Lamproite

Mineralogy

The mineralogy of lamproites is controlled by their peculiar geochemistry, with a predominance of rare silica-deficient mineral species and rare, mantle-derived minerals predominating.

Minerals typical of lamproites include: forsteritic olivine; high iron leucite; titanium-rich aluminium-poor phlogopite; potassium- and titanium-rich richterite; low aluminium diopside; and iron-rich sanidine. A variety of rare trace minerals occur. The rocks are high in potassium with 6 to 8% potassium oxide. High chromium and nickel content is typical. The rocks commonly are altered to talc with carbonate or serpentine, chlorite, and magnetite. Zeolites and quartz may also occur.

Lamproites are characterized by the presence of widely varying amounts (5-90 vol.%) of the following primary phases (Mitchell & Bergman, 1991):

• titanian (2-10 wt.% TiO₂), aluminium-poor (5-12 wt.% Al₂O₃) phenocrystic phlogopite;
• titanian (5-10 wt.% TiO₂) groundmass poikilitic "tetraferriphlogopite";
• titanian (3-5 wt.% TiO₂), potassium (4-6 wt.% K₂O) richterite;
• forsteritic (Mg) olivine;
• aluminium-poor (<1 wt.% Al₂O₃), sodium-poor (<1 wt.% Na₂O) diopside;
• nonstoichiometric iron-rich (1-4 wt.% Fe₂O₃) leucite, and;
• iron-rich sanidine (typically 1-5 wt.% Fe₂O₃).

The presence of all the above phases is not required in order to classify a rock as a lamproite. Any one mineral may be dominant, and this, together with the two or three other major minerals present, suffices to determine the petrographic name.

The presence of the following minerals precludes a rock from being classified as a lamproite: primary plagioclase, melilite, monticellite, kalsilite, nepheline, Na-rich alkali feldspar, sodalite, nosean, hauyne, melanite, schorlomite or kimzeyite.

Geochemistry

Lamproites conform to the following chemical characteristics:

• molar K₂O/Na₂O > 3, i.e., ultrapotassic;
• molar K₂O/Al₂O₃ > 0.8 and commonly > 1;
• molar (K₂O + Na₂O)/Al₂O₃ typically > 1, i.e., peralkaline;
• typically < 10 wt.% each of FeO and CaO, TiO₂ 1-7 wt.%, > 2000 and commonly > 5000 ppm Ba, > 500 ppm Zr, > 1000 ppm Sr, and > 200 ppm La.

Economic importance

The economic significance of lamproite became known with the discovery of Ellendale E4 and E9 lamproite pipes and better known 1979 discovery of the Argyle diamond pipe in Western Australia. This discovery led to the intense study and re-evaluation of other known lamproite occurrences worldwide; previously only kimberlite pipes were considered economically viable sources of diamonds.

The Argyle diamond mine remains the only economically viable source of lamproite diamonds. This deposit differs markedly by having a high content of diamonds but low quality of most stones. Research at Argyle diamond have shown that most stones are of E-type; they originate from eclogite source rocks and were formed under high temperature ~1,400 °C (2,600 °F). The Argyle diamond mine is the main source of rare pink diamonds.

Olivine lamproite pyroclastic rocks and dikes are sometimes hosts for diamonds. The diamonds occur as xenocrysts that have been carried to the surface or to shallow depths by the lamproite diapiric intrusions.

The diamonds of Crater of Diamonds State Park near Murfreesboro, Arkansas are found in a lamproite host.

Nomenclature

Lamproites, as a group, were known by a variety of localised names because their mineralogy is quite variable, and because of their rarity often few examples of the following lamproite variants were known. Modern terminology classes all as lamproites but modifies this term with the mineral abundances as per the standard IUGS rules.

Related rock types

• Kimberlite – Igneous rock which sometimes contains diamonds
• Lamprophyre – Type of ultrapotassic igneous rock
• Ultrapotassic igneous rocks – Class of rare ultramafic or mafic igneous rocks rich in potassium