Notes on theChemical Compositionof some of the Rocks of the Lizard District. By W. H. Hudleston, Esq., M.A., F.G.S.
These rocks may be roughly divided into four principal groups
(without enumerating others of less importance), viz.:—
1. The Serpentines.
2. The "Greenstones" in the Serpentine.
3. The Hornblende Schists.
4. The Gabbros.
Features in common.—In all specimens of the above rocks examined by me, alkalies are extremely scarce, if not altogether absent in most cases. They are all basic, rarely containing as much as 50 per cent, of silica.
Differences.—Chemically they divide themselves into two groups:—Group A (poor in lime and alumina) includes the serpentines; Group B (rich in lime and alumina) includes the "greenstones," hornblende schists and gabbros—rocks which, however much they may differ in appearance, have considerable resemblance in their fundamental constitution.
Group A. The Serpentines.
These rocks, as usual, present an infinite variety as regards external appearance; but there is considerable resemblance in their chemical composition. A specimen has been selected for careful analysis.
Black Serpentine from "near Cadgwith"—Black to greenish black, opaque; fracture splintery; charged with numerous crystals of a highly cleavable mineral of a pale brass-colour and metallic lustre. Sp. gr. of the mass 2⋅587. Freed as far as possible from the crystals, the black matrix yields a greenish-grey powder, which adheres in considerable quantity to the magnet. Composition:—
Two or three kinds of crystalline fragments are noticeable in the undecomposed residue; one is probably enstatite.
As the amount undecomposed is so small, the quantity of unaltered augite, diallage or enstatite, must be very trifling. Any olivine still existing would of course be dissolved; but the quantity of water found, and the fact that no gelatinous silica is noticeable, preclude the possibility of there remaining any considerable amount of this mineral[1].
The quantity of ferric oxide is probably wholly in combination as normal magnetite; this would absorb 2⋅09 out of the 3⋅31 of ferrous oxide, leaving 1⋅22 ferrous oxide for the hydrous silicate which forms the bulk of the mass. There exists, therefore, in all probability, 6⋅75 per cent, of normal magnetite disseminated as a fine powder throughout the mass, causing the blackness and opacity for which this serpentine is remarkable.
The undoubted presence of nickel is interesting in connexion with these magnesian rocks; it may exist partly as nickeliferous pyrrhotite, and partly as a base of the hydrous silicate. It has been calculated as an oxide. Sterry Hunt had previously detected it in the serpentines of Cornwall, Banffshire, and the Vosges, but states that none exists in the ophiolites of the Laurentian rocks of Canada[2].
Red Serpentine from the Balk quarry.—Dull purple, ground streaked with green, and containing crystals somewhat similar to the last, but of a green colour and more blended with the general mass. Sp. gr. 2⋅561.
Though so different in appearance, this rock is in composition singularly like the preceding one.
The undecomposed silicate is less in amount, and the water is greater (13⋅28); the relative proportion of silica and magnesia about the same; but there is only 1⋅37 per cent, of ferrous oxide, the total iron remaining nearly the same. The bulk of the ferric oxide occurs probably as hæmatite, dispersed in fine powder through the mass, just like the magnetite in the last example[3]; in this case the powder does not adhere to the magnet. There is about the same quantity of nickel, rather more alumina; but, on the whole, we have every reason to suppose that we see here a rock whose original composition was very close to that of the black rock. The gradual oxidation of the magnetite into hæmatite, and a slightly more complete hydration of the original basic minerals, constitute the real difference between them.
The Brass-yellow Crystals.—These lose 9 per cent, in ignition (dried at 100° C), and, although composed principally of silica and magnesia, contain more alumina and lime than the matrix, and much less iron. Their chemical composition is similar to that of bastite; and they are probably the result of the hydration of a variety of enstatite[4].
Group B.
The following are very rough analyses of some of the rocks associated with the serpentines, and which, as before stated, present such a remarkable contrast to them in the amount of lime and alumina which they contain. This is due to the presence, in all or nearly all of them, of a kind of alumina-lime felspar (?), whose varieties may be grouped under the general term "Saussurite." The amount of alkali in those specimens examined appears to be so small that it has been queried in some cases when making up the total constituents.
One of the most interesting rocks is a peculiar gabbro occurring near Caerleon Cave[5]. This gabbro is remarkable for crystals of diallage of great size, associated with an opaque white mineral mass, representing the "Saussurite." The external portions of this latter contain much carbonate; but the central mass, or core, consists of a bluish-white compact silicate, or mixture of silicates which, omitting small quantities, has the following composition:—
The compound has more affinity, as regards its percentage of silica, with anorthite than with labradorite; but the presence of so much magnesia and its poverty in alkali must lead us to regard it as a mixture rather than as a mineral species worthy to be classed with the felspars[6].
II.
The diallage associated with the above is a well-defined mineral, and but slightly affected by ignition or acids. Subjoined is a rough analysis:—
No alkali was found in the specimen examined. The amounts of lime and magnesia, being so exactly equal, indicate a mineral which, in its chemical aspect, is almost equally allied to the augites and the hornblendes[7].
III.
Analysis of the Carnpersack Rock (p. 902).—A very hard tough variety, poor in diallage, occurring in the great dyke-like mass of gabbro which comes down to the sea at Karak Clews. The analysis therefore represents the condition of one of the more "saussuritic" varieties of the last rock.
The hornblende schist of the Lizard presents many varieties, which, on analysis, would differ materially as to quantities. The following is from a variety of high specific gravity occurring near the lighthouse. About one fourth of the rock is soluble in acid. It contains a noteworthy amount of pyrites and phosphoric acid.
↑This is confirmed by microscopic examination (see p. 921).—T. G. B.
↑See the description of the microscopic appearance, p. 921.
↑Mr. Hudleston's specimen is very similar to some of the varieties found at the Balk, so this may be taken as a type of the "Saussurite" in this series of gabbros.—T. G. B.
↑See p. 895 for the history of this mineral. Perhaps the magnesia may be accounted for by the presence of pale-coloured hornblende, which, though almost microscopic, is often present in considerable quantities.—T. G. B.
↑This may explain the extreme facility with which this diallage is replaced by hornblende, see p. 895.