

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First Modification.

The first modification is the result of a decrease along the lateral edges of the primitive prism, by which each is replaced by a quadrangular plane.[3] The crystals represented by figs. 6, 7, and 8, are the most common of the whole series.

Second Modification.

This modification consists in a decrease on the terminal edges of the primitive prism, by which each is replaced by a trapezoidal plane, inclining more on the lateral than on the terminal faces; and, as will be obvious on consulting the planes of this modifications numbered 2.2, on figs. 23 and 24, tending to produce a very acute octohedron. On the crystals described by figs. 13, 14, 15, and 16, the lateral primitive planes have wholly disappeared The lines on fig. 9 shew the striæ, which, on the larger crystals, are not only visible, but deep.

Third Modification.

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The third modification consists in a decrease on the solid angles of the primitive prism, by which each is replaced by an isosceles triangular plane, inclining more on the lateral edge than on the terminal face, fig. 17. The length of the crystals delineated by figs. 23 and 24, which shew the approach to the acute octahedron, would, if the apices of the crystals had been complete, have equalled at least four times their breadth; they are deeply striated in the direction shewn on fig. 9. I possess a crystal delineated by fig. 24, on which the planes 2.2, are perfectly brilliant and well defined. Fig. 31 shews the combination of the planes of this with those of the preceding modifications and of the primitive prism. On the crystals described by the four last figures in the series of this modification, two of the four planes of the second modification have disappeared on each pyramid, giving to their common base and to their terminal faces the form of triangular planes.

Fourth Modifications.

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This modification, like the preceding, consists in a decrease on each solid angle of the primitive prism by an isosceles triangular plane, but which, instead of being placed, as in that modification, more on the lateral edges than on the terminal faces, inclines more on the terminal faces than on the lateral edges. On the crystal represented by fig. 42, the planes of this modification are perfectly brilliant.

Fifth Modification.

By this modification the terminal edges of the primitive prism are replaced by trapezoidal planes tending to form an octahedron, fig. 46. The succeeding figure shews the planes of this, in combination with those of the second modification, or the acute octahedron. The crystals described by figs. 46 and 47 are numerous, brilliant, and well defined: they rarely exhibit any lateral striæ, but are so minute as to render it impossible even to approximate the real admeasurement of the angles formed by the meeting of any two of their planes. An attentive examination induces the belief of their being somewhat more obtuse than the regular octahedron; and they are so delineated.

  1. Geol. Trans. Vol. 2. p. 152.
  2. In reply to this observation it may be said that the plane which constitutes the third modification of the annexed series, also occurs on cubes of fluate of lime and sulphuret of iron; but on these two substances, which are remarkable for the beauty and elegance of their crystalline forms, that plane only appears in combination with two other similar planes, placed on the edges, and replacing the solid angle of the cite.
  3. The Count de Bournon in his “ Catalogue” has described a modification which I have not been fortunate enough to discover on any crystal from Cornwall, whence every crystal delineated in the annexed series was brought. The planes of this modification he describes as being in combination with that above quoted as the lint modification, and as replacing the lateral edges of that plane by quadrangular planes.