In the Margala Hill Limestone conversion of micrite to microspar is a common phenomenon and is characterized by the development of isolated patches of inequigranular microspar selectively converting high magnesium calcite (micrite) into low magnesium calcite with heterogeneous texture of middle shelf facies (Plate 1C) and inner shelf facies (Plate 2E).
The allochemical constituents include a wide assemblage of larger foraminifera with dominantly low magnesium calcite skeletal composition.
Microscopic plants called coccolithophores (common alga) surround themselves with protective calcite plates; aragonite is used by pteropods (snail-like molluscs) to build their shells and corals use it to make their skeletons which helps form reefs; while some echinoderms, starfish, sea urchins, brittle stars, utilise magnesium calcite
to form their exoskeletons.
Living on a steady diet of copper and tin, the organisms on the statue had "digested" the metals to produce shells with unusual ratios of magnesium calcite
and aragonite, for example, as well as traces of feldspar and quartz.
Being a highly organized network of microscopic channels, the magnesium calcite
shell of the sea urchin has a configuration similar to that of a photonic crystal.
X-ray diffraction analysis of the residue shows quartz as the dominant residue component, with scarcely more than trace quantities of magnesium calcite
Meteoric environment is characterized by pore fluid that is either supersaturated or undersaturated however mostly undersaturated with respect to metastable carbonate species aragonite and magnesium calcite
The mineral phase is mainly composed of magnesium calcite
(Chave, 1952) with a MgC[O.
The granules are composed of a fibrous organic matrix in which needle-like low magnesium calcite
crystals are deposited (Neff 1971a).
In the latest of these studies, the scientists found that the sea urchins' teeth contain crystals of magnesium calcite
, which are smaller, harder and denser than those of pure calcite.
In the Lockhart Limestone conversion of micrite to microspar is a common phenomenon and is characterized by the development of isolated patches of inequi-granular microspar selectively converting high magnesium calcite
(micrite) into low magnesium calcite
with heterogeneous texture (Plate 2G 2H and 1H).
Microdolomite inclusions in cloudy prismatic calcites: a proposed criterion for former high magnesium calcites