Transactions of the Geological Society, 1st series, vol. 3/On the Corundum of Gellivara
By C. T. SWEDENSTIERNA, or Stockholm,
foreign member of the geological society.
In the spring of 1803, when examining a series of iron ores from Lapland which I had brought with me to Paris, I was greatly surprised to find that the porphyry mortar employed in reducing one of them to fine powder was scratched and lost its polish. Mr. Tennant having nearly at the same time published his discovery on the identity of emery with the sapphire, I was at first induced to believe that the said ore was merely a kind of emery. Upon a further examination however, I found that this ore was as soft as any other from the same place, and was chiefly composed of black and red oxide of iron. These could be separated by the magnet without leaving any earthy residue, which would not have been the case with emery, in which the cutting substance is finely divided and intimately connected with the oxide. I afterwards broke several specimens of the ore, and perceived in some of them very hard crystals the largest of which were of the size of small peas, and exhibited regular faces of an oblique octahedron. I at last succeeded in extracting about half a dozen of them of a perfectly determined form. I then no longer doubted of these crystals being a variety of corundum, which also was ascertained by my teacher in mineralogy, M. Haüy, to whom I presented the purest and most perfect specimens I had got, and some of which have since obtained a place in the museum at Paris, and have been mentioned in the catalogue of Mr. Lucas, p. 132, under the name of Corindon Harmophane basé. The few remaining specimens, being all of an inferior quality, I gave to my mineralogical friends, keeping but a single crystal for my own collection, in the hope that on my return to Sweden I should easily procure a fresh supply from a large stock of Gellivara iron ore in my possession. In this expectation however I have been for the last ten years disappointed, and it is only within a few days that my search has been successful enough to furnish me with sufficient materials for a satisfactory description of this substance. The few but very excellent specimens now in my possession, and which will perfectly serve this purpose, I have been so fortunate as to pick out of several hundred pounds of Gellivara iron ores, for which I am indebted to Bar. Hermelin, the proprietor of the mines. It is rather singular that with the exception of the specimens I have mentioned, I did not discover a trace of corundum in the large quantity which was examined. I am led to believe therefore that it is by no means common, or at all events that the mines which are now worked do I not furnish it in abundance. The specimen which accompanies this short memoir, and which I have the honour to present to the Geological Society, is the best and most complete among them all.
When perfectly cleared of the grayish dust of the iron ore in which the crystals are imbedded, their colour is exactly that of common French flint.
The only form in which it has hitherto been found is that of an oblique octahedron, more or less complete, from the size of a pin's head to that of about three lines across. Very often the crystals are so much compressed that they exhibit to the naked eye only two large faces, the lateral ones being then almost imperceptible. Sometimes the crystals are grouped two or three together, and striped parallel to the great diagonal.
The external lustre is accidental, being mostly hid by the fine powder of the matrix covering the surface, but in a fresh fracture the internal lustre is very brilliant.
The cross fracture is uneven, though approaching to the laminar. The longitudinal fracture is perfect laminar, in which direction it also can be easily divided.
The small fragments are in general undetermined, more or less pointed, often taking the form of a rhomboidal prism, the four faces of which exhibit a specular gloss.
It is semitransparent, though not so much as French flint.
Its hardness is the same as that of corundum from India.
It breaks easily in the direction of the laminæ, but in other directions not without some difficulty.
The specific gravity of two specimens was 4.20 and 4.02.
At the blowpipe it cannot be melted, neither alone nor with borax, &c.
Its most common and immediate matrix is a light grey coloured laminar iron ore, consisting of an intimate mixture of red and black oxide of iron, without any visible earth. This ore (a variety of eisenglanz, Werner,) which has been mentioned by Count de Bournon in the Philos. Trans. 1803, is often accompanied by another coarse grained, half crystallized, loose, and more magnetic ore, by red felspar, red and white grayish phosphate of lime, and silvery coloured mica. The corundum may sometimes be found in the last mentioned iron ore, but never in the other substances, though they often make a great part of the mass. Nor has it been observed in any other variety of the Gellivara iron ores, except in a light grey coloured, fine grained, compact and very hard one, where I suspect it to be in the same close connection with the red oxide of iron as in the common emery, to which it has a great resemblance.
The mines of Gellivara are situated in Swedish Lapland, at 67° 10′ North latitude, about 160 English miles north-west of Torneo. The mountains in which they are wrought is almost in the center of a large country comprized between the shore of the Baltic and the Norwegian Alps, 240 miles in length, and between the two rivers of Torneo and Luleo, from 35 to 100 miles broad. The height above the Baltic is not exactly determined, but may be supposed about 1200 feet at its highest point. It rises gently on the south side, but is more abrupt on the north and east. The surrounding country is but a few hundred feet lower, and a mountain called the Dundary, at a distance of five or six miles to the east, is much higher. The whole mountain, which is about 2600 fathoms in length and from 1000 to 1600 in breadth, may be considered as a large deposit of iron ore, separated in standing layers (Stehende Lager, Werner) of different thicknesses, by a red and almost compact feldspar mixed with several other minerals. This red felspar, in a fine grained combination with quartz and mica, is the common rock of the country, which however is never here found uncovered. It occurs only in detached blocks as well at the foot as at the highest top of the mountain, which is besides partially covered with a fine white sand to a considerable depth; with siliceous gravel and small blocks of amorphous garnet with specks of iron. Slaty quartz is also found in small fragments, but never forming a solid rock.
The above mentioned layers of iron-ore have in all the mines and all over the mountain a general direction from north—east to south-west, dipping about 45° to north-west. They very often stretch several hundred fathoms in length, and from two to three hundred in breadth, merely covered by a slight vegetation of mosses. If we except the red felspar, which at small distances runs parallel with the ore and sometimes crosses its layers, the whole forms almost a solid mass of iron, of a very considerable extent, and of an unknown depth. In those places where the ore is of a loose texture and either coarse or fine-grained, a large quantity can in an hours time be dug up by a spade. The more hard and compact magnetic ores break generally in rhombs, the angles of which are about 45°, or equal to the dipping of the layers,
The ores, of which I hope to give a more detailed account at another time, form the most complete series of iron in different degrees of oxidation, from the almost black magnetic ore to the light gray not the least magnetic., (eisenglanz, Werner, ) the true substance of which shews itself by its red colour when rubbed. The greatest number of ores forming this series, may probably be considered as a more or less intimate though merely mechanical combination, between the two varieties of iron in its minimum and maximum of oxidation. The variety mentioned by Count de Bournon may be taken perhaps as an instance of the combination in which the red oxide is found in the greatest quantity. The minerals which occur at Gellivara, besides the corundum, in the feldspar separating the layers or within them, are chiefly the following.
P. S. Since writing the above memoir I have again examined the Gellivara iron-ore, and have found amongst it some specimens of true emery.
- The specimens were very smell, the one weighing only 1.93 grains English, the other 0.63 of a grain. They were cut and polished in order to free the substance from the iron ore with which the crystals are in general mixed.