| GEMS & MINERALS FROM PEGMATITES II. Miarolitic Pegmatites. David London School of Geology & Geophysics University of Oklahoma 100 East Boyd Street, Rm 810 SEC Norman, OK 73019 Email: dlondon@ou.edu and Managing Editor for the Pegmatite Interest Group at: http://www.minsocam.org/msa/special/Pig/ The term "miarolitic" originates from an Italian word, miarole, which originally referred to small crystal-lined cavities in granite from the famous mineral-collecting regions of Baveno and Cuasso al Monte in northern Italy. Miarolitic pegmatites are those that possess a proportionately large number of crystal-lined cavities, and from which most of the world's great pegmatitic gem & mineral specimens are mined. To start with, a miarolitic pegmatite begins as a silicate melt (or magma, molten rock material with included crystals) generally of granitic composition. The magma also contains gases, primarily H2O but also CO2 and other volatile components, that are dissolved into the melt at high pressure (about 200-300 MPa, corresponding to ~ 30,000 - 45,000 psi at depths of ~ 6-8 km). From this starting point, miarolitic cavities may originate from some combination of the following processes: > As the pegmatite-forming melt rises toward the earth's surface, the pressure imposed on the magma by its surrounding rocks decreases. As the pressure drops, the amount of H2O and CO2 that can dissolve in the melt decreases until the point where the melt can no longer contain all the dissolved gases, and bubbles exsolve from the melt. This process, which geologists term "first boiling", is analogous to the formation of bubbles in carbonated beverages when the top is popped, and pressure is released. > As the pegmatite forming melt crystallizes, the minerals that form - mostly quartz and feldspars - do not contain H2O or CO2. As their mineral-forming constituents are removed from the melt, the concentration of excluded components, e.g., gases, increases again to a point where the melt can no longer hold all of its dissolved gas, and bubbles form. Geologists refer to this as "second boiling". Both of these processes occur during the stages when the pegmatite-forming melt is crystallizing. A third process may operate after the melt is (mostly) exhausted. > The gases liberated at the end of pegmatite crystallization are corrosive to some early-formed minerals (particularly potassium feldspars). As those gases escape from the crystallized pegmatite, they react with minerals along the way, dissolving some and precipitating others in the space created by dissolution. Such miaroles are termed "solution" cavities when their origin can be ascertained. Other types of magma (those that form basalt, gabbro, andesite, diorite, etc.) also contain dissolved gases. It is the release of gas bubble pressures trapped in magma that powers volcanic eruptions. So why aren't most igneous rocks miarolitic and full of crystal-lined cavities? To form crystal-lined miaroles, the gases must be trapped inside the crystallizing magma. The gases should not escape before the magmatic stage of crystallization is complete. That may happen if the gas pressures are so high as to blow the magma to smithereens (e.g., a volcanic eruption). It may also happen if the magma crystallizes slowly, allowing gases to escape by the migration of bubbles through the melt, or simply by the diffusion of the dissolved gases through the melt and out of the magma. Two attributes of pegmatites foster the formation of abundance miarolitic cavities in these rocks. > The pegmatite-forming magmas become highly viscous once they are injected into cooler host rocks. High viscosity impedes the migration of bubbles through the melt and helps to trap them. This explains why miaroles are more common in granites (and their pegmatites) than in other types of igneous rocks, whose magmas are generally less viscous. > When pegmatites solidify, they tend to do so as solid fronts of crystals that advance into the center of a pegmatite from both sides. Hence, the gas bubbles may become trapped between the solid "walls" of advancing crystals and cannot escape. This is why most miarolitic cavities are located near the center of pegmatite dikes; this "pocket line" represents where the advance of crystal fronts from the sides finally met in the interior of the pegmatite. This is also the primary reason why miarolitic cavities are found in pegmatites of all types (not just granitic). The material that forms the crystal-lined cavities is not easy to characterize or explain. It is not water or steam in any conventional sense, as these fluids cannot hold enough dissolved material to form the large amount of silicate crystals (quartz, feldspar, tourmaline, beryl, etc). Nor is it a silicate melt in a traditional sense, as it probably contains much more H2O and other solvents (including fluorine, boron, and phosphorus) than any typical magma. In the 1960s, Soviet scientists found evidence for these fluids as trapped inclusions within crystals from pegmatites, and they termed them "solution-melts", something transitional between the two typical fluids mentioned above. I was the first Western scientist to document them from pegmatites (London, 1986a,b), and I have reproduced fluids of the appropriate composition by laboratory experiments (London, 1999). The crystal-forming fluids may possess properties akin to gels, such as silica gel and gelatin - rigid, viscous, but very rich in water. References Cited London, D. (1986a) The magmatic-hydrothermal transition in the Tanco rare-element pegmatite: evidence from fluid inclusions and phase equilibrium experiments. Invited, American Mineralogist, Jahns Memorial Issue, 71, 376-395. London, D. (1986b) Formation of tourmaline-rich gem pockets in miarolitic pegmatites. Invited, American Mineralogist, Jahns Memorial Issue, 71, 396-405. London, D. (1999) Melt boundary layers and the growth of pegmatitic textures. (abstr.) Canadian Mineralogist, 37, 826-827. |