Proc. Technical Session, 23rd Rochester Mineralogical Symposium, Rochester, New York, April 16, 1999. pp. 11-12. Also : Rocks & Minerals 75 (2000) 172-173.



John A. Jaszczak and George W. Robinson. A. E. Seaman Mineral Museum,

Michigan Technological University, Houghton, Michigan 49931-1295.


Remarkable graphite spheres from 0.1 to 10 mm in diameter occur in coarsely crystalline calcite marble lenses in a highly mylonized area in the south-western end of the Bancroft shear zone (Carlson et al., 1990) in the Central Metasedimentary Belt of the Grenville province. The occurrence is exposed by a roadcut on Haliburton County road 507 approximately 3.6 km south of Gooderham, Ontario, Canada. The spherical graphite at this occurrence was apparently first recognized by Basil Breen in the 1970's.

Optical and scanning electron microscopy reveals that the spheres have a variety of surfaces and internal textures. Surfaces can be well-crystallized, velvety, or smooth and lustrous. Internal textures typically show pronounced zonation of crystallite orientation and grain size. The graphite basal planes are oriented randomly in some zones and dominantly circumferentially in others, particularly near the surface of the spheres.

Many spheres have what appear to be pressure shadows of very finely dispersed graphite crystals or occasionally other minerals (Fig.). Naturally broken graphite spheres are common and make dramatic shear sense indicators.

Stable carbon isotope analysis for the spherical graphite in three different calcite lenses shows an average D13Ccal-gr = 4.30.6 PDB. Assuming a homogeneous isotope composition throughout the spheres, the calcite-graphite thermometer of Dunn and Valley (1992) suggests peak metamorphic temperatures in the different calcite lenses ranging from 595 to 690 C. Flake graphite in a mica-rich layer adjacent to a calcite lens shows significantly lighter carbon (average d13C = 10.4) than the graphite spheres (average d13C = 4.1), indicating that they may have formed under different conditions.

Possible growth models, originally developed to explain spherical graphite growth in nodular-graphite cast iron (Lee, 1997; Double and Hellawell, 1974, 1975, 1995) are illustrated in Figure 2. These models rely on structural defects to promote growth of graphite normal to the basal planes and lead to a predominantly circumferential alignment of the basal planes around the sphere.

Other unusual graphite aggregates, which appear to be genetically related to the graphite spheres, also are present (Fig.). Some exhibit a 3-fold pattern one might call a 3-dimensional Celtic triskelion. Other aggregates appear to be combinations of the triskelion and a sphere. Still other aggregates are combinations of a sphere and a tetrahedral shape. Monte Carlo computer simulations performed in another context (Lee, 1997) suggest that these aggregate morphologies may result from influences of elastic stress energy of an initially spherical graphite aggregate in calcite matrix.


Literature Cited:


CARLSON, K.A., VAN DER PLUIJM, B.A. and HANMER, S. (1990) Marble mylonites of the Bancroft shear zone: Evidence for extension in the Canadian Grenville. Geological Society of America Bulletin 102:174-181.

DUNN, S.R. and VALLEY, J.W. (1992) Calcite-graphite isotope thermometry: a test for polymetamorphism in marble, Tudor gabbro aureole, Ontario, Canada. Journal of Metamorphic Geology 10:487-501.

DOUBLE, D.D. and HELLAEWLL, A. (1974) Cone-helix growth forms of graphite. Acta Metallurgica 22:481-487.

DOUBLE, D.D. and HELLAEWLL, A. (1975) Growth structure of various forms of graphite. In: The Metallurgy of Cast Iron, eds. B. Lux, I. Minkoff, and F. Mollard. Georgi Publ. Co., St. Saphorin, Switzerland. p. 509-528.

DOUBLE, D.D. and HELLAEWLL, A. (1995) The nucleation and growth of graphite- the modification of cast iron. Acta Metallurgica et Materialia 43:2435-2442.

LEE, J.K. (1997) Morphology of coherent precipitates via a discrete atom method. Materials Science and Engineering A 238:1-12.

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Fig. 2. Models of possible mechanisms for the growth of spherical graphite proposed by Double and Hellawell. (a) Cone-helix model (modified after Double & Hellawell 1975). (b) Twist-tilt grain-boundary model (modified after Double & Hellawell 1974).