The elements listed in the chemical formulas of minerals aren't the only ones found in them, as you know from Homework 1 and our class discussions: any cation of similar Ionic Radius and Valence may substitute for a major mineral-forming cation, as it will coordinate similarly with oxygens, and both "fit" into available structural sites in minerals, and provide charge balance for the mineral. Such substitutions are called Camouflage substitutions, meaning that during the crystallization of the mineral no discrimination occurs between the major elements and these minor species.

However, camouflage substitutions are rare: most of the time cations are either larger and smaller than is ideal for mineral sites, and often they are of the wrong valence. Yet, we find significant, if small abundances of such "poorly fitting" elements in any mineral that we chemically analyze. Where are these species found?

A rather labor-intensive way to assess where in a mineral a given element might be was attempted by Onuma and coworkers in Japan in the 1960's and 1970's. The specific interest of Onuma was in how low abundance Trace Elements distributed between precipitating minerals and melt in natural igneous systems. So, he (actually, his students!!) would collect 10+ kg samples of volcanic rocks from Japanese volcanoes that contained large and abundant crystals (a texture called Porphyritic), crush the samples, and separate out several grams of each of the Phenocryst minerals and of crystal-free rock matrix. They would then chemically analyze each of the mineral and matrix splits for a large variety of major and trace elements, and assess the affinities of each element based on a parameter called a Mineral/Matrix Partition Coefficient , which was just the ratio of the measured concentration of each element in the minerals versus its concentration in the matrix.

A) What we're going to do is interpretive part of Onuma's work. Below I have given you concentration data for several minerals and their associated matrix. I want you to calculate mineral/matrix partition coefficients for each element in each mineral, and tabulate this data for yourself.

ii) Compare the same elements in different minerals: are the coefficients similar? Why or why not?

 n.a.: unable to analyze.

b) I have also provided a list of the "octahedral" ionic radii for each of the elements analyzed. (admittedly, these elements all can't coordinate octahedrally with oxygen, but using radii for a specific coordination number facilitates comparison, and anyway, it's what Onuma did!) Below is a semi-log plot, loaded as a JPEG image. (You can download and copy this, or you can buy some semi-log paper, but either way, you're going to need graph paper of this sort!) What I want you to do is put Ionic Radii on the "x" axis, Partition Coefficients on the "y" axis, and plot the different elemental partition coefficients for a specific mineral (in other words, plot all the olivine/matrix partition coefficients on one graph, then put all the pyroxene/matrix coefficients on a second graph, etc. You'll end up with a graph for every mineral). Label the different points with the symbols for the element.

c) The table above also lists the common valence states of the elements we're working on. Label the elements with their valences.

d) What I want you to do now is a modified "connect the dots":

i) On each graph identify the data for all plotted elements of similar valence

ii) connect elements of similar valence on each graph via a smooth curve. What you should end up with is a graph with a series of curves on it for +1, +2, +3, and +4 valence elements.

1) Where do trace elements "fit" in mineral structures?

2) is ionic radius or valence more important in determining the "fit"?

3) Do some minerals more readily accept trace constituents than others? If so, why do think this is the case?