Theralite

Theralite (from Greek "to pursue") is, in petrology, a group of plutonic holocrystalline rocks consisting of nepheline, basic plagioclase, augite and olivine, and so called because of its rare occurrence.

Its discovery was looked forward to with interest as completing the series of basic rocks containing nepheline as an essential constituent.

The term Theralite is rarely used within modern igneous petrology to describe a group of rocks, as the rocks which define the group name Theralite are referred to as alkaline igneous rocks.

The term Theralite is still used in modern petrology within the IUGS classification scheme as equivalent to nephaline gabbro. Teschenite is no longer considered to be classified under Theralite, as it contains analcime.

Theralite is the intrusive equivalent of tephrite. As feldspathoid content decreases it becomes a gabbro (pyroxene, plagioclase, olivine), and as it increases into nepheline monzonite (with loss of olivine). As such, theralite is an adjunct term between these two modern terms and is obsolescent.

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Teschenites

Teschenites were once considered part of the Theralite group. They contain essential pyroxene, olivine and analcime.

Modern petrology would classify the teschenites as an analcime bearing gabbro. They are petrogenetically unrelated to nepheline gabbros as analcime is unable to be produced by the same genetic processes.

Essexites

Essexite is a form of silica-undersaturaed pyroxene gabbro, containing nepheline and orthoclase as essential constituents, though often in small proportions. Essexites can be considered to represent the intrusive equivalent of an alkaline basalt.

Essexites are usually associated with other alkaline, silica undersaturated to monzonitic intrusive rocks and coeval high-alumina basalts.

Essexites are no longer considered part of the theralite group and are more properly known as nepheline monzogabbro or nepheline monzodiorite. The presence of orthoclase indicates a petrogenetic difference with nepheline gabbros.

Shonkinite

Shonkinites are rare high-alumina, silica-undersaturated mafic to ultramafic rocks comosed primarily of augite pyroxene, olivine, nepheline, orthoclase and phlogopite. They are more properly considered to be part of the kalsititic and melilitic group of rocks.

The type locality of shonkinites is at Shonkin Sag in the Highwood Mountains of Montana.

References

• Russian alkaline igneous rock occurrences, including "theralite"]