Insight into the fascinating world of magmatic crystals –

Light and electron microscopy of accessory zircon

 

By Robert Sturm

 

Elsbethen, Austria

 

 

 

 

 

ircon may be regarded as a remarkable and, from the geoscientific point of view, as a very interesting mineral worthwhile for extensive investigations, because it represents one of the very few mineral phases occurring in a wide spectrum of magmatic, metamorphic, and sedimentary rocks. Zircon thereby belongs to the so-called accessory minerals which in contrast to the so-called main minerals are characterized by a rather low abundance. In a granitic rock sample with a mass of 10 kilograms, the respective mass of zircon crystals amounts to about 1 gram. Since zircon mainly occurs as a mineral phase being included into the main minerals, its size exceeds 1 mm only in exceptional cases.  Normally, a single crystal has a length between 50 and 300 µm and a width between 20 and 80 µm. Besides its ubiquitous occurrence, accessory zircon is marked by some further characteristics enhancing its scientific value significantly. First, the mineral has the unique ability to survive several cycles of erosion, sedimentary transport, diagenesis, and metamorphism, thus representing an excellent protolithic indicator containing information of the original rock, within which it was formed. Second, zircon contains minor but measurable amounts of U and Th and therefore may be subject to any dating procedures yielding ages of crystallization, magma cooling as well as element redistribution within the mineral phase. By using this dating technique it could be shown that some zircon crystals have an age of more than 1 billion years (1,000,000,000 years) and thus were involved into several cycles of orogenesis (formation of mountain belts). A third reason underlining the fascination and scientific importance of zircon concerns its way of crystallizing out of the magma. Only in very few cases, crystal growth may be evaluated as continuous, with nearly equal amounts of chemical components being added over the whole duration of crystallization. In most cases, however, crystal growth is carried out in a discontinuous, more stepwise fashion due to more or less rapid changes of chemical or thermal conditions in the direct vicinity of the crystallizing mineral phase. As a consequence of this ‘step-by-step’ growth, respective growth bands or zones are established which sometimes can be already observed under the light microscope but are best visualized with electron microscopic techniques (e.g. backscattered electron imaging or cathodoluminescence). By chemically investigating the growth zones of a zircon crystal, important information on chemical and physical changes of the magma during the cooling process may be obtained. What is the chemical composition of accessory zircon? In the ideal case the mineral with the chemical formula ZrSiO₄ consists of 67.1 % ZrO₂ and 32.9 % SiO₂. In natural zircon about 50 more elements occur in the crystal structure, from which Hf, Y, P, U, Th, La, and Rare Earth Elements are most prominent. As will be shown in detail in the next chapter, the morphology of zircon crystals is marked by two types of prisms and three types of pyramids, being combined in several different ways (Speer, 1980).

 

 

 

efore studying zircon crystals under the light microscope they have to be separated from their host rocks and prepared according to well defined procedures. Successful extraction of accessory zircon from the host rock includes rock crushing with a hammer, milling, sieving, floatation (coarse separation of single mineral phases according to their specific weight), magnetic separation, and, finally, separation using so-called heavy liquids (Unfortunately, these liquids are uniformly classified as extremely hazardous!). After this time-consuming procedure, grains have to be prepared for light microscopy which is best carried out by placing some zircon crystals on a glass specimen, adding some Canada balsam or resin with high light fraction, and covering the crystal-liquid mixture with a thin cover slip.

 

Figure 1. Light microscopy of zircon crystals.

With this rather simple microscopic technique it is already possible to visualize the extern morphology of single zircon grains, respective growth zones (see crystals B, E, and H), and mineral inclusions of different size.

 

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As exhibited in Figure 1, light microscopy enables the detailed study of the outer or external morphology of accessory zircon. While some grains are characterized by steep pyramids (e.g. crystals A and G), others show a predominance of flat pyramids (crystals B, D, E, F, and H). Most crystals separated from granitic rocks contain both types of pyramids, being marked by a more or less remarkable difference in size. It has been found that in granites with high contents of Al, termed ‘peraluminous’, steep pyramids dominate over flat pyramids, whereas in granites with high contents of Ca, K, and Na, termed ‘calcalkalic’, the opposite case may be observed (e.g. Vavra, 1994; Sturm, 1999). Besides the pyramidal morphology also the prism morphology plays an important role concerning the crystal shape of accessory zircon. Differing between the two prism faces is indeed not easy under the light microscope but can be realized in the following way: 1) The crystal is positioned on the prism {100}, if the angle of the two pyramidal faces crossing on the top amounts to 96° (grains B, D, E, and F in Figure 1). 2) The crystal, on the other hand, is positioned on the prism {110}, if this angle has a value greater than 112° (grains A, C, and G; see also Figure 2).

Another typical characteristic of accessory zircon being already noticeable under the light microscope is its growth zoning which in numerous cases is represented by a set of more or less concentric growth shells (grains B, E, and H). If the growth zones are too weak for an appropriate visualization under the light microscope, electron-microscopic techniques have to be applied (see below).  Zircon is normally characterized by a wide spectrum of inclusions ranging from magmatic fluids to mineral phases, from which apatite occurring as small needles (grains C, D, and F) is most prominent. With light microscopy these inclusions can be categorized and quantified.

 

 

Figure 2. Typology diagram introduced by Pupin and Turco (1972) for the systematic classification of the zircon morphology.

A statistical evaluation of the average zircon morphology within a zircon population consisting of all zircon crystals separated from a specific magmatic, metamorphic or sedimentary rock was outlined by Pupin and Turco (1972) as well as Pupin (1980). Within this two-dimensional classification scheme, eight rows and eight columns defining 64 crystal shapes are given. Crystals of the first row containing no prism faces and crystals of the 8^th column containing the very steep pyramid {311} do not occur with high frequency in natural rocks. From row 2 to row 8 the ratio of size between the two main prisms {100} and {110} is continuously changed. While in row 2 zircon crystals exclusively develop the {110} prism, in row 8 only the {100} prism occurs. In row 3 to 7 intermediate forms can be observed. The same arrangement is given for the pyramids, i.e. in column 1 only the {211} pyramid is developed, whereas in column 7 the {101} pyramid is not accompanied by a further pyramidal form. In columns 2 to 6 the ratio of size between the two pyramids is continuously changed with {211} becoming smaller and {101} becoming larger. In column 4 the two pyramids are equally sized. A very specific crystal type is that in column 4 and row 5, because it develops not only pyramids but also prisms with the same size.

To give a further impression of the various crystal shapes resulting from the combination of the prism and pyramid forms, another, let’s say, 3-dimensional Typology diagram is illustrated below. From the examples drawn in this diagram, the high variability of zircon morphology can be well estimated. The diagram also shows a subdivision into several subtypes (A, B, C, etc.), which, indeed, is important for a scientific analysis of a specific zircon population, but would exceed the aim of this small contribution.

 

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ith electron microscopy further detailed information on single zircon crystals can be obtained. While scanning electron microscopy (SEM) mainly serves for the investigation of the crystal surface, which is important, if zircon underwent e.g. an extensive mechanical metamorphosis, electron microprobe analysis of oriented crystal sections provides essential information on crystal growth and chemistry. For SEM single zircon crystals are picked out of the separated zircon fraction and are afterwards mounted on a glass slide using resin (e.g. Epon). After coating of the crystals with carbon (This is necessary for the leakage of electrons hitting the sample.) SEM procedure can be started. Respective results of this very impressive microscopic technique are presented in Figure 3.

 

 

Figure 3. Scanning electron microscopy (SEM) of selected zircon crystals.

The morphology of single crystals is very well recognizable, whereby most grains show a clear predominance of {211} over {101} and of {110} over {100}. Crystal morphology mainly composed of {211} and {110} can be frequently separated from granitic rocks characterized by an enhanced concentration of aluminium (Al) and a lower content of calcium (Ca). Such rocks are usually termed ‘peraluminous’.

Regarding the crystal surface, significant differences among the exhibited zircon grains can be determined. Crystals A-G are marked by a well developed surface which only shows some small scratches and roughening. The pyramidal tops are not mechanically damaged. Another picture is given for crystals H-J, where roughening and scratches become more remarkably intensified and are added by so-called corrosion pits. Cracks running over the crystal faces (grain I) give evidence for an increased mechanical influence due to brittle or ductile shearing.

 

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For obtaining information on the growth development of individual crystals, electron microprobe analysis (EMPA) has to be regarded as a preferential technique. An appropriate application of this technique, however, requires an extensive and time-consuming preparation of single zircon grains. A scientific study of the pyramidal growth is only enabled, if individual crystals are sectioned parallel to their crystallographic c-axes, defining the axes running trough the pyramidal tops (‘longitudinal section’, Figure 4, 5). Prism growth, on the other side, can only be investigated in detail, if respective crystals are sectioned perpendicular to their crystallographic c-axes (‘cross section’, Figure 4, 5). Good results with the microprobe are guaranteed by using a accelerating voltage of 15 to 20 kilovolts (kV) and a electron beam current of 30 to 40 nanoamperes (nA).

 

 

Figure 4.  Preparation of zircon crystals for backscattered electron imaging and chemical analysis using the electron microprobe.

A selected crystal can be sectioned either parallel or perpendicular to its crystallographic c-axis, resulting in so-called longitudinal or cross sections. As demonstrated in the following figure, longitudinal sections are most appropriate for the study of the pyramidal growth development, whereas cross sections are used for the investigation of prism growth. Before starting crystal preparation, selected grains have to be mounted on a glass slide and embedded in a layer of resin. Afterwards starts the grinding and polishing procedure (Sturm, 1999). For obtaining best results, crystals have to be perfectly cut in the middle (see the cutting planes in the sketch), because otherwise numerous effects may complicate growth analysis immensely. Surfaces of the crystals must be polished perfectly to guarantee an ideal interaction with the electron beam.

 

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Figure 5.  Backscattered electron imaging of zircon crystals sectioned parallel and perpendicular to their main crystallographic axes.

Crystals A-F are sectioned parallel to their crystallographic c-axes, so that respective sections provide useful information on the pyramidal growth. As can be observed for most crystal, ratio of size between the two pyramids {211} and {101} is not constant over the whole growth period but changes continuously. This may be primarily regarded as a result of magma chemistry changing permanently during crystallization. Most impressive in this case is crystal C, where early growth stages are characterized by nearly equally sized pyramids, while the outer or extern morphology (marked by the outline of the crystal) shows a clear predominance of the steep pyramid {211}.

Crystals G-L are sectioned perpendicular to their crystallographic c-axes. Respective sections give an impressive insight into the prism growth. Similar to the development of the pyramids, also prism growth may not be classified as constant or static in many cases.  Concerning crystals G-J, ratio of size between the prisms is subject to a continuous change, resulting in morphological differences between early and late growth stages.

 

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For chemical analysis crystal sections prepared for backscattered electron imaging are investigated by electron microprobe analysis (EMPA). With this chemical analysis method besides main chemical components of zircon (zirconium (Zr) and silicon (Si) also chemical components represented with lower concentrations can be detected (see introduction). The main question standing behind the measurement of chemical profiles like those provided in Figure 6 is, whether there can be detected a relationship between crystal chemistry and crystal growth or not.

 

 

Figure 6. Chemical profiles of the elements hafnium (Hf), yttrium (Y), and uranium (U) measured on a cross section of a selected zircon crystal.

The profiles should demonstrate that magmatic element concentrations are not constant over the whole growth period. Some elements like U show a somewhat oscillating concentration, i.e. growth zones with low U-content are followed by growth zones with higher U-content and vice versa. As could be found out in the past, concentration of certain elements in the magma is an essential factor controlling prism growth of accessory zircon.

 

 

 

 

 

rom the brief study presented here it can be concluded that magmatic crystals and particularly accessory zircon represent extremely interesting objects for geological and mineralogical research. With the help of traditional light microscopy on the one side and modern electron microscopy on the other side, numerous results concerning magmatic mineral growth and its control by environmental factors may be obtained. In future, questions regarding the relationship between magma chemistry and zircon morphology will be subject to a detailed review and to further investigations.

 

 

 

• Pupin, J. P. (1980): Zircon and granite petrology. Contrib. Mineral. Petrol. 73, 207-220.
• Pupin, J. P. & Turco, H. (1972): Une typologie originale du zircon accessoire. Bull. Soc. Fr. Minéral Cristallogr. 95, 348-359.
• Speer, G. (1980): Zircon. In: Ribbe, P. H. (ed.), Orthosilicates, MSA, Washington DC, 67-112.
• Sturm, R. (1999): Longitudinal and cross section of zircon : a new method for the investigation of morphological evolutional trends. Schweiz. Mineral. Petrogr. Mitt. 79, 309-316.
• Vavra, G. (1994): Systematics of internal zircon morphology in major Variscan granitoid types. Contrib. Mineral. Petrol. 117, 331-344.

 

Any comments on the study or questions concerning mineralogical research are very welcome by the author Robert Sturm.