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The Pikes Peak region is renowned for its rare mineral specimens. It is a favorite collecting area for rock hounds and serious collectors alike. Scientists from around the world come to Colorado to study the minerals of this region. Because the granite covers a large portion of the Colorado Front Range, there are good mineral collecting areas scattered all over the Pikes Peak region. The collecting localities range from near Colorado Springs on the south to just west of Denver on the north. ==Geological history== The Pikes Peak granite formed in several stages. In the initial stage, about 1.02 billion years ago, a large mass of magma intruded into what is now the Front Range of Colorado. Although there may have been volcanoes overlying the intrusion, the majority of the magma never reached the surface, but formed and cooled at a depth of about 1 to 2 miles (1.6 to 3.3 km). Ultimately, the underground extent of the magma covered an area of . Later on, over the next 40-million years, newer pulses of magma pushed into the existing granite, re-melting and pushing it aside to form, what are now called "intrusive centers." As the magma flowed into these intrusive centers, it tended to form a circular masses of rock with concentric rings of varying composition that look something like a bulls eye. The granite in the intrusive centers is different from the original granite, with higher concentrations of rare elements and more exotic minerals. Some of the best mineral collecting areas in Colorado can be found around the edges of these intrusive centers. When a new pulse of magma forced its way upward, the edges in contact with the older magma cooled rapidly forming a kind of "skin" of fine-grained granite. Since fine-grained granite is less porous, this "outer skin" sealed the magma body and helped to trap rare elements and water inside. Over time, the water and rare elements were concentrated along the edges of the magma. While the magma cooled and crystallized, the water content of the remaining liquid magma increased rapidly. This is because molten magma can hold water, but the solid crystals cannot. As a result, as each crystal is formed, the water that was contained in the original molten magma is forced out of the crystal. As more crystals are formed, more water is concentrated in less and less magma. As the water and certain elements were concentrated along the edges of the magma, even larger crystals began to form. At the same time, the water helped produce open pockets called ''miarolitic cavities.'' The cavities are important because they allow the crystals to grow freely without interference from other crystal faces or the surrounding rock. Although good crystal occur within solid masses of rock, the most perfectly formed specimens usually come from miarolitic cavities. Because the intrusive centers were often circular and the cooling took place from the outside in, certain minerals were concentrated in ring-shaped zones around the edges of an intrusive center. For example, in the Tarryall region, most of the topaz is found near the edges of the intrusion. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Mineralogy of the Pikes Peak Region」の詳細全文を読む スポンサード リンク
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