BOWEN'S REACTION SERIES John J. Thomas Purpose Frequently, people cannot visualize the mineral associations that form the sequences of igneous rocks that you find in the earth's crust and what happens to them once they are exposed at the earth's surface. Norman L. Bowen experimented with laboratory melts of igneous rocks in the 1920's and 30's, and discovered an experimental crystallization sequence of minerals that matched what you can observe in nature. This sequence is called Bowen's Reaction Series and it includes most of the common rock forming minerals. Definitions Magma - molten igneous rock Extrusion - magma extruded or placed on the surface of the earth to cool. Intrusion - magma intruded or emplaced beneath the surface of the earth. Feldspar - the family of minerals including microcline, orthoclase, and plagioclase. Mafic - a mineral containing iron and magnesium Aphanitic - mineral grains too small to be seen without a magnifying glass. Phaneritic - mineral grains large enough to be seen without a magnifying glass. Reaction Series - a series of minerals in which you mineral reacts to change to another Rock Forming Mineral - the minerals commonly found in rocks. Bowen's Reaction Series lists all of the common ones in igneous rocks. Specific Gravity - the relative mass or weight of a material compared to the mass or weight of an equal volume of water. Teacher Information Dating back to the earliest geology textbook, De re Metallica, written in medieval times, miners realized that they could observe a sequence of mineral crystallization in the rocks from early to late formed minerals. However, no relationship was codified previous to the work of Norman L. Bowen at the Carnegie Institute in the 1920's and 30's. Bowen ground up actual igneous rocks, along with mixtures of chemicals that could make up igneous rocks and experimented with their melting relationships. He would put the samples into what chemists call a "bomb," or a very strong enclosed sample holder, which can withstand very high temperatures and pressures without exploding (although sometimes they do). He would heat them up until they melted, 1600 o C or more, then cool them to a known temperature, for example 1400 o C. He would hold them at that temperature long enough to allow crystal formation (minutes, hours or days) and then cool them immediately by throwing them into a bucket of water, thus locking into the samples the minerals that had formed at the chosen temperature. Non- mineralized material would be glass. To find out what minerals, if any, had formed, he would analyze the samples with X-ray Diffraction. If there are minerals in the sample, X-rays will be diffracted, or bent, as they pass through the mineral. Glass causes no refraction. The angles to which the X-rays are bent are dependent upon the crystal structure of the mineral. Because every mineral has a different structure, every mineral has a different X-ray diffraction pattern. If there are no minerals are in the sample, X- rays go straight through it without being diffracted. When Bowen performed these experiments, he discovered that there was a regular mineral crystallization sequence exactly like the one that geologists observe in nature. In the course of his analysis, he realized that the sequence was related to measurable physical and chemical