This mineral will also change to limonite, if exposed to the weather.

Pyrite. This is another mineral, which is not uncommon in some clays, and can be often seen by the naked eye. It is sometimes called iron pyrites, or sulphur, and, chemically, is a sulphide of iron (FeS2). It has a yellow color and metallic lustre, and occurs in large lumps, in small grains or cubes, or again in flat rosette-like forms. Not infrequently it is formed on or around lumps of lignite, and is a familiar object to all the clay miners of the Woodbridge district, and other districts where clays of the Raritan formation are dug, but it is rare in other New Jersey clays.

When exposed to weathering action, pyrite is a rather unstable compound, that is to say, it tends to alter, and changes from the sulphide of iron (FeS2) to the sulphate of iron (FeSO4), by taking oxygen from the waters filtering into the clay. This also destroys its form, the yellow metallic particles changing to a white powdery mineral, which has a bitter taste and is soluble in water. Clays containing pyrite are not, as a rule, desired by the potters. (See Soluble Salts.)

Glauconite. This mineral, which is sometimes termed greensand, and in bulk greensand marl or simply marl, is an important one in some of the New Jersey clays. Chemically, it is a compound containing silica, potash, iron and water (a hydrous silicate of potash and iron), occurring in the form of greenish sandy grains. Its composition is often somewhat variable, and it may contain other ingredients as impurities. Thus a sample from New Jersey1 analyzed: Silica, 50.70 per cent.; Alumina, 8.03 per cent.; iron oxide, 22.50 per cent.; magnesia, 2.16 per cent.; lime, 1.11 per cent.; potash, 5.80 per cent.; soda, 0.75 per cent.; water, 8.95 per cent. It is an easily fusible mineral, and hence a high percentage of it is not desired in a clay. Greensand is restricted2 to the clays of the Clay Marl series, and is most abundant in Clay Marl I.

Kaolinite. This mineral is a compound of silica, alumina and water (a hydrated silicate of alumina), represented by the

formula of Al2O3, 2SiO2, 2H2O, which corresponds to a composition of Silica (SiO2), 46.3 per cent.; Alumina (Al2O3), 39.8 per cent.; Water (H2O), 13.9 per cent. It is rarely found in pure masses, but when isolated is found to be a white, pearly mineral, the crystals forming small hexagonal plates, which are often found to be collected into little bunches, that can be separated by grinding. When the mineral kaolinite forms large masses, the name of kaolin is applied to it.1 It is plastic, and is also highly refractory, fusing at cone 36. The amount of kaolinite present in clays varies, some white kaolins containing over 98 per cent., while other sandy impure clays may have less than 20 per cent.

Associated with kaolinite, there have been found one or more species of allied minerals, which are all hydrated silicates of alumina. They are known as halloysite, rectorite, newtonite, allophane, etc. Some have been found in the form of crystals, and others have not.

Rutile. The oxide of titanium (TiO2) rutile is of widespread occurrence in clays, and is usually found on chemical analysis, when proper tests are made. Rutile grains can be seen under the microscope in many fire clays, and the analyses show the presence of titanium oxide to the extent of nearly two per cent. The presence of this mineral, however, is unfortunately too commonly ignored in the analysis of clay, and yet, as will be shown later, its effect on the fusibility of the clay is such that it should not be neglected in the higher grades, at least.

Calcite. This mineral is composed of carbonate of lime, and, when abundant, is found chiefly in clays of recent geological age, but some shales also contain considerable quantities of it. It can be easily detected for it dissolves rapidly in weak acids, and effervesces violently upon the application of a drop of muriatic acid or even vinegar. When in grains large enough to be seen with the naked eye, it is found to be a translucent

'The material termed kaolin, which is found in the Woodbridge district of New Jersey, is not such, but a quartz sand with a considerable percentage of white mica and clay.

See the Fire Clays and Fire-Brick Industry, Chap. XVI.

mineral with a tendency to split into rhombohedral fragments, due to the presence in it of several directions of splitting or cleavage; it is also soft enough to be easily scratched with a knife. Few clays contain grains of calcite sufficiently large to be seen with the naked eye, although in some the calcite, as well as some other minerals, may form concretions.1 In some swamps, clay beds are found which are highly charged with lime carbonate, and known as marl (not to be confused with the Greensand marls of the Cretaceous). Very little lime carbonate is found in the New Jersey clays, except in those of glacial origin. Gypsum. This mineral, the hydrous sulphate of lime, contains lime (CaO, 32.6 per cent.), sulphuric acid (SO3, 46.5 per cent.) and water (H2O, 20.9 per cent.). It may occur in clays, even in large lumps, but, so far as known, these have not been found in New Jersey. Gypsum, when present in clay, and large enough to be visible without the use of a microscope, forms crystals or plate-like masses. It is much softer than calcite and can be scratched with the finger nail, has a pearly lustre, is transparent, and does not effervesce with acid or vinegar. When heated to a temperature of 250° C. (482° F.), the gypsum loses its water of combination, and, when burned to a still higher temperature, at least a part of the sulphuric acid passes off.

Hornblende and Garnet.-These are both silicate minerals of complex composition, which are probably abundant in many impure clays, but their grains are rarely larger than microscopic size. Both are easily fusible and weather readily, on account of the iron oxide in them, and, therefore, impart a deep red color to clays formed from rocks in which they are a prominent constituent.

Dolomite.-Dolomite, the double carbonate of lime and magnesia, and also magnesite, the carbonate of magnesia, may both occur in clay. They are soft minerals resembling calcite, and either alone is highly refractory, but, when mixed with other minerals, they exert a fluxing action, although not at so low a temperature as lime.

1 See Limonite, Siderite.

THE CHEMICAL ANALYSIS OF CLAYS.

There are two methods of quantitatively analyzing clays. One of these is termed the ultimate analysis, the other is known as the rational analysis.

The ultimate analysis.1-In this method of analysis, which is the one usually employed, the various ingredients of a clay are considered to exist as oxides, although they may really be present in much more complex forms. Thus, for example, calcium carbonate (CaCO3), if it were present, is not expressed as ⚫ such, but instead it is considered as broken up into carbon dioxide (CO2) and lime (CaO), with the percentage of each given separately. The sum of these two percentages would, however, be equal to the amount of lime carbonate present. While the ultimate analysis, therefore, fails to indicate definitely what compounds are present in the clay, still there are many facts to be gained from it.

The ultimate analysis of a clay might be expressed as follows:

In most analyses, the first seven of these and the last one are usually determined. The percentage of carbon dioxide is usually small, and commonly remains undetermined, except in very calcareous clays. Titanic oxide is rarely looked for, except in fire clays, and even here its presence is frequently neglected. Since the sulphur trioxide, carbon dioxide and water are volatile at a

'The method followed was in general that given by Hillebrand, in Bulletin 176 of the United States Geological Survey.

red heat, they are often determined collectively and expressed as "Loss on ignition." If carbonaceous matter, such as lignite, is present this also will burn off at redness. To separate these four, special methods are necessary, but they are rarely applied, and, in fact, are not very necessary, except in calcareous clays, or black clays. The loss on ignition in the majority of dry1 clays is chiefly chemically combined water. The ferric oxide, lime, magnesia, potash and soda are termed the fluxing impurities, and their effects are discussed under the head of iron, lime, magnesia, etc., and also under Fusibility, in Chapter IV.

As can be seen from the experiments described in Chapter IV, all clays contain a small but variable amount of moisture in their pores, which can be driven off at 100° C. (212° F.). In order, therefore, to obtain results that can be easily compared, it is desirable to make the analysis on a moisture-free sample, which has been previously dried in a hot-air bath. This is unfortunately not universally done.

The facts obtainable from the ultimate analysis of a clay are the following:

1. The purity of the clay, showing the proportions of silica, alumina, combined water and fluxing impurities. High-grade clays show a percentage of silica, alumina and water, approaching quite closely to those of kaolinite (pp. 46, 47).

2. The refractoriness of the clay, for, other things being equal, the greater the total sum of fluxing impurities, the more fusible the clay.

3. The color to which the clay burns. This may be judged approximately, for clays with several per cent. or more of ferric oxide will burn red, provided the iron is evenly and finely distributed in the clay, and there is no excess of lime. The above conditions will be affected by a reducing atmosphere in burning, or the presence of sulphur in the fire gases.2

4. The quantity of water. Clays with a large amount of chemically combined water sometimes exhibit a tendency to

1

This means dried at 100° C. until their weight is constant. See under Moisture.