Volatile elements:

Since the late 1940s, important advances have been made in the chemical analysis of both meteorites and the Sun. A remarkable result has emerged from this work. Although at one time there appeared to be major differences between the Sun and typical meteorites in the ratios of elements to one another (e.g., iron to silicon), these differences tended to disappear as the accuracy of the measurements improved. It turned out that, for most meteorites and most elements, the solar and meteoritic values of the element ratios relative to silicon (taken as a standard) agreed to within better than a factor of two.

Two kinds of exceptions to this rule were found. Relative to the Sun, the meteorites were deficient in the more volatile elements. For the most volatile elements, hydrogen and the noble gases, the deficiencies were gross--more than a factor of 10,000. For less volatile elements, the deficiencies were smaller; and for the nonvolatile elements, or "refractory" elements, such as iron, magnesium, aluminum, and calcium, the meteoritic and solar abundance ratios were identical within the accuracy of the data.

Excerpt from the Encyclopedia Britannica without permission.