Chemical Composition of Felspars.
| Silica. | Alumina. | Potash. | Soda. | Lime. | |
|---|---|---|---|---|---|
| Orthoclase | 64.6 | 18.5 | 16.9 | ||
| Albite | 68.6 | 19.5 | 11.8 | ||
| Oligoclase | 63.7 | 23.9 | 1.20 | 8.1 | 2.0 |
| Labradorite | 52.9 | 30.3 | 4.5 | 12.3 | |
| Anorthite | 43.0 | 36.8 | 20.1 |
Orthoclase felspar, in addition to the above, frequently has small proportions of lime, iron, magnesia and soda. Amongst other things it is an essential constituent of granite, and on the decomposition of that rock is the first mineral to become affected. When attacked in the open air by rain and the ordinary agents of denudation, granite ultimately gives way by the dissolution of the felspar, and on being removed, the felspathic matter may accumulate in convenient situations to form kaolin. If we now compare the chemical constitution of orthoclase felspar with that of kaolin as previously given, we notice that the potash has disappeared in the decomposing process; it has been dissolved and taken away by rivulets, and the like, or washed by rain direct into the sea. We also observe that there has been a re-distribution, so to speak, of the relative proportions of silica and alumina—following well-known laws.
Of the remaining felspars the commonest for our purposes is oligoclase, a mineral found in nearly all British “granites” in a greater or less degree. That contains a higher percentage of alumina than orthoclase, and there is a fair proportion of soda and little lime, but much less potash. The lime-soda felspar, labradorite, and its near ally, anorthite, are not often met with in a recognisable form in clays. If present, they are generally as “kaolinised matter,” too highly decomposed to exhibit their characteristic optical properties.
It is pretty generally stated, and too often assumed by some, that pure china-clay is derived from the direct decomposition of rocks containing “orthoclase” felspar. Yet, this cannot really be so, if we reflect on the mineral composition of many of the rocks, which, obviously, have yielded the china-clays in question. Take the china-clays of Devon and Cornwall; they have undoubtedly been derived from the “granites” of those counties. To some extent, as previously remarked, the orthoclase is attacked, and provides the material of which china-clay is made. But in the “West of England,” we have yet to learn that some of the other felspars are not also involved in the process. If we examine a fresh piece of granite from the flanks of Dartmoor, or from the neighbourhood of Liskeard, or St. Austell, we find no difficulty in recognising a fair proportion of triclinic felspar (one or more of those mentioned in the table except orthoclase) in it. There is a difference in the composition (and therefore the commercial applicability) of a china-clay derived from a rock containing orthoclase alone, and one from a rock having orthoclase and one or more triclinic felspars in addition. The latter minerals are more easily decomposed than orthoclase, especially the lime and lime-soda varieties. We should not have raised this point only that, by reason of the granites being to some extent mechanically as well as chemically decomposed, a large proportion of “kaolinised particles” and “kaolinised matter” is introduced into certain china-clays, which render them different in their behaviour under intense heat from those china-clays in which orthoclase alone has been principally concerned. In other words, great practical advantages accrue from an accurate knowledge of the constitution and origin of the china-clays in question. Two clays of the same chemical composition often behave in a different manner in the kiln; the cause of this is frequently to be found in the prevalence of “mechanical fragments” of felspar in one of the clays; and the absence of these, but the presence of “kaolinised particles” of the same chemical composition, in the other.
Another point to which we may draw attention is the erroneous supposition that granites which have yielded china-clay have in all instances been reduced to the condition in which we now find them by the action of atmospheric agents of denudation alone. Granites, as a matter of fact, yield very slowly to the action of the atmosphere, and taken as a whole no building stone is as durable as they. How comes it, then, that they have decomposed to such an extent as to have formed extensive deposits of china-clay in a very short space of time, geologically? We think the answer is to be sought, at any rate in some instances, in the alteration the rock as a whole has undergone in certain situations, whereby it became more easily decomposable. Take the rotten china-stone of the neighbourhood of St. Austell, for example. In that material we clearly see a stone from which the “life” has been sapped, and instead of a bright, sparkling, porphyritic granite, as it once was, we now notice only a ghost of its former self. The large orthoclase felspars may be seen in it as skeletons, the mica is reduced to mere iron-stains (when present at all), whilst the quartz is also slightly affected. This altered and comparatively rotten material (although sometimes hard enough to be used as building stone) extends to an enormous depth from the surface; it has not been bottomed in some parts of the district. Such an extensive transformation could not possibly be due to ordinary agents of denudation which do their work at and near the surface of the rock only. It seems to arise from an enormous regional alteration, acting underground to an unfathomable depth, and which may not be unconnected with the mineral veins so common in, and in the immediate vicinity of the workings.[4]
Yet another thing to be remembered is that, under certain conditions, as near St. Austell, china-clay has been formed in situ, and has therefore not been deposited by the action of running water, as have the majority of china and other clays. Mr. Collins remarks that this china-clay is very irregular in its occurrence. It seems to be formed of various granite masses decomposed in place; it often occupies considerable surface areas, and extends to a depth unknown. He remarks that at Beam mine, and also at Rocks mine, both near St. Austell, china-clay was found to a depth considerably exceeding 60 fathoms from the surface. This china-clay, in its natural condition, is very much the same as china-stone; but the decomposition has proceeded further, the felspar being completely changed into clay; and nothing more is necessary for extracting the clay than the disintegration of the whole mass by a stream of water directed upon it, when the clay is carried away in suspension and collected at convenient spots. Thus there is every gradation between the true crystalline orthoclase and triclinic felspars, through china-stone into china-clay formed in situ, so into china-clay deposited from water by natural or artificial means, and into a pure clay containing a large proportion of kaolin crystals, “kaolinised particles” and “kaolinised matter.” But although we can state that much, a great deal yet remains to be done in connecting mineral structure with chemical composition of the purer clays, and in defining the various grades scientifically, in order that full advantage may be derived from them in a commercial sense.