Defects and Microstructures in Feldspars

BY A. C. McLAREN

1 Introduction

The minerals of the feldspar group are probably the most important of all rock-forming substances since they make up between 50 and 60 weight per cent of all igneous rocks and, in addition, they occur under a wide range of geological conditions. In fact, the classification of rocks is based to a large extent on the quantity and kinds of feldspars present.

In view of this, the feldspars have been studied in greater detail than any other group of minerals. The literature is, therefore, voluminous but fortunately there are good summaries in many elementary text books on mineralogy, as well as more extended reviews. In addition, there is the monumental work by J. V. Smith.

Optical microscopy and X-ray diffraction studies in particular have shown that the feldspars are an extremely complicated group of minerals and that specimens as we find them are very rarely homogeneous single crystals with grown-in or stress-induced crystals defects such as dislocations and twins. In general, feldspar specimens consist of a complex microstructure which is the product of order-disorder and structural transformations, as well as diffusion controlled processes such as exsolution (solid state precipitation). In addition, feldspar specimens which have been strained (deformed) in response to externally applied stresses develop characteristic micro-structures.

Before proceeding further it is necessary to consider briefly the nature of the feldspar series of minerals and their basic crystal structure.

The feldspars fall into two main series: (i) the alkali feldspars KA1Si3O8 to NaA1Si308 and (ii) the plagioclase feldspars NaA1Si3O8 to CaA12 Si2O8. These end members are referred to as orthoclase (Or), albite (Ab), and anorthite (An), respectively.

The chemical composition of any feldspar mineral is usually given in terms of the mol per cent of Or, Ab, and An. The alkali feldspars usually contain less than IO per cent An, but the Na-rich members (such as anorthoclase) may contain more. Similarly, the plagioclase feldspars usually contain less than IO per cent Or. The composition of any specimen is written as AnxAbyOrz: where x,y, and z are the concentrations in mol per cent.

The feldspar structure is based on a framework of (Al,Si)O4 tetrahedra, with the metal ions (K, Na, Ca) occupying positions in the interstices of the framework. The idealized feldspar structure is monoclinic with space group C2/m and there is complete disorder in the occupancy of tetrahedral sites by Al and Si. This is the structure of the K-rich mineral sanidine which has 4 KA1Si3O8 per unit cell. However, the fully ordered structure of KA1Si3O8 is triclinic, with space group C]bar.1]. This mineral is known as (maximum) microcline and has a structure similar to that of low-temperature fully ordered ablite.

With the exception of monalbite (a high-temperature, monoclinic, disordered form of albite), the plagioclase feldspars are all triclinic. Because the Al/Si ratio in anorthite (An100) is 1:1, ordering requires a regular alternation of Al and Si in the framework and this produces a body-centred structure I[bar.1] with a doubled c-axis. For this reason, it is usual when considering the plagioclase series to use this larger unit cell containing eight formular units.

The structural type exhibited by any particular feldspar specimen and its unit cell dimensions are clearly a function of the chemical composition and the degree of ordering which itself is dependent upon the temperature of crystallization and the subsequent thermal history of the specimen. For example, feldspars in volcanic rocks which have crystallized at high temperature followed by quenching to a low temperature may retain their high-temperature disordered state. On the other hand, feldspars in rocks which have cooled slowly may become ordered into a low-temperature state.

However, when any feldspar specimen is cooled from a high temperature, other significant changes may also occur, as indicated above. It is these changes, together with changes directly related to ordering, which are responsible for the micro-structures observed. Sometimes the microstructure is on a coarse enough scale for it to be observed directly by optical microscopy. For example, alkali feldspar specimens of intermediate composition which are homogeneous at high temperatures, exsolve at low temperatures into periodically-twinned domains of Na-rich feldspar and untwinned-domains of K-rich feldspar. On the other hand, the existence in some specimens of microstructures of exsolution and/or twinning on a very fine scale was implied originally from X-ray observations. But such observations provide little or no information about the size, shape, and distribution of the domains.

The first successful use of transmission electron microscopy to obtain such information on an alkali feldspar was made by Fleet and Ribbe. They showed that a K-rich moonstone from Ceylon consisted of alternating lamellae of monoclinic orthoclase and triclinic albite approximately parallel to (601). The lamellae were of the order of 1000 Å wide and the albite lamellae were periodically twinned on the albite-twin law. These observations gave a detailed explanation of the features of the associated diffraction pattern and further explained the origin of the well-known 'schiller' (or interference colours) exhibited by moonstone.

These observations were, of necessity, made on the thin edges of tiny crushed fracture fragments of the moonstone. In spite of the obvious disadvantages of this method of specimen preparation, many useful TEM studies of feldspars and other minerals were made. However, most of these disadvantages have been overcome by the development of ion-bombardment thinning. With this technique it became possible to produce consistently extensive thin areas of a wide range of non-metallic materials, and to relate TEM and optical microscope observations directly. As a consequence, there has been a spectacular increase in the application of TEM to mineralogical and petrological problems. McLaren has reviewed the TEM observations of feldspars which had been carried out up to the beginning of 1972. Since then a number of important observations have been made. In the plagioclase feldspars extensive work has been carried out on (i) antiphase domains; (ii) the coexistence of domains of different structural type; (iii) exsolution; iv) the occurrence and nature of the superlattice structure in specimens of intermediate composition (An25 to...