MINERALOGY

The nepheline syenite at Table Mountain is light to medium gray with a glassy luster, holocrystalline, very fine to fine with a pronounced trachytic texture. It is an igneous plutonic sill, tabular in shape, which intruded sedimentary rocks. The ne-syenite fine texture indicates a fast cooling process making it resemble a volcanic rock (phonolite) but still presents an aphanitic grain or crystalline structure.

General Mineral Composition.
In general, the nepheline syenite is composed of alkali feldspar Ab90-Or10 75-80%, nepheline 5% to 14%, analcime 5%, aegirine 10%, Riebeckite-arfvedsonite 3%, olivine <1%, biotite <0.5%, opaque minerals plus apatite <0.5%. The alkali feldspar Ab90-Or10 constitutes 75-80% of the nepheline syenite with a sub parallel alignment of laths (trachytic texture). Nepheline occurs as euhedral and subhedral crystals and as anhedral small grains interstitial to feldspar. Analcime occurs in association with nepheline. The mafic minerals surround the nepheline and analcime crystals and are found intergrown with alkali feldspar.*

Mineral Composition for Three Samples and Weather Alteration.
In the process of determining the distribution of the iron content in the rock, the following mineral assemblage was found for 3 samples.

Mineral composition and comparative Ne% for three samples with low, mid, and high Loss On Ignition (LOI) values.

Weathering exposure causes the leaching of nepheline out of the rock. Loss On Ignition (LOI) values are an indication of the weathering effect on a sample: the fresher the rock the lower the LOI value. The chart at right shows three selected samples, for the high, medium and low LOI values, to demonstrate this inverse relation contrasting the ne values. Most Altered (NS-13) / High LOI-----in the mid-stream (NS-10)-----Least altered (Bulk B)/ low LOI.
Source: Dr. Stan Mertzman from the laboratory at Franklin & Marshall College.

*Descriptions and analysis of eight new USGS rock standards. Nepheline Syenite, STM-1, from Table Mountain, Oregon. By P.D. Snavely, Jr., N.S. MacLeod, F.J. Flanagan, Sol Berman, H.G. Neiman, and Harry Bastron.