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Syenite

An igneous rock composed primarily of alkali feldspar together with other minerals, such as hornblende.

A phaneritic (visibly crystalline) plutonic rock with granular texture composed largely of alkali feldspar (orthoclase, microcline, usually perthitic) with subordinate plagioclase (oligoclase) and dark-colored (mafic) minerals (biotite, amphibole, and pyroxene). If sodic plagioclase (oligoclase or andesine) exceeds the quantity of alkali feldspar, the rock is called monzonite. Monzonites are generally light to medium gray, but syenites are found in a wide variety of colors (gray, green, pink, red), some of which make the material ideal for use as ornamental stone. Syenite is an uncommon plutonic rock and usually occurs in relatively small bodies (dikes, sills, stocks, and small irregular plutons).

Feldspathoid syenites are one of the most common and most variegated (both mineralogically and texturally) groups of alkaline rocks. They are broadly defined as plutonic rocks in which feldspathoid minerals account for 10-60% of all felsic minerals, and alkali feldspars are at least 10 times more abundant than Na-Ca plagioclase. Depending on the predominant feldspathoid mineral, this group of rocks can be further subdivided into nepheline syenites, sodalite syenites, cancrinite syenites, etc. Nepheline syenites and "mixed" sodalite-nepheline syenites are relatively common, whereas such rocks as kalsilite syenites are petrographic oddities. In addition to nepheline and other foids, these rocks contain potassium feldspar, albite (commonly as perthitic intergrowths with K feldspar), sodic amphiboles, aegirine, biotite, titanite and an assortment of rare minerals (like eudialyte, lamprophyllite or loparite).

Nepheline syenite is a medium to coarse-grained, light- to medium-gray, igneous rock that is composed predominantly of a silicate mineral called orthoclase (KAlSi₃O₈) and has a granite-like appearance. It may be distinguished from granite by little or no quartz content (free SiO₂). In Arkansas, nepheline syenite has been intruded into the sequence of Paleozoic sedimentary rocks. Locally, quarrymen recognize "blue granite" and "gray granite" varieties.

Material Notes:
Igneous rock. Very acidic. Predominent mineral is orthoclase feldspar (ca. 80%); may also contain quartz or plagioclase feldspar. Syenites are comparatively rare rocks, being found chiefly in a few areas of the United States and Germany. They are occasionally substituted for granites as building stones.

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