BH250-207

BH250-207

Siderite Carbonatite

Major Minerals: siderite, calcite
Minor Minerals: perovskite, pyrochlore, strontianite (?), witherite, oxides

Optics: Numerous siderite and calcite grains are oriented nearly perpendicular to the c-axis, making them ideal for obtaining optic axis interference figures. Both minerals exhibit perfect cleavage. Their contrasting refractive indices allow for clear comparison of relief differences between siderite and calcite. Additionally, non-pseudoisotropic crystals of perovskite, pyrochlore, and various opaque oxides are present, each displaying distinct optical characteristics under crossed polars.

Chemical composition, origin, and geologic distribution of carbonatites:

Sample BH250-207 is carbonatitic igneous rock with an exceptionally low SiO₂ content (3.13 wt.%) and a rare earth element (REE) pattern marked by strong enrichment in light rare earth elements (LREE) and depletion in heavy rare earth elements (HREE)—a geochemical signature characteristic of carbonatitic magmas.

Carbonatitic lavas are extremely rare volcanic rocks defined by low silica content (<5 wt.%) and high concentrations of carbonate minerals, such as calcite, dolomite, or ankerite. Unlike common silicate magmas, carbonatitic lavas form through low-degree partial melting of metasomatized mantle peridotite enriched in CO₂ and incompatible elements. Their distinctive chemistry offers important insights into deep mantle processes, metasomatism, and magmatic differentiation. Two primary models explain their genesis: (1) direct partial melting of a carbonated mantle source, as seen in modern eruptions at Oldoinyo Lengai in Tanzania, and (2) liquid immiscibility, where carbonate melts separate from coexisting silicate magmas, such as nephelinites.

Carbonatites span Earth’s geologic history, from the Archean (>2.5 Ga) to the present, and are typically associated with continental rift zones, intraplate settings, and alkaline igneous complexes. Their composition ranges from calcitic and dolomitic to sodium-rich natrocarbonatites, like those erupted at Oldoinyo Lengai the world’s only active carbonatite volcano. Geochemically, carbonatites are enriched in incompatible elements, particularly REEs, and commonly exhibit pronounced LREE enrichment and HREE depletion, as demonstrated in the REE plot linked below.

Location: Iron Hill, Colorado, USA
GPS Coordinates: 38° 15' 11'' N , 107° 3' 11'' W

Bereket Haileab

From the rock collection of Bereket Haileab. Sample BH250-207. Housed at Carleton College in Minnesota.

Thin section

View on ArcGIS Online here





























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