Orbicular Granites

Orbiculites are a special form of granite or related rock which contain spheres built up in layers.  The spheres are so strongly bound to the matrix rock (granite, gneiss, andesite, etc.) that they cannot be separated out from them, or only with difficulty.  Their size varies between several up to about 20 cm (some exceptions up to 40 cm).  They have often undergone ductile deformation.  The layers contain, concentrated, the same minerals (plagioclase, alkali feldspar, quartz, mica, hornblende) as the matrix material.  These sorts of orbiculites, which display strong evidence of differentiation, growth, deformation and re-dissolving, are found in only a few dozen quarry locations on earth, often in the border regions between granite, gneiss, diorite or other crystalline complexes.  Some come from middle and south Finland from the Baltic shield (Archaen and Proterozoic).  The orbiculite usually is confined to small areas, and is only occasionally extracted, generally for geological rarity pieces for collections and museums.  In part they are protected by law.

The small-sphered Rapakivi granite does not belong to the genuine orbicular granites.  It is found over wide stretches of east Finnland, and is sold by the natural stone industry.   The present-day science has had great difficulty explaining orbicular granite.  There is one theory of origin at present which is somewhat satisfying, but when compared to certain phenomena still leaves an uncertainty.  These phenomena will be discussed here. . Orbiculite from Finnland (Kangasala).  One should note the rock fragments, which can form the cores of spheres, and the broken or dissolved shell on the right-hand picture, through which the granite has been pressed in from outside.

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Left: Finland (Kangasala).  An unusual combination of core fragment, a separate small sphere and numerous layers.
Right: Finland (Savitaipale).  Lighter zone with larger crystallization which had penetrated the border of one sphere.

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Finland (Virvik - Porvoo). On the right with strong signs of dissolving
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Finland (Pengonpohja – Kuru).  Strong ductile deformation, reddish orthoclase layers, in part loosened shell fragments.
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Finland (Pallokivilevyt).  Separated shell sections, individual spheres with feldspar core.
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Origin in Scandinavia, found in glacially-transported material in Lower Saxony, Germany.
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Left: Australia.                                                               Right: Chile (Caldera)

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Finland.  Rapakivi granite, obtained commercially (spheres up to approx. 5 cm) actually not a true orbicular granite.
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The details in the pictures indicate soft, ductily-deformed structures, in part broken off, so that the outer material could flow in, or the inner material out.  In part the outer shells come loose, and can float as fragments in the surrounding matrix.  Individual spheres have centers composed of fragments of the shells of other spheres, or pieces of gneiss.  To some extent one can discern a series of growth and dissolution phases.

Orbiculites were early to come under scientific scrutiny, and numerous published articles have attempted to explain the phenomena.  In 1966 the American D.J. Leveson brought together numerous works and undertook his own researches, reaching finally, among others, the following conclusion:  the spheres oiginated in magmatic plutons which have intruded cooler rock complexes, under particular physical and chemical conditions.  They can exhibit very different features in chemically-similar rock formations, and likewise those found in formations with very different chemistry can be quite similar.  Deformed and partly-dissolved spheres led Leveson to the conclusion that their rarity may be due to their frequently dissolving shortly after their formation.  He also drew a connection to the experiments of R.E. Liesegang, who in 1896 produced concentric ring formations with silver nitrate in colloidal gels.  Leveson nevertheless did not conclude that the orbiculite matrix was colloidal, but rather that the Liesegang-effect might possibly also take place in magmatic-plutonic melts and solutions.

At the end of the 1980's H.P. Meyer of Heidelberg, described a precise model of origin in his dissertation.  He described the occurrences in the vicinity of the growing orbicules with the model "undercooling with heterogeneous seed-formations and growth controlled by diffusion".  According to this, quickly-cooled magma intruded into a foreign rock body, whereby certain substances crystallized out in radial "starlike" form from a multiplicity of crystallization centers, which led to a local thinning-out of these slowly-diffusing substances in the magma.  At a particular point in time the thinning-out is so strong, that the predominance of another substance leads to its crystallization around the growing orbicule, creating a second step.  After a certain time-period, the balance swings in the other direction, leading to a third layer,a nd so on.  In comparison with earlier models, this one appears able to reconcile itself to the phenomena, and has thus become generally recognized.

Despite this, one can have the impression that certain aspects of the phenomena have not been taken sufficiently into account:  puzzling are above all the peeling off and floating of outer shells (comparable to the same phenomena in certain agates), the breaking-open of spheres with the subsequent penetration by the surrounding matrix material, and not least the soft deformation caused by spheres pressing against one another.  With conventional crystallization, one would expect spheres bordering on one another to either grow together or simply form rigid contact surfaces.

At this point one can make the thought-experiment of trying to understand the phenomena of orbiculites under the presupposition of a colloidal-gelatenous consistency of the "pluton".  One must thereby take the step from a magmatig pluton to a hot, nearly "gelatenous" mineral mass.  The "diffusion-controlled crystallization" postulated by Meyer would then become a diffusion-controlled layered deposition in a colloidal groundmass, which will also be described in the section dealing with agates.  The actual crystallization, resulting in the present condition of the orbiculites, would then have occurred later, after the complete forming and frequent ductile deformation of the orbicules.


The photos on the left show experiments with eggs, which have hardened in a colloidal material under pressure, and on the right sections of orbicular granites for comparison.

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Concerning the orbiculite matrix, one could perhaps think of a substance somewhere between the two conditions of "magmatic" and "colloidal", of a hot, colloid-like, watery-silicious rock precursor.  It must be assumed, to be sure, that under the present chemo-physical conditions of the earth such a substance is hardly possible, and also hardly to be reproduced in experiments.  It is perhaps significant that no orbiculites of this type are found in younger rocks, and that the phenomenon seems restricted to a particular period of the earth's history (late Archaen, Proterozoic, a few from the Paleozoic).  In later sections, for example Self-organization, the question will be gone into of whether certain processes of rock formation are strongly conditioned by time and also irreversible.