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Minerals of Igneous Rocks 1 – The Ferromagnesian Minerals (Contain iron (Fe) and magnesium (Mg) [May or may not contain aluminum (Al)] Olivine (Fe,Mg)2SiO4 Olivine is always some shade of green. Lighter green indicates greater Mg content (and lower density), darker color more Fe (and higher density). Large crystals uncommon, almost always occurring as dominant or sole mineral in a rock, as small granular crystals. No cleavage – conchoidal fracture (like glass). Coin is ~2.5cm (1”) in diameter. Included for scale Pyroxene (Augite, most commonly) (Ca,Na)(Fe,Mg,Al)(Si,Al)2O6 Pyroxene can be green (usually dark green) or black depending on the exact composition. There are two directions of cleavage that meet at 90°, though one or both may be difficult to see. The luster is usually less bright and the surface texture less splintery than amphibiole, with which pyroxene is easily confused. Pyroxene generally occurs in igneous rocks with another mineral – a feldspar. Amphibole (Hornblende, most commonly) Ca2Na(Fe,Mg)4(Al,Fe,Ti)3Si8O22(O,OH)2 Amphibole may be dark green but is more typically black. There are two directions of cleavage that do not meet at 90°, The cleavage tends to step rather than continuing across a smooth face. This gives the mineral a splintery appearance. The luster is usually quite bright and reflective – sparkly in fact. Amphibole generally occurs in igneous rocks with another mineral – a feldspar. Biotite (black mica) K(Fe,Mg)3(AlSi3O10)(OH)2 Biotite is a “black” mica, but usually appears dark brown if the “book” is thin enough to see through. There is one perfect direction of cleavage that allowt thin sheets to be peeled off of a crystal. The luster is quite bright and reflective. Biotite is not common in igneous rocks and when it occurs feldspar and quartz are usually the dominant minerals. 2 – The Non-Ferromagnesian Minerals (Contain little or no iron (Fe) or magnesium (Mg) [Usually contain aluminum (Al)] Muscovite (white mica) KAl2(AlSi3O10)(OH)2 Muscovite is a “white” mica, but may appears brown if the “book” is thick. There may also be some Fe substituted for Al, giving it some compositional similarity to biotite and making it darker. As with all micas thin sheets to can be peeled off. This character has allowed it to be substituted for glass, particularly in high-temperature oven doors, for example. The luster is quite bright and reflective. Muscovite on mountain stream beds has fooled many children (including me) into mistaking it for gold. Muscovite occurs uncommonly in the same sorts of igneous rocks as biotite, with the same other minerals. Which (if either) occurs depends on the availability of Fe. Plagioclase Feldspars (Ca Plagioclase) (Ca,Na)Al2(Al2Si2O8) (Ca and Na freely substitute in plagioclase. If Ca predominates we call it a Ca plag or Ca feldspar) The feldspars are a very important group of minerals, occurring in almost every igneous rock many metamorphic rocks, and some sedimentary rocks. Feldspars are the most common mineral in the Earth’s crust. Because the “backbone” of every feldspar (and all other common igneous minerals) is a silicon oxygen molecule with the formula SiO4, that makes oxygen the most common element in the crust. (In the mantle too). Silicon is second most abundant, followed by Al, Fe, Mg, and then the other cations you’ve seen in the mineral formulas. The feldspars have two directions of cleavage at about 90°, one of which is usually easier to spot than the other. Plagioclase often has obvious parallel lines on the obvious cleavage face called “striations”. The most common color for all feldspars is white. Often, however, Ca-rich plag is dark gray to black, like the one shown. For our purposes we will consider this a diagnostic color, though it really isn’t. Plagioclase Feldspars (Na Plagioclase) (Na,Ca)(AlSi3O8) (Ca and Na freely substitute in plagioclase. If Na predominates we call it a Na plag or Na feldspar) Sodium plag is typically white. Because K and Na are both +1 cations there can be some substitution of K for Na, giving the mineral a flesh-pink tinge. Before we leave the plagioclase feldspars we need to make sure to understand the composition. This mineral group is called a “solid solution” – there can be any proportion of Ca or Na in the crystal structure – 100%Ca/0%Na, 99%Ca/1%Na … 0%Ca/100Na. The series is arbitrarily broken into segments and names are applied to various ranges of composition, but these are not truly distinct minerals, only convenient names to pin onto a thing without borders. Potassium Feldspars (Kspar) K(AlSi3O8) Potassium (K) feldspar comes in several forms and can be several colors, most commonly white. We are going to pretend in this class that pink is the characteristic color because it is fairly common. Remember that it isn’t really the diagnostic color, we are just pretending. Notice that the specimen shown has what appear to be striations across it. These are not actually striations as you can tell from a couple of things. First, they are not all continuous across the specimen as striations are, and second, the edges (and the “zones” themselves) are not parallel to each other. The lighter bands are actually K-rich (Na) plagioclase that has intergrown with the Kspar. Quartz (SiO2) Quartz can have many colors depending on impurities, but in igneous rocks it is usually perfectly clear. Of course if the crystal is embedded inside a rock and you are looking at it from the surface you won’t be able to see through it, so it will look clear only if it is a very thin piece. The thicker it is the darker gray it will look. Still, it is never as dark as a ferromagnesian mineral and so you shouldn’t confuse it with those. The give-away for quartz on the broken face of a rock is that it doesn’t cleave. It’s crystal faces are never evident (for reasons we will get into) and it breaks with a conchoidal fracture (like glass). So in a rock you will see lots of feldspars and other minerals with flat cleavage faces sparkling at you. The quartz will be the stuff with the non-flat broken edges. The picture shows the crystal faces of a well-grown crystal (right) and the fracture of a broken piece (left)