Illite

The study of illite may seem rather narrow in scope, but we hope to demonstrate that this is not completely true. In fact this assemblage of published and unpublished research tries to analyze the accumulated knowledge and interpretations in order to make a summary of illite occurrence, to define more precisely what illite is and then to establish its place in a geological-environmental context. The information gathered since the 1950s has made proper identification and interpretation much easier as time has passed and more work has been done in fields where little information was previously available. However, research on the identification and diagnostic of illite occurrence and its related clay mineral suites stagnated somewhat in the 1980s. It seemed that further investigation would bring little new information. This was especially true in studies of soil clay mineralogy. The research activity in agronomy departments in institutions over the world has almost come to a halt. This is largely due to the difficulty in further pursuing clay mineral identification with the tools at hand, as well as a shift of interest from soil to plants in agronomic research. The statics of clay mineral identification and the slow mineral change observed in studies of sedimentary series, occurring on the scale of millions of years, seems to have discouraged people from observing the changes possible in active soil sequences. Silicates were assumed to be stable, or at least of such slow kinetic reaction that their existence was a given in soil clay problems. Interest in clay minerals shifted to problems related to sedimentary basins subjected to burial diagenesis and related problems of petroleum generation. Here the general scheme was established and one had only to apply the recipe to a given problem.

The purpose of this chapter is to try to establish a guide line among the numerous uses of the term “illite”, considering first historical definitions, then recent advances in the fields of mineralogy and thermodynamics. The goal is to describe the variability of the crystal structure of illite in its different states of crystallization through X-ray diffraction, particle morphology and HERTM observations. If illite is “a phase”, we must understand how it can grow. From the examination of our present knowledge we hope to propose a “working” definition of illite before considering the way in which illite forms in natural environments.

The geology of illite is surprisingly diverse since it forms in soil as well as in metamorphic or peri-magmatic conditions. The goal of this chapter is to give an overview of this diversity in order to outline the most important facts concerning illite formation. This information gives a basis to determine the transformation of minerals into illite according to the factors of kinetics (Chap. 3) and to apply the knowledge of illite formation to practical problems of mineral resource development and some environmental problems.

The occurrence of illite in sediments and sedimentary rocks is closely related to the transformation of smectite to illite under conditions of burial metamorphism. Our objective at this point is to identify the origins of illite (PCI and WCI) in a context of changing temperature. This process is related to the effects of time (t), temperature (T), and composition or chemical activity of certain elements (x) in sediments affected by burial diagenesis. This transformation reaction is the origin of a large portion of the illitic materials found in different geological contexts.

There are two fields of current human endeavor where illites play an important role. Other clays have been and some are still used in industrial processes, smectite and kaolinite being the chief minerals. Smectites have been used as thixotropic agents to fluidify paints, drilling muds in oil exploration, or as filling agents in hot dogs and other foods. Illite is not adapted to most of such industrial uses. In the past it was a major component of common clays used for the production of cooking pots, plates, and, continuing to this day, especially in Europe, tiles and bricks, but this is a diminishing field of usage. Hence we will not try to find a use for illite in factories. However the importance of illite will perhaps be even greater in the future.

Illite, and potassium clays, are essential for agriculture. In the recent past (since about 1950) use of potassium fertilizer became a very common practice in agriculture. Its origin dates back to the late 19th century, but almost the complete use of this material to increase or maintain plant fertility is more recent. Use of potassium fertilizer is necessary because intensive farming practices deplete much of the available potassium from agricultural soils. If one is to decrease the dependence on artificial means of maintaining fertility one needs to understand how farming techniques in the past kept fertility at a reasonable level without the addition of potassium. For example, the major portion of France has been farmed for approximately the last 4,000 years. The spectacular decrease in fertility is only a recent phenomenon. It has occurred as productivity increased dramatically with the use of hybrid plants and artificial fertilizing agents. If the two are interrelated by necessity, there is no going back to traditional methods of farming or at least in ameliorating these methods. However, if there is a mean of increasing the efficiency of traditional methods of farming and maintaining fertility, there is a great hope for the future of the world’s soils. Most of this hope lies in better understanding the release and re-capture of potassium by illites and the role of ammonium, a replacement ion for potassium in illite and the second major fertilizing agent (out of three). The stability of potassium illite layers in soil clay minerals and their relation to the ammonium form is the key to future research in soil clay mineralogy. Increasing fertility and suppressing dependence on nitrate-forming ammonium fertilizer will solve many problems of water resource procuration.

A second, and potentially as important, aspect of illite in man’s activities is that of the storage of high energy radioactive waste. Illite has an important role to play here. If one can devise a safe and long-lasting barrier to the migration of nuclear waste elements in the surface environment, the use of atomic energy can proceed and the use of petroleum, the major producer of gas causing the greenhouse effect and major climate change, can be slowed to a reasonable consumption. This is a major challenge at present, and one that will decide the future development of human activity.

Illite has played and still plays a major role in the recovery of primary resources such as petroleum and ores. This exploration potential resulted in a correct identification of illite occurrence which is of primary importance to the efficient discovery of porphyry-type ore deposits. It is also important for evaluating the production potentials of sandstone reservoirs for petroleum deposits and the thermal history of sediments. The aspects of illite occurrence are less pressing, perhaps, but none the less important to the future of resource exploration and exploitation.

The importance of illite to prospecting in geothermal energy fields is also very great. A mastered use of this knowledge can increase the efficiency of finding and using this little exploited energy source which can be used to develop electricity in using the natural emanations of heat and vapour escaping from the earth and not adding to the pollution of the atmosphere.

These are some of the more evident aspects of the use of illite mineralogy in the activity of modern societies. We develop several of these aspects below.