The half life for the radioactive decay of rubidium-87 to strontium-87 is 4.88 × 10 years. Suppose nuclear chemical analysis shows that there is 0.728 mmol of strontium-87 for every 1.000 mmol of rubidium-87 in a certain sample of rock. Calculate the age of the rock. Round your answer to 2 significant digits.

Most igneous, metamorphic, and sedimentary rocks contain rubidium (Rb) and strontium (Sr) in detectable amounts. However, the concentrations of these elements are almost always less than 1 percent, and they are therefore rarely determined in routine chemical analyses. Neither rubidium nor strontium is a major constituent in the common rock-forming silicate minerals, although strontium does form a carbonate (strontianite) and a sulfate (celestite) which are found in some hydrothermal deposits and certain sedimentary rocks, particularly carbonates.

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