Return to Alkaline Rocks
Just a hundred years ago the idea that carbonate rocks could crystallize from magma would seem preposterous. In the late 1950s-1960s, the discovery of carbonate lavas in Africa, high-temperature experimental work, and careful examination of carbonate rocks associated with alkaline silicate intrusions around the globe dispelled the last remaining doubts that carbonatitic magmas existed. These magmas and their derivative fluids give rise to a wide spectrum of rocks composed of carbonate minerals (usually, calcite, dolomite or ankerite), ferromagnesian silicates (clinopyroxenes, olivine, biotite, amphiboles, monticellite and melilite), apatite, magnetite, perovskite, ilmenite and various accessory constituents enriched in "incompatible" elements. Such minerals as baddeleyite ZrO2, calzirtite Ca2Zr5Ti2O16, lueshite NaNbO3, ancylite (Sr,Ca)REE(CO3)2(OH)·H2O, strontianite SrCO3, monazite REEPO4, pyrochlore, and Ti-Zr garnets are considered "hallmark" minerals of carbonatites. Carbonatites (this name is applied to both intrusive and extrusive varieties) and related silicate-oxide-rich rocks (silicocarbonatites and phoscorites) are typically associated with ultramafic rocks and foidolites, feldspathoid syenites, melilitolites or their volcanic equivalents. It is still debated whether these silicate rocks are genetically related to carbonatites (e.g., through crystal fractionation or liquid immiscibility), or if they are derived from discrete sources in the upper mantle. The relative significance of igneous, hydrothermal and wallrock-assimilation processes in the development of carbonatites and their mineral resources also remains to be determined. The astonishing textural, mineralogical and chemical diversity of carbonatites, phoscorites and silicocarbonatites attests to the complexity of geological processes involved in their formation. Shown on this page are:
[Upper right] Devonian carbonatite dike (white) crosscutting Precambrian granitoids, Turiy Mys (Kola, Russia).
[Middle right] Coarse-grained calcite carbonatite (sövite) emplaced in alkali feldspar syenite, Eden Lake (Manitoba, Canada).
[Below] Mineralogist Anatoly Zaitsev examines freshly erupted natrocarbonatite lava (black) at one of the hornitos in the crater of Oldoinyo Lengai, Tanzania.
[Above] Relict lenses of ijolite (one type of foidolite rock) in sövite, Oka (Québec, Canada); small black crystals in the carbonatite are magnetite.
[At left] Forsterite-phlogopite-calcite carbonatite from Kovdor (Kola, Russia) as seen under the optical microscope (in crossed polars). This rock consists of calcite (Cal), zoned phlogopite (Phl) and resorbed crystals of forsterite (Fo) and fluorapatite (FAp).