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There are many other isotope series utilized in determining rock ages, the history of magmatic source rocks, the conditions of sedimentation and/or the age of metamorphic events. Some of these are discussed in the chapters that follow in contexts where they are especially important.

1 What three conditions favor the complete substitution of different atoms/ions for one another in a particular coordination site as a mineral grows? What three conditions limit such substitutions? Why are some substitutions simple and other substitutions paired?

2 For an initial solid plagioclase of composition An60, use ssss1 to:Describe the proportions of Na+ and Ca+2 in the large cation substitution site.Describe the proportions of Si+4 and Al+3 in the small cation substitution site.Write a formula for this plagioclase in the form (NaxCax) (SixAlx)AlSi2O8, similar to the formula for An35 shown in the figure.

3 In addition to the β‐quartz—cristobalite—liquid triple point show by the letter X in ssss1, identify the two other triple points shown in the figure in terms of the three phases that exist in equilibrium and the unique temperature and pressure at which the three phases are equilibrium. Which of these sets of coexisting minerals might be expected to exist deep below the surface and which might be more likely to occur much nearer the surface? How might you recognize a rock that formed under triple point conditions?