Spectrographic analyses of a group of igneous rocks and their constituent minerals from Jamestown, Boulder County, Colorado, revealed:

Qualitative Analyses

Rocks: Each, regardless of age, is distinguishable by variations in content of certain elements (Sc, La, Ce, Nd, Zr, V, Cr, Co, Ni, Pb, and Zn). Comparison with several Massachusetts rocks emphasizes the province relationship.

Minerals: Minor elements occur systematically. Sr is highest in K and Ca minerals (particularly feldspars); Ba in K minerals (potash feldspar and hornblende). All quartz and sphenes contain Al. Sc is most abundant in Mg and ferrous Fe minerals (micas and hornblende). Sphenes contain a remarkable proportion of rare earths. Ti, Mn, V, and Cr are concentrated in the dark minerals and muscovite. Ni and Co occur almost exclusively in biotite.

Quantitative Analyses

Quantitative analyses substantiate all conclusions based on the qualitative work.

Minerals: Characteristic minor elements of each mineral vary from rock to rock. The SrO : BaO ratios decrease numerically in the order of plagioclases, biotites, K-feldspars, and muscovites. Pegmatite K-feldspars have much larger SrO : BaO ratios than parent rock samples. This is explained by temperature-crystal structure relations. Qualitatively and quantitatively each pegmatite mineral bears an average of less of almost every minor constituent than the same mineral in the parent rock. This also is explained by temperature-crystal structure relations.

The minor constituents are probably present in solid solution.

First Page Preview

First page PDF preview
You do not currently have access to this article.