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Lab 2. Igneous Rocks and the Gems Produced from Them

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Lab 2. Igneous Rocks and the Gems Produced from Them Lab 2. Igneous Rocks and the Gems Produced from Them A) Introduction to Igneous Rocks Igneous: Gems can occur in any type of rock. In the next few exercises we will look at all three types of rock, igneous, sedimentary, and metamorphic. Each forms in a different way. Rocks formed from cooling melted (molten) earth materials are called igneous rocks (rocks full of fire). Gems formed in these rocks crystallize as the molten rock cools (in most cases). The molten material is called magma below the surface and lava when it flows on the surface. The igneous rocks are primary and formed the crust as the earth cooled. They still are being formed today both above and below ground. The melts are mainly composed of silicon and oxygen. These two elements bond together making the back bone of a group of minerals called the silicates. Along with silicon and oxygen (these two element make up 76% of the crust), aluminum, iron, calcium, sodium, magnesium, and calcium make up about 99% of the earth’s crust by weight. So in a melt, these elements usually predominate and make up most of the rock forming minerals (dozen or so common minerals). Though they are abundant and important not all that many gems are silicates, most common of these are emerald (beryl group), quartz, garnets, and tourmaline. A lot of gems, even in of silicate family, have exotic elements. These exotic elements, such as beryllium, boron, lithium, chromium, vanadium, and zirconium need to be concentrated. So gem formation often requires unusual geologic circumstances. Sometimes water-rich hot fluids generated from the cooling rock separate from the main magma and bring with them the exotic elements that do not easily fit into the common silicate mineral structures (that is ferromagnesian silicates, quartz, feldspar, mica, etc.). With all the water and exotic elements excluded from the main mass of igneous magma, this very-fluid, exotic-element rich “gem soup” forms gem pockets on the outer margin of the larger igneous rock bodies. These very fluid magmas create pegmatites. Figure 1 Igneous rock formation by intrusion into layered sedimentary rocks. Note pegmatite is to the outside of the intrusion. B. 1 Classification of Igneous rocks Some examples: Igneous rocks are of two main types volcanic (extrusive) rocks formed on the surface and plutonic (intrusive) formed below the surface (see Figure 1 above). Volcanic rocks can form from flowing lava or may be fragmental meaning that they are caused by explosive volcanism that can have horrific consequence. The city of Pompeii was destroyed in AD 79 by a volcanic eruption of Mt. Vesuvius. Some gems such as diamonds and pyrope garnet are carried to the surface by volcanic eruptions referred to as diatremes (Literally, a hole filled with breccia [broken pieces of rock], formed by a subterranean gaseous explosion). The rock carrying diamonds to the surface is kimberlite. It is fragmental in nature (see pyroclastic below), but no one has directly witnessed a kimberlite eruption and the fragmental texture may in some cases even be intrusive having never reached the surface. Other gems that are carried up with lava outpouring include peridot (olivine) which may be carried upward from where it crystallized at a much slower rate than diamonds in kimberlite. Diamonds and peridot carried up as solids from a deeper rock are called, Xenoliths (which means foreign rocks). The peridot crystals have to be broken out of the lava or glassy obsidian that oozed out with them in tow. Obsidian itself, a volcanic glass caused by rapid cooling, is occasionally used as an ornamental gem. Snow flake obsidian is very attractive when tumbled or polished. The snowflakes in the obsidian are crystals of feldspar that grow in the magma or lava as it rises and are not xenoliths. They grow in the same melt and are larger than the rest of the surrounding rock. Sometimes these are called phenocrysts. Phenocrysts differ from xenoliths in that phenocrysts form from the same magma as the surrounding rock creating a texture called porphyritic texture, while xenoliths did not crystalize in the magma they are now found in. Some gems form after beds of ash and lava settle because the ash is soluble in the hot fluids that may rise from hydrothermal (hot water) vents emanating from below (see geyser in Figure 1). Hot gases also rise and can carry mineral forming elements. Because the hot water is filled with dissolved minerals, on cooling a precipitation of crystals into spaces in the rock may occur as the hot gases and water rise and cool. Some opals and nodules such as thunder eggs form in this way. Though often called geodes, many quartz and other mineral lined pockets are really formed is cavities between pillow lavas. Much of the Brazilian amethyst and citrine “geodes” may have this origin. Pillow lavas form when lava flows into water (see Figure 2, 3 & 8) below. Gold and other minerals also may precipitate from hydrothermal solutions that emanate from igneous activity and much prospecting for metals takes place in igneous regions as well. However, some people consider these to be metamorphic rather than igneous processes. For geologist to have conversations about rocks special vocabulary and descriptive terminology has been developed. 2 Figure 3. The pillows have hollows between them that can be mineralized. Similar to Figure 2. Pillows formed as lava erupts into geodes. water. Igneous rocks are classified and named by their texture and color. These properties help us to interpret their cooling history and the source of the magma. Knowing the history of the rock allows us to consider its economic potential as well. Volcanic rock cool rapidly The terminology can be complex. We will say that fine-grained rocks are made of microscopic crystals. Geologist call this texture aphanitic. Aphanitic rocks and glass (no crystals/minerals) are mostly of volcanic origin as are fragmental rocks (sometimes called pyroclastics). Pyroclasts are literally “fire fragments. ” Pyroclastics rocks are explosively emplaced. Usually this means a volcanic eruption. Sometimes the rock material comes out of fissure (crack) or just explodes without a prior volcano. Fine-grained rocks may not have formed on the surface, but slightly below it (close to the surface), they are considered volcanic because of their microscopic interlocking crystal texture. The reason why fine-grained rocks (and volcanic glass) are considered volcanic is that they must have cooled relatively quickly, not giving enough time for larger crystals to form. Sometimes however, a finer-grained rock will include coarser crystals that rose with the lava or glass. The larger crystals called phenocrysts encased in the finer groundmass, give the rock a special composite texture called porphyritic. The constituents of porphyritic rocks indicate two separat e rates of cooling in differe nt environments (below the surface for the Figure 4. Porphyritic texture pheno crysts, and nearer the is due to two rates of cooling. surfac e for the groundmass). Larger crystals are called phenocrysts that are surrounded by ground mass. 3 Phenocrysts and xenoliths (foreign rocks) carried up can be of value, consider peridot and diamonds. Both may exist in large quantities in the mantle but are rare on the surface and in the earth’s crust. As well, corundum, ruby and sapphire and moonstones can be carried up with volcanic eruptions (such as alkali basalt) that flood the earth’s surface with large volumes of volcanic rock making gem-rich regions as in Thailand’s Chantbaburi-Trat area that includes the Hill of Gems (rubies) area, the Pailin ruby and sapphire gem field in Cambodia. Peridot Mesa near San Carlos, Arizona, and Australia’s sapphire and zircon-rich Ankie and New England districts (Yellow and blue sapphires) have similar origins. The liquid magma acts like an elevator bringing these precious crystals to the surface. Plutonic Rocks Cool Slowly Plutonic rocks on the other hand must have cooled relatively slowly because they have large, visible crystals (This texture is called phaneritic texture = coarse-grained crystals). The growth of larger crystals takes time and indicates slower cooling below the surface. The surrounding rocks act as a sort of thermos only allowing the heat to escape slowly. Since a hot fluid allows for more fluidity, the elements that make up crystals can travel more freely to hook up with each other and the crystals have more time to grow large. For this reason course-grained rocks, such a pegmatite (which is a textural term meaning having crystals larger that 1 inch [2.5 cm]) are more likely to have large mineral grains/crystals that potentially could yield gems of sufficient size for cutting. Some pegmatites apparently form from very fluid remnants of cooling magma with incompatible elements that are driven off of the main cooling plutonic rock body. These mineralogically complex pegmatites are especially sought after targets for gem hunters and may contain large crystals of quartz, tourmaline, beryl, and spodumeme, and many other rare minerals that fascinate the mineral collector. They are also economically important for certain chemicals, such as lithium and beryllium that are mined from them. Exercise Begins Here After a discussion of the rocks, you will be given a box of rocks and asked to identify their origin and name them based on a brief description. You should learn to recognize these rocks for an exam. Your instructor will emphasize certain rocks and will help you to correctly identify the 4 samples. List of rock numbers for you to learn. Rock numbers Igneous Look at the texture and Composition (color). Texture types of igneous rocks You should locate, 1) glassy texture, 2) fine-grained (aphanitic) texture, and 3) coarse texture (phaneritic), 4) pegmatitic texture which has crystals larger than about an inch in diameter, 5) the bubbly or vesicular texture, and 6) lastly pyroclastic texture made of broken pieces of rock, such as pumice, and ash mixed together.
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