camptonite - hand/display specimen of a very unusual lamprophyre

Camptonite is a rare lamprophyre. Lamprophyres are ultrapotassic silica-depleted rocks with K₂O : Na₂O at least 2:1. They form small intrusions such as dikes, stocks and laccoliths. The most likely origin of this magma type is partial melting of mantle material. Lamprophyres are similar to kimberlites and can contain diamonds.

Camptonite has a composition similar to nepheline diorite (foid-bearing diorite). A camptonite is a porphyritic alkaline igneous rock dominated by essential plagioclase and brown amphibole, usually hornblende, often with titanaugite. Plagioclase occurs in the groundmass. 

This camptonite came from a famous dike exposed in a roadcut on U.S. Highway 93, roughly eight miles south of Hoover Dam. The exposure was almost completely removed when the two-lane highway was widened to four lanes during the construction of the bypass around Hoover Dam. The bypass includes the longest single-span concrete arch bridge in the western hemisphere, spanning Black Canyon to the immediate south of Hoover Dam. Only a thin band of camptonite remains in a tall roadcut beside the new highway. These specimens were collected from material removed during highway construction. You can still trace deeply weathered outcrops of this dike southwest and northeast of the highway.

The formerly four-foot wide dike was well exposed on both sides of the original road, a short distance to the east of the two-lane route. Both the original route and the two lane roadway are now gone. The margins of the dike were aphanitic in texture, with small white areas of calcite and calcite-analcime amygdules and veinlets. Small rust-colored pseudomorphs of olivine are occasional. Toward the center of the dike, large black phenocrysts of an amphibole close to kaersutite are abundant, with their abundance increasing symmetrically from both walls of the dike. 

This camptonite is from the inner portion of the dike. Its composition is shown on the QAPF diagram. It fits in the green polygon. Click and click again to enlarge the diagram.

Cut on both sides. Both sides are shown.

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