Wärmeleitfähigkeiten anisotroper Gesteine

Abstract

Ca. 70 — mit wenigen Ausnahmen orientiert im Gelände bzw. unter Tage entnommene — Gesteinsproben wurden bei einer Temperatur von ca. 94° C und Atmosphärendruck auf ihre Wärmeleitfähigkeiten in verschiedenen Richtungen untersucht. Insgesamt wurden einschließlich der Wiederholungsmessungen ca. 500 Einzelmessungen der Wärmeleitfähigkeit durchgeführt. Bei den untersuchten Proben handelt es sich zum großen Teil um Tonschiefer aus dem Rheinischen Schiefergebirge und dem Harz, zum kleineren Teil um Kalksteine aus dem Rheinischen Schiefergebirge, Metamorphite aus den Ostalpen und anderen Gebieten sowie um einige Magmatite. Bei den meisten untersuchten Proben, insbesondere denen mit schon äußerlich erkennbaren Planar- und Lineartexturen, sind die Wärmeleitfähigkeiten streng richtungsabhängig; diese Proben sind bezüglich der Wärmeleitfähigkeit (und anderer gesteinsphysikalischer Eigenschaften) anisotrop. Die Meßergebnisse werden in den Tabellen 2 bis 5 vorgestellt. The heat conductivities of some 70 oriented rock specimens taken from the field or subsurface were determined in various directions at about 94° C and atmospheric pressure. Some 500 individual measurements, duplicate measurements included, were carried out. The majority of the tested samples are slates from the Rheinische Schiefergebirge and the Harz (“Rhenoherzynikum”), a minor portion is made up of limestones from the Rheinische Schiefergebirge, metamorphic rocks from the central Eastern Alps and some other areas, and only some magmatic rocks were tested for comparison. The results of the heat conductivity measurements are presented in the tables 2–5. In most of the rocks investigated, especially in those showing planar and linear textures at the mesoscale, the heat conductivity is strongly controlled by the direction of measurement. These rocks are anisotropic with respect to the heat conductivity (and to other physical rock properties as well). The measurements of the heat conductivity in slates from the Rheinische Schiefergebirge showed considerable differences in different directions of the rock samples. In those slates, containing a transverse (“slaty”) cleavage (s1) besides the bedding planes (ss) (the typical roofing slates, e g.), the plane of the slaty cleavage turns out to be the best heat “insulator”, i. e., the direction of the least heat conductivity of the whole rock (k33) is oriented normally to s1 The direction of the greatest heat conductivity (k11 of these rocks lies within the plane of s1 and parallel to the intersection line of ss and s1, or parallel to the “Faser” (“longrain”), a lineation down dip of the slaty cleavage planes, only in some regionally restricted areas. The direction of the mean conductivity (k22) is oriented normally to k11 in the plane of s1. The anisotropy of heat conductivity in these slates (as well as in the mica schists and gneisses investigated) shows a orthorhombic symmetry. The tensor of the heat conductivity for these rocks, referred to the main axes, has the form In other “slates”, or better “shales” in the cases concerned, from the upper part of the Rhenohercynian sedimentary column, lacking any transverse cleavage at the mesoscale and rearrangements and new crystallization of the phyllosilicate minerals at the microscale, the bedding plane (ss) turns out to be the best heat “insulator”. Consequently, the direction of least heat conductivity (k33) in these rocks is normal to the bedding plane (ss). Within the bedding plane there is no preferred direction of heat conductivity (k11 = k22 > k33). Both slate types differing with respect to the anisotropy of heat conductivity with k33 ⊥ s1 and k33 ⊥ ss, resp., are linked by transitional stages. The results of heat conductivity measurements in slates from different tectonic frameworks (tectonic “environments”) of the Rhenoherzynikum presented here, reflect — in an indirect way — the various finite stages of the progressive fabric development within a relatively “shallow” orogen such as the Rheinische Schiefergebirge and the Harz, according to the author (Langheinrich 1964, 1976, 1978). Measurements of the heat conductivity of limestones from the Rheinische Schiefergebirge showing marked evidence of finite internal tectonic deformation demonstrated that these rocks are isotropic with regard to heat conductivity, presumably as a result of posttectonic annealing recrystallization of the rock forming calcite. Within the mica-schists and gneisses investigated the heat conductivity is always at a minimum normal to the s-planes (foliation) of these rocks, and at a maximum within the s-planes, mostly parallel to a lineation at mesoscale. Within this group of rocks the highest (k11) and lowest heat conductivity (k33) can differ by a factor of nearly 3 (table 4). All specimens of magmatites, only measured for comparison, showed near-perfect isotropy of heat conductivity. The whole-rock anisotropy of heat conductivity — considered under the conditions of experiment — is controlled by a number of factors. But the essential factors controlling this strong whole-rock anisotropy of the slates, mica-schists, and gneisses are the presence in considerable amounts of rock forming minerals with high individual anisotropies of heat conductivity, and the statistically preferred orientation of these minerals. The phyllosilicate minerals illite/muscovite with an “anisotropy factor” (k11/k33) of about 6 and chlorite (with a presumably similar “anisotrop factor”) play a major part. Quartz which shows some preferred crystal lattice orientation in some cases only plays a subordinate role with respect to the whole-rock anisotropy of heat conductivity. The “anisotropy factor” of quartz is about 1.7 (at room temperature). The statistically preferred orientation of some crystal lattices of the rock forming minerals with high individual anisotropies of heat conductivity was studied for a series of representative specimens of slates using a X-ray texture goniometer. The X-ray studies revealed excellent geometric correlations between the symmetries of the X-ray textures and the anisotropy of heat conductivity. Some 50 specimens of slates were selected for measurements of the propagation velocity of longitudinal ultrasonic pulses in various directions. Here, a perfect geometric correlation of the anisotropy of heat conductivity and the elastic anisotropy was found. Comparisons of the degree of crystallinity of illite and the anisotropy of heat conductivity in some slate specimens revealed no clear-cut correlation of these two parameters (see table 2). Before extending the data of heat conductivity and anisotropy of heat conductivity of rocks, determined under laboratory conditions, to “in situ” conditions a number of correction factors must be taken into account. These correction factors involve temperature conditions, the porosity of rocks, the kind of the pore fluids, the crack content (again a function of the stress conditions “in situ”), and others. Près de 70 échantillons de roches pris sur le terrain ou en subsurface, et orientés sauf quelques exceptions, ont été examinés en vue de leurs conductibilités thermiques dans des directions différentes à une température de 94° C et à pression atmosphérique. En tout 500 mesures individuelles de conductibilité thermique ont été exécutées, y compris les mesures doubles. Les échantillons examinés sont pour la plupart des schistes argileux du Massif Schisteux Rhénan et du Harz, le reste provient de calcaires du Massif Schisteux Rhénan, de métamorphites des Alpes orientales et d'autres régions, ainsi que quelques magmatites. Les conductibilités thermiques de la plupart des échantillons examinés, surtout de ceux qui montrent des textures planaires et linéaires déjà reconnaissables à l'extérieur, dépendent strictement de la direction; ces roches sont anisotropes du point de vue de la conductibilité thermique (ainsi que d'autres caractères physiques de roches). Les résultats des mesures sont présentés dans les tableaux 2 à 5. Определили. теплопро водность пород (70 прод) с известной ориентировкой при температуре прим ерно в 94 С и при нормаль ном атмосферном давлени и. В общем, при учете повт орных измерений, пров ели примерно 500 измерений теплопроводности. Ис следованные пробы яв лялись в большенстве своем гл инистыми сланцами из Рейнских сланцевых гор и Гарца, в меньшей мере известняками Рейнских сланцевых г ор, метаморфитами из в осточных Альп и других районов, а такж е магматитов. В большенстве исслед ованных проб, в особен ности в таких, где уже внешне видны линейные и план арные текстуры, теплопроводность то чно связана с направлением; эти пор оды являются относит ельно теплопроводности — и других физических св ойств — анизотропным и. Результаты измерени й представлены в нескольких таблица х (2–5).