EARTH AND PLANETARY SCIENCE LETTERS 11 (1971) 312-316. NORTH-HOLLAND PUBLISHING COMPANY
IONIC ETCHING OF TITANOMAGNETITE GRAINS IN BASALTS Heinrich SOFFEL and Nikolai PETERSEN Institut fi~r Angewandte Geophysik, Universitat Mi~nchen, Germany Received 26 February 1971 Revised version received 19 April 1971 An etching technique for minerals in rock samples is presented which is based on a bombardment of a mechanically prepolished surface with ionized air. Under certain conditions oxidation of the mineral surfaces can be avoided. The ionic etching enhances the contrast for the study of size and structure of submicroscopic lamellae in exsolved titanomagnetites with both light and electron microscopes. Ionic etching is also a suitable technique for producing stress-free surfaces of natural ferrimagnetic minerals in rocks such as magnetite, titanomagnetite, chromite, goethite and pyrrhotite for the observation of their ferrimagnetic domain configurations.
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Mechanical polishing with diamond powders is a widely used technique for the preparation of polished sections of rocks. However, due to the friction between the sample surface and the polishing disk the uppermost layers of the crystals are partially melted and distorted [ 1 }. This so-called Beilby layer (fig. 1)has a thickness of several 10 -6 c m [ 2 ] . It is optically homogeneous and isotropic and is characterized by large statistically distributed internal stresses. A removal of the Beilby layer is possible by etching the samples with an appropriate liquid. The relief which is produced hereby depends on the different resistivity of adjacent minerals against the etchant. Carriers of magnetic properties of basalts are grains of titanomagnetite embedded in a matrix of non-ferromagnetic silicates. These titanomagnetites vary strongly in composition. There are basalts with homogeneous grains but there are also basalts where the ore grains are completely decomposed and consist of several systems of lamellae of different breakdown products (exsolution lamellae). The breakdown phenomena vary widely in different basalts. The magnetic properties of basalts are strongly dependent on the internal state of exsolution, the nature of the breakdown products and the size and
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Fig. 1. Lattice deformation of a mechanically polished surface (schematic). and shape of the exsolution lamellae. Their knowledge is of special importance for the suitability of a basalt for paleomagnetic field direction and intensity studies. In the following section we describe a method using ionic bombardment which enhances the contrast between groundmass and exsolution lamellae in titanomagnetite grains. It should be helpful in the detection of very fine systems of lamellae. In contrast to the etching methods the relief produced is influenced by the differences in hardness of the minerals involved.
11. Soffel, N. Petersen, Ionic etching of titanomagnetite grains in basalts
2. The ionic polishing
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The polishing device described in this section (and elsewhere [4] in a 15riefversion considering different apects) is essentially a bombardment of a mechanically polished surface with ions in order to remove the Beilby layer that masks the undisturbed crystallographic layers of the minerals and to produce a relief between the different phases within one ore grain. To simplify the device, ionized air was used instead of ions of rare gases like argon. The problem of a possible oxydation of the sample surface during the bombardment will be discussed later. A schematic view of the apparatus is shown in fig. 2. It consists of a commercial bell jar which is evacuated by a mechanical pump down to pressures of about 10 -2 mm Hg. The pressure is regulated by a needle valve. Two electrodes in the form of tungsten plates, 40 mm in diameter are placed within the evacuated system at a distance of 40 mm opposite each other. The cathode has a circular hole behind which the sample is placed at a distance of about I mm from the cathode. The (mostly) positive ions (mainly N+, N 2+, O÷, O 2+) which are produced by collision in a gaseous discharge in the evacuated system are accelerated between the electrodes. They form a stationary flux of ions through the hole in the cathode where they impact on the sample and erode its surface. Details are given elsewhere [6]. bell jor
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Fig. 3. Variation of the temperature of the sample surface during ionic polishing. The bombardment of the surface with ions increases its temperature. It is therefore necessary to cool the samples to avoid oxidation of the surface and/or to heat the samples close to or above the Curie temperature of their ferrimagnetic minerals. This would destroy their natural magnetic properties. Samples with a good thermal conductivity (metals, single crystals of magnetite) can be cooled directly (e.g. thermal contact with circulating water). Samples with bad thermal conductivity (silicates) or samples containing minerals with good thermal conductivity (i.g. titanomagnetites) within a matrix of bad thermal conductivity can be cooled or at least kept at moderate temperatures by a periodic interruption of the bombardment. The surface temperatures of the basalts were determined by inserting a thermocouple (copperconstantan, thickness of the wires: 0.1 mm) into a hole of 1 mm diameter in a polished section. The hole was then closed by araldite and the top of the thermocouple was polished mechanically. This simulates an ore grain 200 microns in diameter. The temperature of the surface was measured as dependent on time, gas pressure and voltage between the electrodes. The result is shown in fig. 3 for a constant gas pressure o f p = 8 X 10 -2 mm Hg, varying
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H. Soffel, N. Petersen, Ionic etching of titanomagnetite grains in basalts
voltage between the electrodes and periodic interruption of the bombardment every 20 min for a period of 20 min. The steep increase of the temperature with time shows that by a permanent bombardment the sample would attain very high temperatures even with low voltages between the electrodes. There seems to be a relationship between the voltage U applied and the average temperature T of the sample surface of the form: U = l n T . A decrease of the gas pressure also decreased the average temperature of the sample surface [6]. In order to keep the maximum temperature of the sample surface below 40°C an air pressure in the system of 2 X 10 -2 mm Hg and 3.5 kV d.c. between the electrodes was chosen for the ionic polishing of titanomagnetites and magnetite in a rock matrix. The same periodicity for the bombardment was used as shown in fig. 3. Under the above mentioned conditions the polishing rate is very small compared with chemical etching procedures and is in the order of several hours, as later seen in fig. 4. With the above-mentioned polishing conditions no signs of an oxidation of the surface of the iron oxides due to chemical reactions with the oxygen i.:, *.he impacting ionized air could be detected with reflecfivity studies under the ore microscope. Such oxidations, however, occurred on the surface of magnetite (blue coating due to the formation of maghemite and subsequently brown coating due to hematite) and of titanomagnetites (blue to violet coating due to formation of titanomaghemites) when the surface temperature significantly exceeded 40°C caused by much higher gas pressure and voltage between the electrodes (see [6]).
3. Ionic polishing of titanomagnetite grains in basalts As an example illustrating the effect of ionic polishhag, we selected a basalt sample from the Vogelsberg area (Germany) containing titanomagnetite grains with very f'me exsolution lamellae of hemoilmenite. A more detailed description of the magnetic properties of this sample has been published elsewhere [5]. First a section of the sample has been polished using diamond paste of different grades. Figs. 4a and 5a show two titanomagnetite grains after this state of preparation. Exsolution lamellae within the ore
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grains are only faintly visible, even under crossed nicols. The polished section was then subjected to ionic polishing for 12 hr withp = 2 X 10 -2 mm Hg and U = 3.5 kV. Fig. 4b and fig. 5b show the same ore grain after ionic bombardment. The structures of the exsolution lamellae are now much more clearly visible. The formation of a distinct relief has considerably increased the optical contrast. A further ionic bombardment of 12 hr under the same conditions (fig. 4c and fig. 5c) gives a more pronounced relief, however the contrast for optical observations is no more enhanced. This example shows that a characteristic relief can be produced by ionic bombardment. This relief enhances the optical contrast and is useful for any observation of size, number and shape of exsolution structures in titanomagnetite grains.
4. Further applications of the ionic polishing method The ionic polishing under similar polishing conditions to those described above turned out to be successful with many kinds of minerals such as silicates, pyrrhotite and iron oxides, and such hard and chemically resistant minerals as chromites and zircon. The polishing method has also been applied successfully to the preparation of surfaces for the replica technique in electron microscopy [6]. Its suitability for the production of stress-free surfaces of magnetite and titanomagnetites in a rock matrix for the observation of magnetic domains has already been published elsewhere [4]. It has also been used to prepare the surfaces of ferrimagnetic chromites, pyrrhotites and goethite in order to study their domain structures. On etched surfaces the dislocation density of the minerals can be investigated [6, 7], which has some influence on the magnetic properties of minerals and on the rheological properties of rocks.
Acknowledgements The studies were carried out in the Institut flit Angewandte Geophysik, Universit/it Mtinchen, Germany. We thank its director, Prof. Dr. G. Angenheister and the Deutsche Forschungsgemeinschaft for their generous support.
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H. Soffel, N. Petersen, Ionic" etching o f titanomagnetite grains in basalts
References [1] G. Beilby, Aggregation and Flow of Solids (Macmillan, London, 1921). [2] M. Berek, Optische Messmethoden im polarisierten Auflicht, Fortschr. Min. 22 (1937) 1. [3] T. Nagata, E. Larson, Mituke Ozima, Minoru Ozima and D. Strangway, Stability of remanent magnetization of igneous rocks, Geophys. J. 17 (1969) 263. [4] H. Soffel, Die Beobachtung yon Weiss'schen Bezirken auf einem Titanomagnetitkorn mit einem Durchmesser von 10 Mikron in einem Basalt, Z. Geophys. 34 (1968) 175.
[51 G. Angenheister and C. Turkowsky, Die Verteilung der induzierten und natiirlichen remanenten Magnetisierung innerhalb einiger Basaltlagen des Vogelsberges, Boll. Geof. Teor. Appl. 6 (1964) 285. [6] H. Soffel, Die Bereichsstrukturen der Titanomagnetite in zwei terti~en Basalten und die Beziehung zu makrty skopisch gemessenen magnetischen Eigenschaften dieser Gcsteine, Habil. Schrift, Naturwiss. Fak. Universit~it Mtinchen (1968). [7] ll. Soffel, The influence of the dislocation density and inclusions on the coercive force of multidomain titanomagnetites of the composition 0.65 Fe2TiO 40.35Fe304 in basaits as deduced from domain structure observations, Z. Geophys. 36 (1970) 113.