Transactions of the Geological Society, 1st series, vol. 1/On the Laumonite
|←On the Pitch-lake of the Island of Trinitad||On the Laumonite
|On the physical Structure of Devonshire and Cornwall→|
By M. Le Comte de Bournon, F.R. & L.S. &c.
Foreign Secretary of the Geological Society.
[Translated from the original French Manuscript.]
This substance, which has been long known under the name of efflorescent zeolite, has been termed Laumonite by Werner, in honour of M. Gillet de Laumont, to whom we are indebted for our first knowledge of it, as well as for many other important contributions to the science of mineralogy.
The former name of this mineral (efflorescent zeolite) was given to it, in consequence of the property it has of not being able to bear, even for a short period, exposure to the air without undergoing disintegration and finally falling into powder; and the opinion most generally entertained with respect to it was, that it belonged to the species of mesotype. It is however altogether different from that substance, nor does it belong to any of the numerous series of zeolitic minerals, which have hitherto been made known to us.
Highly calculated however as this substance is to excite the interest and the curiosity of the mineralogist, we have as yet had but very slight and imperfect descriptions of it. The author who has spoken of it most in detail is the Abbé Haüy.
In the first appendix to his “ Traité de Minéralogie ” that celebrated mineralogist, who was then disposed to consider it as a variety of the mesotype, states that he was induced to regard its primitive crystal as a rectangular tetrahedral prism, having indications of subdivision in the direction of its two diagonals; but since in his subsequent work, entitled, “ Tableau comparatif des résultats de la Crystallographie et de l'Analyse chimique,” &c. p. 49. he has changed his opinion, and has declared its primitive crystal to be a rectangular octahedron, having its faces unequally inclined. The observations, which I have myself made on this substance, prevent my assenting to either of these two forms as its primitive crystal; and they have enabled me at the same time to present to the Geological Society a more complete examination and description of it. The possibility of my doing so I owe to the friendship of M. Gillet de Laumont, who lately sent me several specimens, among which was one of very considerable magnitude. On its way to me it was broken. Chance, which very frequently assists the observer when he is prepared to profit by it, produced by that accident what I should certainly never have attempted myself: it furnished me with an immense number of fragments, and of perfectly regular crystals, and at the same time by exposing to view the central part of the mass, which had never yet been acted upon by the atmosphere, and had therefore been preserved entirely unaltered, it enabled me to examine this substance before it had undergone any kind of change, a circumstance which must of necessity be but of very rare occurrence.
1. Primitive Crystal. A rhomboidal tetrahedral prism with rhombic bases, the sides of the prism meet at angles of 92° 30′ and 87° 30′; the bases are inclined upon the edges of 92° 30′, so as to form with them angles of 55° and of 125°, fig. 1. The height of the prism is to the edges of the terminal faces, in the ratio of eight to seven. This prism divides in a direction parallel to all its planes, but much more easily longitudinally than on its terminal surfaces; this division takes place also with greater facility on two of the opposite sides than on the two others. It is likewise more readily effected, but at the same time less neatly, when the substance has undergone alteration.
2. Integrant Molecule. This slightly rhomboidal tetrahedral prism is besides divisible parallel to its axis, and to the greater diagonal of its rhombic terminal planes. I have not perceived any natural joints in the direction of the other diagonal. This second division shews that the integrand molecule of the laumonite is a trihedral prism, the exact half of the rhomboidal tetrahedral prism, fig. 2.
3. Fracture. Lamellar.
1. Specific gravity. Taken with a piece slightly changed, but still preserving in some degree its transparency 22,34. The variety in which disintegration had commenced, having a dull white appearance, and opaque, but not sufficiently changed to break and divide of itself, did not shew any sensible difference with respect to this character.
2. Hardness. When the laumonite has not been altered, it cuts glass with ease; but in proportion as it becomes disintegrated, this hardness diminishes, and ultimately the least pressure reduces it into small delicate prismatic fragments.
3. Electricity by friction. None.
1. Action of acids. This substance is reduced to the state of a jelly by the action of acids.
2. Action of heat. Under the blowpipe, and without addition, the laumonite is fusible with a slight degree of ebullition, and affords a perfectly opaque and beautifully white enamel.
3. Analysis. As far as my knowledge extends, no analysis has yet been made by which the constituent parts of this substance have been determined.
4. Natural alteration. This character is so peculiar, so striking, and so constant in this substance, that it is perfectly entitled to be placed in the number of its essential specific characters. From the moment that the laumonite is exposed to the atmosphere, it begins to alter, and this alteration, as well as the progress of it, is proportional to the warmth of the air; it advances with such rapidity when the temperature is high, that having inadvertently undertaken during a very hot day the examination of its crystals, many of these, in which the alteration, although begun, was not yet very considerable, so quickly disintegrated that it became impossible, even in the short space of time necessary for their examination, to touch them without reducing them to powder. By this mere inattention I have to regret the loss of many very beautiful crystals. As this alteration proceeds, the mineral loses its transparency and at length becomes of a shining white appearance; then the faces which belong to the longitudinal planes of the primitive tetrahedral prism, assume a slightly pearly aspect. In proportion to the extent of this alteration the degree of cohesion subsisting between the crystalline molecules diminishes. The effect of this first state of alteration is to render the natural joints of the laminæ, which are parallel to the longitudinal planes of the prism, much more evident; and sometimes also, but more rarely, those which are parallel to their terminal faces. The alteration increasing, the diminution of cohesion between the molecules becomes more considerable; and the separation of the laminæ of crystallisation may be easily produced by simple pressure between the fingers. The crystal at length separates spontaneously according to this direction into prismatic fragments, of which many are perfectly regular; these again subdivide, and ultimately reduce the crystal to a mere powder. This alteration may be prevented, or arrested, by lightly covering the surface of the crystals with gum or with varnish, or by preserving the specimen in distilled water.
1. Colour. When it is not altered, this substance has hitherto occurred perfectly colourless. The alteration which it undergoes, renders it opaque, and of a shining white aspect, giving it at the same time a slightly pearly lustre on the longitudinal faces of the prism.
2. Transparency. The laumonite in its perfect and unaltered state is beautifully transparent, but it is extremely rare to meet with it in this condition, in consequence of the great facility and rapidity with which its alteration proceeds.
3. Phosphorescence. This substance does not possess any sensible phosphorescence; however, as it is accompanied, and even intermixed, with a highly phosphorescent lamellar carbonate of lime, the luminous appearance of which is, a bright reddish yellow or orange, it is liable to have some portions of this carbonate of lime interposed throughout its substance, which is the cause of its being sometimes observed, that fragments of it when submitted to trial give traces of a slight phosphorescence.
If this substance offer a striking peculiarity, and one very proper to entitle it to a distinct place in the classification of mineral substances, by the facility with which simple exposure to the atmosphere causes it to undergo disintegration, and finally to fall into powder; its crystallisation, which cannot be referred to that of any other known mineral, adds still more to the singular characters which the laumonite exhibits. This crystallisation generally appears in the state of crystalline masses, often of considerable size, and which at first sight present only a deeply striated or fasciculated surface; but these same masses, which apparently have no determinate crystalline forms, on being broken become immediately a rich field of observation by the great quantity of extremely perfect and diversified crystals, which the portions into which they divide, afford. They are therefore, at least with regard to a great number, merely a confused aggregation (arranged however in the direction of their prisms) of crystals perfectly formed, and of different shapes, piled one above the other on their ends. This structure, if I may be allowed the comparison, is like the bud of a flower, in which the form of the petals cannot be seen except by tearing it open and unfolding them. May it not be the same in many other substances which occur also in fascicular masses of more or less considerable size, such as tourmaline, thallite, prehnite, analcime, stilbite, &c. May not also the irregularity of the interior of these masses be at the same time owing to the difference in the variety of forms which these aggregated crystals present? and if it were possible to separate in these masses each of the component crystals, with as much facility as they are separated in the masses of the laumonite, might we not obtain the same result?
There is a striking irregularity in the manner in which the planes of substitution are situated on the laumonite, an example of which may be seen in fig. 3, with respect to those by which the edges of 92° 80′, in the prism are replaced. The same irregularity occurs in the position of the planes which are the result of the other modifications; for example, in the crystal delineated in fig. 28, there occurs in the place of one of the two obtuse angles of the terminal faces, a plane belonging to the 9th modification, whilst the plane of substitution for the other angle belongs to the 8th. In fig. 30. there exists a still greater irregularity in the planes of substitution of these two angles, one of them being replaced by two planes, which belong to the 8th and 9th modifications, and the other by a single plane of the 10th. This fresh example, if it were necessary, might serve to shew the difference which exists between the retrogradations on primitive crystals, where the geometrical form is one in which all the pants are perfectly symmetrical, and those on primitive crystals, in which the parts are not so circumstanced. A symmetry is established between the new planes produced on the first, while the planes which occur in the second, are more or less distant from it.
The difference which exists between what has been said by the Abbe Haüy in his “ Tableau Comparatf, &c.” with regard to the form of the primitive crystal of the laumonite, and that which I have myself established respecting it, will no doubt excite surprise. This difference for a long time detained me. The estimation, in which this celebrated mineralogist is so justly held, made me redouble my attention and care in the examination of this substance, but the several results of my inquiries have always afforded me the fullest confirmation of what I have said of the form of its primitive crystal, and of the measure which I have given of its angles. The circumstances that probably led the Abbé Haüy into an error, are the different varieties of crystals I have mentioned as forming the aggregations to which the fasciculated masses of the laumonite belong, but which, when this substance has undergone any alteration, are detached with much facility, producing so many isolated crystals, a great number of which present varieties differing one from the other. He would without doubt have obtained in this way a crystal analogous to that represented in fig. 14, as I have myself done, and which in reality presents a rectangular octahedron with its faces unequally inclined, and, regarding all the faces of this crystal as arising from a natural cleavage, he may have adopted it as being the primitive form of this substance. But two of the faces of this crystal are by no means primitive; they belong to the sixth modification. In no instance could the angles of this crystal be such as the Abbé Haüy has given them: the four faces which belong to the longitudinal ones of the primitive rhomboidal tetrahedral prism, meet each other two by two at an angle of 92° 30′; and the other four, of which two belong to the terminal faces of the primitive crystal, and the two others to the planes of the sixth modification, meet at an angle of 127° 40′; but these angles are never 98° 12′ and 121° 34′ as is said in the work which I have just noticed. The rhomboidal tetrahedral prism of 92• 30′, and 87° 30′ of this substance, approaches so nearly to the rectangular form that it is extremely easy to conceive how the Abbé Haiiy in his first determination of the primitive crystal of the laumonite (a determination which he has given us to understand was merely a first glance of the subject) might have been led to consider it as being in reality rectangular; but he certainly would not have committed that error, if the angles of this prism had really differed more than eight degrees from a right angle. It is on the supposition of the octahedron being the primitive form, that the Abbé Haiiy has stated, that the crystal, fig. 40, pl. III. of his “ Tableau Comparatif,” is a variety produced from that figure. This form is the only variety which he has given. I have represented it at fig. 38, keeping on its planes the letters of indication which he employs. In this figure the planes M belong to the longitudinal planes of the primitive rhomboidal tetrahedral prism; one of the planes P to the terminal face of this prism, and the other to the plane of the sixth modification; The plane I belongs to the first modification, and that indicated by S, which I have never perceived in any of the crystals that have come under my own observation, would belong to a fifteenth modification, which would be the result of a retrogradation by a single row along the edges of 87° 30′ of the prism.
This difference in opinion between the Abbé Haüy and myself with regard to the true primitive form of the laumonite may perhaps again give rise to the unjust reproaches which a similar diversity under circumstances nearly alike has already occasioned. By rendering the science responsible for the errors which those who cultivate it may commit, an attempt has been made to insinuate its uncertainty and even its inutility. But what science is there which would not be annihilated the moment that we made its truth and its usefulness to depend on the exact degree of correspondence that might subsist between the opinions of those philosophers who make it the subject of their study? If there exist some difference in the opinions entertained by the Abbé Haüy and myself on certain points in crystallography, what conclusion ought to be deduced from this circumstance? simply that this science, which on the one hand is supported by physics, and on the other by mathematics, and will perhaps at some future day become equally exact with the latter, has not yet obtained that certainty. Let us allow it to proceed towards this point, without obstructing its course. Difference of opinion when maintained with candour and decorum is perhaps not without advantage to the security and promptitude of its progress.
The laumonite has never hitherto been discovered except in a crystallized state, either in separate crystals, which is the most common appearance, or in an aggregation of crystals, forming masses of more or less considerable size for the most part irregular, and deeply striated externally.
Till the present time the laumonite had been observed only in the lead mine of Huelgoet in lower Brittany, in which it was discovered about twenty-five years ago by M. Gillet de Laumont.
But on further examination I find this substance occurring in specimens from various other places. I have in my possession a specimen from the Island of Ferröe, in which pretty large crystals, of a dull white aspect, and almost pulverulent, but of which the figure is still perfectly discernable, are grouped with stilbite in larger crystals, and not in the least altered, on a layer of quartz about three lines thick, enclosing a nucleus of that sort of argillaceous and earthy rock, well known as the gangue of the zeolites of Ferröe, and from which it is separated by a thin layer of green ferruginous earth. The exterior surface of this quartzose layer, on which the crystals of laumonite are placed, is covered by a vast quantity of small crystals of stilbite, differing in figure from those of the same substance that accompany the laumonite, the forms of both of which belong to the very numerous series of crystals of this substance, that have not yet been described.
I have another small group of laumonite also from Ferröe, the crystals of which, unmixed with any other substance, are placed on a small layer of granular quartz of a loose texture, the grains of which are all hexahedral prisms, terminated at each extremity by a hexahedral pyramid. These crystals, all of which belong to the elongated variety of the primitive crystal, arc placed edgeways on the quartz, and though this specimen has been several years in my collection, several of the crystals of laumonite still retain some degree of transparency.
In the collection of Mr. Richard Phillips I have seen a very fine specimen from Paisley in Renfrewshire, in which the laumonite is grouped with analcime, and I have a small specimen from Portrush in the County of Antrim, in which it occurs with stilbite and analcime.
I possess a specimen of amygdaloid having avery argillaceous and earthy base, from the Venetian States, the nodules of which are all hollow, and have their interior surface lined with very small crystals of this substance, of a dull white appearance, and very friable.
Lastly, I have a specimen of prehnite, of a slightly yellowish green colour, the crystals of which are grouped together and penetrate each other in such a manner as to form spheroids; these have their surface covered by a dull white and pulverulent laumonite. This specimen, which was given me as coming from China, has for its gangue an earthy argillaceous rock of a greenish grey colour.
Thus, with this, as with many other mineral substances, it has ceased to be found the exclusive production of a peculiar district, as soon as attention has been particularly directed towards it. It may be observed however, from what I have stated, that the laumonite affects the zeolitic rocks more than any others; and that wherever these exist, we may hope to meet with it.
I have said, under the head of specific characters, that the laumonite had not exhibited to me any sensible difference with respect to its specific gravity, whether this was taken whilst this substance had experienced only a very slight change, or whether when it was much more considerably altered. This fact which I did not expect, attributing, according to the opinion generally entertained, its disintegration or efflorescence to the loss of its water either of composition and consequently combined with it, or of crystallisation, and in that case simply interposed between its particles, this fact, I say, greatly surprised me. But is it true that in the laumonite, as well as in all the salts which effloresce on exposure to the atmosphere, this phenomenon is to be attributed to the loss of water? as far as the laumonite is concerned it appears to me very probable that this destruction is in reality, as I have said, nothing more than the simple result of disintegration. That this is the case will appear from the changes which occur in this mineral, and principally from the greater or less regularity of the primitive form which many of the smallest fragments preserve, when this alteration has even arrived at such a point that the substance divides of itself. If it is to be attributed to the loss of its water, this can only be the case with respect to that of crystallisation or of simple interposition. But if so, ought not this substance, as happens with regard to hydrophanous bodies, at some period during the loss of its water to have, in consequence of the same affinity which placed it there originally, a great tendency to resume it, and thereby, re-establishing the refractive power which belongs to it in its unaltered state, to recover its transparency, which it never does ? may not its alteration rather be occasioned by a strong attraction of its integrand molecules for caloric, and by the separation produced between them from the introduction of this fluid in larger quantity? I do not mean this as an assertion, but propose it simply as a question.
Primitive Crystal. A tetrahedral prism slightly rhomboidal of 92° 30′ and 87° 30′, with rhombic bases inclined on the edges formed by the meeting of the sides of the prism at an angle of 92° 30′, so as to form with them angles of 125° and 55°. The height of the prism is to the edges of the terminal faces in the ratio of 8 to 7.
|Retrogradations along the edges of the prism formed by the meeting of its sides at an angle of 92° 30′.|
|Number of the modifi-
|Figure of the crystal||Angles formed by the meeting of the new planes with the side of the prism on which they are inclined||Angles formed by the meeting of the new planes with the sides opposite to those on which they are inclined||Angles formed by the meeting of the new planes with those of the 1st modification.||Angles formed by the meeting of the new planes with those of the 2d modification.||Angles formed by the meeting of the new planes with those of the 3d modification.||Nature of the retrogradations|
|1st||Primitives Crystal with planes of substitution on the edges of 92° 30′||130°, 15′||136°||Retrogradation by a single row|
|2d||144°, 2′||128°, 28′||172°, 13′||Retrogradation by 4 rows in breadth, and 3 laminæ in height|
|3d||168°, 48′||103°, 42′||147°, 27′||155°, 14′||Retrogradation by 5 rows|
|4th||171°, 55′||100°, 35′||139°, 52′||147°, 44′||176°, 23′||Retrogradation by 7 rows|
|Retrogradations upon the acute angles of the terminal faces, answering to the acute solid angles of the primitive crystal.|
|Number of the modifi-
|Figure of the crystal||Angles formed by the meeting of the new planes with the terminal faces||Angles formed by the meeting of the new planes with the edges of 87°, 30′||Angles formed by the meeting of the new planes with those of the 5th modification.||Angles formed by the meeting of the new planes with those of the 6th modification.||Nature of the retrogradations|
|5th||Primitive crystal with the planes of substitution on its acute solids angle||98°, 5′||136°, 55′||Retrogradation by a single row|
|6th||124°, 56′||110°, 4′||153°, 4′||Retrogradation by 5 row in breadth and 3 laminæ in height|
|7h||134°, 22′||100°, 38′||143°, 26′||170°, 34′||Retrogradation by 2 rows|
|Retrogradations on the obtuse angles of the terminal face|
|Number of the modifi-
|Figure of the crystal||Angles formed by the meeting of the new planes with the terminal faces||Angles formed by the meeting of the new planes with the edges of 92°, 30′||Angles formed by the meeting of the new planes with those of the 9th modification.||Angles formed by the meeting of the new planes with those of the 9th modification.||Nature of the retrogradations|
|8th||Primitive crystal with the planes of substitution on its acute solids angle||105°, 7′||164°, 53′||Retrogradation by 1 row in breadth and 2 laminæ in height|
|9th||118°, 22′||151°, 29′||166°, 45′||Retrogradation by a single row|
|10h||148°, 23′||121°, 37′||136°, 44′||156°, 9′||Retrogradation by 3 rows|
Retrogradations along the edges of the terminal faces formed by the incidence of these faces on the sides of the prism, at an angle of 66° 38′.
|Number of the modifi-
|Figure of the crystal||Angles formed by the meeting of the new planes with the terminal faces||Angles formed by the meeting of the new planes with those of the prism||Angles formed by the meeting of the new planes with those of the 11th modification.||Nature of the retrogradations|
|11th||Primitive crystal having the edges of its terminal faces of 66°, 38′ replaced by a plane||139°, 10′||107°, 28′||Retrogradation by two rows|
|12th||145°, 9′||101°, 29′||174°, 1′||Retrogradation by three rows|
|Intermediate retrogradations upon the acute angles of the terminal faces that concur in the formation of the acute solid angles of the primitive crystal|
|Number of the modifi-
|Figure of the crystal||Angles which the edges uniting the new planes forms with the terminal faces||Angles which the edges uniting the new planes forms with the edges of 92°, 30′||Angles of incidences between the new planes||Angles of incidences between the new planes anbd the terminal faces||Nature of the retrogradations|
|13th||Primitive crystal having its acute solid angles replaced by two planes||98°, 26′||136°, 34′||136°, 58′||110°, 20′||Retrogradation by a single row, which while it tkaes one molecule from one of the sides of the acute angles, takes two from the other|
|14th||81°, 59′||153°, 1′||134°, 64′||82°, 32′||Retrogradation by a single row in breadth, and two in laminæ in height which, while it takes one molecule from one of the sides of the acute angles, takes two from the other|
- Water being considered as 10,00.
- It lines the walls of the vein, conjointly with a lamellar carbonate of lime belonging to that variety in which the rhomboidal fragments are striated in the direction of the greater diagonal of two of the opposite faces. This carbonate of lime, which is perfectly colourless, is one of the most phosphorescent which I have ever seen. It I may judge of the rock from the portions of it remaining attached to the pieces which I have examined, the walls of the vein, to which this substance, with the carbonate of lime which accompanies it, adheres, consist of a ferruginous argillaceous schist of a deep blackish-grey colour, of a very loose texture, and traversed by small veins of carbonate of lime. A moderate action of the tire changes this colour to a reddish-brown, and at the same time renders this schist extremely attractable by the magnet; its texture likewise becomes more loose, and when examined with a glass and by the light of the sun, this substance appears to be formed of a mass of small and extremely thin scales: its aspect is then much like that which would be presented by a mass of chlorite of the same colour, and of a fine grain. I am indebted to M. Gillet de Laumont for another very fine specimen of this mineral which I have received since that mentioned in the first part of this paper, in which the crystals of laumonite are very large and regular and beautifully grouped with crystals of carbonate of lime. The base of this specimen bears very evident marks of its having been detached from the schist which I have just described.
- In all the shove specimens, however, of this substance the rapidity of its spontaneous efflorescence is less remarkable than in those from Huel-goet.