ABSTRACT

The morphological and optical properties of crystal aggregates which develop by nucleated growth from a plane substrate are modelled by crystal growth diagrams. Such diagrams show that fabric maturation occurs through three growth stages: 1) isolated, 2) competitive, and 3) parallel.

Diagrams are presented which are constructed using calcite rhombohedra of equant {1011}, obtuse {0112} and acute {4041} form. The acute form develops, in the most mature parallel growth stage, length-fast columnar crystals. This graphical model is matched by a natural, calcite-cemented neptunian dyke. The obtuse form develops, in the most mature parallel growth stage, approximately length-slow, bladed crystals. This graphical model is matched by a natural, calcite-cemented dilational vein.

Diagrams are presented which show seeded, in contrast to nucleated, growth. Seeding produces overgrowths which are divisible into an early epitaxial stage of rapid growth and a later idiomorphic stage. The volume of epitaxial growth depends on surface area, shape and lattice perfection of the seed, and on the crystal habit of the overgrowth. The overriding importance of echinoderm skeletal elements in stimulating epitaxial growth in limestones is related to their large size relative to other seeds and to their lattice properties. Polycrystalline seeds with parallel c-axis orientation develop overgrowth aggregates which omit the competitive stage because the crystals are constrained to develop normally to seed surfaces even when these are not the directions of most rapid growth. Growth from polycrystalline seeds with random c-axis orientation produce crystal aggregates similar to those developed by nucleation from a passive substrate.

Crystal growth diagrams are presented to show the effect of a change in crystal habit during continuous growth. The acute to obtuse change examined results in the crystal's greatest growth vector(s) being reorientated, which in turn causes a new episode of fabric maturation. This graphical model is matched by a calcite cement from a South Wales grainstone. T-shaped crystals, growth hiatuses on some crystal surfaces, and enfacial junctions are created in both model and natural example.

The use of crystal fabrics in the diagnosis of cement, the correlation between crystal fabric and saturation state, and the standard of crystallographic observation in studies of calcite cement are criticized.

The match between accurately drawn crystal growth diagrams and natural crystal aggregates is excellent. These diagrams are an underused aid in the stereological assessment of crystal aggregates and reveal features that are not intuitively obvious.