Chapter 4 More Q & A about Quartz crystals 4 Q. I am very confused about something. For instance, can quartz form in all of the rock types: ie metamorphic, sedimentary and igneous, or are they specific to only forming in one type? A. Hi Cassandra, quartz can exist, form and reform in all of these environments because quartz is a stable form of SiO2 (silica) over a broad range of temperatures and pressures. Let's consider the mineral quartz and the rock cycle. If you need to, get an earth science text book and find the rock cycle diagram in it. Now, quartz forms as a mineral in certain igneous rocks, especially those high in silica, say greater than 55% as given in a chemical analysis. Granites are up in the 65 to 70 % silica range. Anyway, you have quartz in a granite. Let's follow it through the rock cycle. Weathering of granite changes the other dominant mineral of a granite, feldspar, into clay and frees the quartz grains to be transported some distance by wind, water, or ice. Once deposited as a sediment, the rock-forming process, termed lithification, compacts and cements the sediment together to become a sedimentary rock. Quartz is relatively hard and chemically resistant usually, so it becomes particles in a sedimentary rock. Let's consider a shale as our sedimentary rock. Shale is often composed of a lot of clay-sized material, usually clay minerals (derived from the weathering of igneous rocks) and some very fine-grained silica as silt-sized particles. Now the quartz that is present here did not form as a mineral in this situation. It was simply transported into this dilemma! Now we begin to bury the rock by additional material being deposited on top of it. Heat and pressure begin to work on the rock. The minerals composing shale will begin to change. First the clay particles will become interlocked as the shale alters to slate, then some will begin to be dissolved and reorganized into mica as more heat and pressure is applied. If the process stops here, the quartz has not changed, but the resultant rock in a phyllite. Continuing with greater heat and pressure, the minerals of phyllite change. The fine-grained mica becomes coarser grained and some of the quartz is dissolved and regrown on to quartz grains that are better oriented for growth to occur. The resultant rock is a schist. Then with increasing heat and pressure, the minerals reorganize into light and dark bands, dark containing the mica and light containing the quartz. This rock is a gneiss. Then finally as we apply heat and pressure, the minerals cannot rearrange anymore, so the rock melts and forms a granite again. We have completed the rock cycle. Now during this entire metamorphic process I have described, some water that was trapped with the sediment and in some of the minerals is given off. This water with dissolved chemicals is transported away from the location where the heat and pressure is being applied. Then it encounters different physical and chemical conditions and becomes chemically unstable depositing minerals in cracks and fractures that it passes through. This is how the quartz veins from Arkansas formed. But that's not the end of the story for quartz. When a granite composition magma is intruded into a region, silica-rich fluids often go off of the magma as it crystallizes in the ground. These fluids care their dissolved minerals out into nearby cracks and fractures and form quartz veins, just like the metamorphic fluids. Usually the igneous origin fluids from granites may carry metal values, such as gold or silver or both. What I have described for quartz crystals and quartz veins is but two processes by which quartz may form -- from molten rocks and from hot fluids. But quartz in the form of chert, flint, agate, etc may also form from cold water. Notice that I said above that quartz is normally stable in the weathering environment. Quartz is soluble in alkaline environments, like are commonly encountered in limestone and dolostone terranes. In this instance, fine-grained quartz may form as nodules in pockets or voids by the deposition of colloidal silica, which slightly recrystallizes over time. Also, in some instances like around Herkimer County, New York, nice crystals of quartz may form in these pockets from cool waters. So here is another way quartz may form! Notice that I keep talking about quartz only. That's because books have been written about all the different ways minerals can form. Minerals exist under certain conditions. These conditions are defined as their stability fields, i.e. the range of temperatures and pressures that the minerals remain stable and do not break down. Most mineralogy books give these parameters. Is it the same for most other crystals as well? I can't seem to find the answer to this. I am assuming that all crystals can form in any of the rock stages - provided the right elements are present. Is that accurate? No, not so accurate. I think now you understand that not only do you have to have the right chemicals present, but also the right set of conditions of temperature and pressure. Oh yes, I forgot to mention about time. Time plays as important a role as temperature and pressure, because without significant time, the reactions don't happen and the minerals do not form. These are not instantaneous changes in minerals I have been talking about, but changes that take time to occur. Migrating elements through solid rock or even with some fluids takes a while geologically. I would like to give you another example of how a single element exists as two minerals. Carbon. Carbon held at a temperature of 1700 degrees F and under a pressure of several 1000 lbs./square inch becomes the mineral diamond. The elemental carbon is packed as atoms as tightly together as is physically possible. How can a mineral formed under such conditions exist at room temperature and pressure? Well, if it was transported rapidly to the present conditions, then it has the inclination to alter to graphite, but did not have the time! Mineralogists call diamond a meta-stable mineral. Contrary to what DeBeers advertizes, diamonds are not forever! Now diamonds crystals that are transported slowly to the earth's surface are dissolved by the transporting material and disappear if the material they are traveling in reaches the surface too slowly. Graphite is the stable form of carbon under the conditions that exist on the earth's surface and in the earth's crust. Only in the mantle do the necessary conditions exist for diamond to have formed. Now, let's take organic carbon, trapped in a sedimentary rock and plunge that rock into the upper mantle. That carbon will first be converted to graphite on the trip down to the mantle and then to diamond as it encounters those conditions of heat and pressure necessary for diamond to form. If we then slowly bring that mass of ultrametamorphosed rock back to the earth's surface, then the diamond present in that rock will alter back to graphite, but perhaps retain the external crystal form of diamond. A piece of such a rock has been discovered and documented occuring in China. It contains graphite pseudomorphs after diamond in it!   Well I know this is a lot more than you asked for, probably you are now thinking. But you ask some humdinger questions that I can't help but try and answer!   Ch 4, page 4 mailto:jmichaelhoward@sbcglobal.net revised July 1999 © Rockhounding Arkansas 1998 http://rockhoundingAR.com