What is the difference between light and dark silicate minerals

In olivine, the —4 charge of each silica tetrahedron is balanced by two divalent i. The divalent cations of magnesium and iron are quite close in radius 0. Cut around the outside of the shape solid lines and dotted lines , and then fold along the solid lines to form a tetrahedron.

If you have glue or tape, secure the tabs to the tetrahedron to hold it together. If you are doing this in a classroom, try joining your tetrahedron with others into pairs, rings, single and double chains, sheets, and even three-dimensional frameworks.

In olivine, unlike most other silicate minerals, the silica tetrahedra are not bonded to each other. This allows them to substitute for each other in some silicate minerals. In fact, the common ions in silicate minerals have a wide range of sizes, as shown in Figure 2.

All of the ions shown are cations, except for oxygen. The structure of the single-chain silicate pyroxene is shown on Figures 2. In pyroxene, silica tetrahedra are linked together in a single chain, where one oxygen ion from each tetrahedron is shared with the adjacent tetrahedron, hence there are fewer oxygens in the structure. The result is that the oxygen-to-silicon ratio is lower than in olivine instead of , and the net charge per silicon atom is less —2 instead of —4 , since fewer cations are necessary to balance that charge.

Pyroxene can also be written as Mg,Fe,Ca SiO 3 , where the elements in the brackets can be present in any proportion.

In other words, pyroxene has one cation for each silica tetrahedron e. These silicates can be generally divided into light and dark silicates. The dark silicates are also called ferromagnesian because of the presence of iron and magnesium in them.

Begin typing your search term above and press enter to search. Press ESC to cancel. Skip to content Home Physics What are the four basic types of plutons? Ben Davis January 14, New pulses of magma added more layers. Besides silicates, chromite and other oxide minerals accumulate in mafic complexes. Rarely, plagioclase forms a cumulate layer at the top of a magma chamber when it floats on denser magma. Because of their high metal content and natural separation into concentrated layers, mafic complexes often host rich ore deposits.

They are especially important for production of chrome and platinum group metals ruthenium, rhodium, palladium, osmium, iridium, platinum. The Bushveld complex is, perhaps, the most valuable ore deposit in the world, producing significant amounts of platinum group metals, as well as chromium, iron, tin, titanium, and vanadium. The photo seen in Figure 6. The metallic minerals are pyrrhotite and chalcopyrite that contain significant amounts of platinum, palladium, osmium, and other generally rare elements.

Besides separation of crystals and melt, disequilibrium occurs for other reasons. Sometimes large mineral grains do not remain in equilibrium with a surrounding magma. For example, because diffusion of elements through solid crystals is slow, different parts of large crystals may not have time to maintain equilibrium compositions. In principle, mineral crystals should be homogeneous, but in zoned crystals, such as the tourmaline crystals seen in this photo Figure 6.

Consequently, different zones have different compositions and sometimes different colors. Marked chemical zonation often occurs if a magma begins to cool at one depth and then rapidly moves upward to cooler temperatures. The result is often a porphyritic rock with large zoned phenocrysts. The zones may be visible with the naked eye, for example the tourmaline in Figure 6. Other examples of visibly zoned minerals include the tourmaline in Figure 4.

If the zoning is not visible to the naked eye, it may be visible when a crystal is viewed in thin section with a petrographic microscope. The thin section photo seen here Figure 6. The colors are artifacts and are not real mineral colors. Zoning can also be detected using a scanning electron microscope. See, for example, the electron microscope images of zoned plagioclase in Figure 4.

Silicate minerals dominate igneous rocks because silicon and oxygen are the most common elements in magmas. So, in the following discussion we systematically consider the important silicate minerals and groups.

Recall from Chapter 1 that the fundamental building block in silicate minerals is an SiO 4 4- tetrahedron with oxygen at the corners and silicon in the center. Individual tetrahedra bond to other tetrahedra or to cations to make a wide variety of atomic arrangements. The top drawing in this chart Figure 6. Sometimes aluminum substitutes for silicon, so silicate minerals may contain both SiO 4 and AlO 4 tetrahedra.

A key property of both silicon and aluminum tetrahedra is that they can share the oxygen anions O 2- at their corners.

When they do this, they form polymers that are various kinds of rings, chains, sheets, or three-dimensional arrangements. For a slightly different view, see Figure 1. In some minerals, SiO 4 4- tetrahedra are not polymerized. They do not share oxygen atoms, and instead are joined together by bonds to other cations.

These minerals, called isolated tetrahedra silicates , or island silicates , include most importantly minerals of the olivine group. In a few minerals, especially minerals of the epidote group, tetrahedra join to form pairs. These are the paired tetrahedral silicates , sometimes called sorosilicates or butterfly silicates.

In the pyroxenes and other single chain silicates , tetrahedra link to create zigzag chains. The amphibole minerals are double chain silicates. Micas and clays are sheet silicates. In sheet silicates, tetrahedra share three of their four oxygen with other tetrahedra, creating sheets and minerals with layered atomic arrangements. Finally, the feldspars and quartz are examples of network silicates , sometimes called framework silicates , in which every oxygen is shared between two tetrahedra, creating a three-dimensional network.

The drawing in Figure 6. The most important framework silicates are quartz and other SiO 2 minerals, and the feldspars. In the feldspars, and a related group of minerals called feldspathoids , alkali and alkaline earth elements — mostly Na, K, or Ca — occupy large sites between tetrahedra. We will start our review of silicate minerals by looking at the SiO 2 polymorphs, the feldspars, and the feldspathoids, and then work our way upwards in the figure above, towards minerals with less polymerization.

Silica Minerals SiO 2. Quartz, like many other minerals, is polymorphic. Mineralogists and chemists have identified more than 10 different silica SiO 2 polymorphs, but some do not occur as minerals. We briefly looked at the most common of these polymorphs in Chapter 4.

All the silica minerals except stishovite are framework silicates; the differences between the minerals are the angular relationships between the tetrahedra that comprise them. The blue spheres are oxygen atoms and silicon atoms are at the centers of every gray tetrahedron.

For some spectacular scanning electron microscope images of several silica polymorphs, see Figure Common quartz, more properly called low quartz because it has lower symmetry than high quartz , is the only polymorph stable under normal Earth surface conditions, but it has many different appearances.

The anhedral specimen seen in Figure 6. But euhedral crystals, such as those shown in Figure 6. Some of the different quartz varieties have specific names, such as milky quartz , rose quartz , Herkimer diamond , amethyst , and citrine. For several decades, petrologists have understood that different silica polymorphs occur in different geological settings because they are stable under different pressure-temperature conditions.

This phase diagram Figure 6. The horizontal axis is temperature. The vertical scale on the left gives pressures in gigapascals GPa , and the scale on the right shows the depths in Earth corresponding to those pressures. Pressure-temperature P-T phase diagrams such as the one seen here show which mineral is stable for any combination of P-T.

Low quartz is therefore the most common polymorph. If all rocks maintained and stayed at equilibrium, we would have no samples of any other silica polymorphs to study.

Although given enough time, they usually change into low quartz. They are usually associated with meteorite impact craters. Tridymite and cristobalite only exist in certain high temperature silicic volcanic rocks. Although not shown in this diagram, just like quartz, tridymite and cristobalite have both high- and low-symmetry polymorphs.

The red field in the phase diagram above Figure 6. Melting temperature is greatest at high pressure, and is different for the different polymorphs.

Consider what happens when a volcano erupts and silica-rich magma cools. This happens most of the time but occasionally metastable polymorphs can be found in volcanic rocks. Essential minerals are minerals that must be present for a rock to have the name that it does, and quartz is an essential mineral in silicic and intermediate igneous rocks, many sediments, and many metamorphic rocks. Quartz is not normally found in mafic igneous rocks because crystallization of mafic minerals such as olivine or pyroxene generally consumes all silica that is available, so there is none left over to form quartz.

Granites contain essential quartz. In silicic plutonic rocks such as granite, quartz is always associated with K-feldspar, commonly in a mosaic pattern similar to what is seen in this photograph Figure 6. The largest of the grains in this view are pinkish K-feldspar about 1 cm across. Quartz is glassy gray. White plagioclase and black biotite are also present. Quartz is also an essential mineral in sandstone and some other sedimentary rocks.

Quartz is the only mineral present in some sandstones or cherts. But, sandstone may also contain significant amounts of other minerals including feldspar or clay, and sometimes pebbles or rock fragments.

Quartz cannot exist in rocks containing corundum Al 2 O 3 , because the two minerals would react to form an aluminosilicate mineral of some sort. It cannot exist in rocks containing feldspathoids leucite, nepheline, or analcime because quartz and feldspathoids react to give feldspars. For similar reasons, quartz is absent or minor in many alkali-rich igneous rocks and in rocks containing the oxide mineral spinel MgAl 2 O 4.

The photograph on the left above Figure 6. The quartz formed when hot hydrothermal water infiltrated the rock along fractures before the rock was uplifted to the surface and eventually weathered. Temperature need not be high, however, for quartz to precipitate. For example, the amethyst purple quartz in the geode shown in Figure 6.

Figure 4. Most quartz crystals are twinned, but the twinning can be impossible to see or easily overlooked. The drawings seen here Figue 6. Brazil twins and Dauphine twins are penetration twins, and Japanese twins are contact twins. All three are generally growth twins but can also form in other ways. Some quartz crystals exhibit more than one kind of twinning. Brazil and Dauphine twins are distinguished by symmetry relationships between crystal faces of particular shapes, sometimes by identifying striations fine lines on crystal faces that developed when the crystal formed , but sometimes are difficult to tell apart.

Click on the crystal drawing right to see a 15 second video showing Dauphine twinning; the striations on the crystal faces are also apparent. The two large quartz crystals in Figure 6. They are widespread and are essential minerals in many igneous, metamorphic, and sedimentary rocks. Rarely, they contain significant amounts of other elements such as Ba, Sr, B, or Fe.

For most purposes, we consider them to be ternary solutions, which means we can describe their composition in terms of three end members and plot them on diagrams such as the one shown in Figure 6. In Figure 6. Natural feldspars form two distinct series, the alkali feldspar series and plagioclase , both labeled on this triangular diagram.

We call any feldspar with composition near NaAlSi 3 O 8 , albite , and one with composition near CaAl 2 Si 2 O 8 , anorthite , even if other components are present. Intermediate plagioclase compositions are commonly given specific names labeled in the figure : oligoclase , andesine , labradorite , and bytownite. Labradorite may also contain a small amount of orthoclase. Compositions between plagioclase and alkali feldspar that would plot in the white part of the triangle are rare or do not exist.

Confusion sometimes arises because the names of some composition ranges are the same as the names of feldspar end members albite, anorthite, orthoclase. Orthoclase, for example, is the name given to end member KAlSi 3 O 8. The triangular diagram in Figure 6. This feldspar is an example of anorthoclase. It has a composition of about Ca 0. Often, we describe the compositions of feldspars by using abbreviations with subscripts.

Thus, the Grorud feldspar has composition An 3 Ab 62 Or Like quartz, feldspars are framework silicates. Unlike quartz, feldspars contain both SiO 4 and AlO 4 tetrahedra. As an example, Figure 6. The purple atoms are sodium, and the yellow and green tetrahedra are SiO 4 and AlO 4 , respectively.

In all feldspars, Na, K, or Ca occupy spaces between tetrahedra. Most igneous rocks contain feldspar of some sort, but the kind of feldspar varies with rock composition. In silicic igneous rocks, such as granite, plagioclase is absent or subordinate to K-rich alkali feldspar. If plagioclase is present, it is always albite-rich. Similarly, in mafic rocks, alkali feldspar is not normally present but plagioclase is common. Because mafic rocks contain much more Ca than Na, the plagioclase in them is generally anorthite-rich.

Intermediate igneous rocks nearly always contain both feldspars. Vesuvius, Italy. The two photos above show light colored albite crystals from a classic locality in Austria and anorthite crystals from near Naples, Italy. These are both examples of plagioclase. But, K-feldspar has three polymorphs with slightly different atomic arrangements: orthoclase , microcline , and sanidine.

Photos of each are shown below in Figures 6. In these photos the polymorphs have different colors, but color is not a good diagnostic property because the different polymorphs all come in many colors. Note the well developed penetration twins in the sanidine specimen. Sanidine forms in high temperature rocks; orthoclase and microcline form in lower temperature rocks Figure 6.

Sanidine commonly crystallizes from silicic lava but may change into orthoclase and, perhaps, microcline if the lava cools relatively slowly. Just about all microcline forms by recrystallization of sanidine or orthoclase. Like K-feldspar, Na-feldspar forms different polymorphs, with different atomic arrangements, depending on temperature Figure 6. Three other polymorphs exist at lower temperatures, all generally called albite. Unlike the SiO 2 polymorphs, the differences in atomic structure between the feldspar polymorphs are not great and the boundaries on phase diagrams are poorly known.

Distinguishing between the different Na-feldspar polymorphs can be difficult or impossible without X-ray analysis. The striations derive from a type of polysynthetic twinning called albite twinning. The atomic arrangements in alternating domains are slightly different and are reflections of each other.

See the drawing of albite twinning in Figure 4. Twinning occurs in all kinds of feldspars but is generally not visible with the naked eye.

So, although a feldspar crystal appears homogeneous, it may be composed of two or more twin domains. We most easily see feldspar twinning when using a polarizing microscope to view a feldspar in a thin section. The different domains are visible because they have different optical properties. Feldspars twin in several different ways that have different names, for example Carlsbad, Baveno, or Manebach twinning, shown in the three photos above Figures 6. The crystals seen in these figures are about 4 cm, 4 cm, and 7 cm tall, respectively.

These three kinds of twins are simple twins involving only two twin domains. Carlsbad twins are penetration twins common in orthoclase, and sometimes in plagioclase. The photo above left shows Carlsbad twinning with two orthoclase crystals that appear to have grown through each other. Figures 4. Baveno twins are contact twins that occur most often in orthoclase and microcline.

The photo above center shows a feldspar with a Baveno twin. A vertical composition plane separates the left and right sides of the crystal. Manebach twins are most common in orthoclase. The photo on the right, above, is a crystal with a Manebach twin. The reentrant angle two small crystal faces forming a vee shape that points into the crystal in its top shows where the near-vertical composition plane passes through the crystal.

Both Baveno and Manebach twins are rarer than Carlsbad twins. When both are present, Carlsbad and Manebach twinning are diagnostic for orthoclase. Polysynthetic twins, such as the twins in Figure 6.

Albite twinning is one kind; pericline twinning is another. The difference between the two is that domains in albite twins are related by reflection, and domains in pericline twins are related by rotation.

Albite twinning is a diagnostic property for plagioclase. Sometimes albite twinning combines with Carlsbad twinning to produce complex crystals, but it is still generally recognizable. Microcline is the stable K-feldspar polymorph at normal Earth surface conditions. So, other K-feldspars may turn into microcline. Pericline twinning is produced during the polymorphic transformation of sanidine or orthoclase to microcline, and most microcline exhibits both pericline and albite twinning.

The combined twinning produces typical microcline twinning , sometimes called cross-hatched twinning or tartan twinning , but we can only see this twinning with a polarizing microscope. The colors are artifacts and are not the true colors of the domains. The field of view is 2 mm across, and the veins that cut across the field of view contain albite. At high temperature, as long as they are not so hot that they melt, these feldspars can have any composition between sanidine KAlSi 3 O 8 and albite NaAlSi 3 O 8 , and may be anorthoclase part way between the end members.

However, as shown in this phase diagram Figure 6. At lower temperatures, instead of having one intermediate feldspar, anorthoclase unmixes to produce two separate feldspars, one K-rich and the other Na-rich. This happens because a miscibility gap exists between albite and orthoclase. A miscibility gap is a composition range within which no single mineral is stable under a particular set of pressure-temperature conditions. It is the gray region shown in the phase diagram.

The curve above and around the miscibility gap is called the solvus. Miscibility gaps are common in mineral systems because some elements do not mix well under all conditions.

Many mineral series show complete miscibility at high temperatures, meaning that all compositions are stable. At low temperatures, partial or complete immiscibility restricts possible compositions.

We might make an analogy to a pot of homemade chicken soup that separates into two compositions fat and chicken broth with cooling Figure 6. We call the process of a single feldspar separating into two compositions exsolution , equivalent to unmixing.

If an intermediate-composition alkali feldspar cools rapidly, it may not have time to exsolve. Thus, we have examples of anorthoclase to study. On the other hand, if cooling is slow, the feldspar will unmix. This may result in separate grains of K-feldspar orthoclase or microcline and Na-feldspar albite developing in a rock. More often, it results in alternating layers or irregular zones of orthoclase and albite within individual crystals.

If the layers or zones are planar or nearly so appearing long and thin in thin section , we call them exsolution lamellae. The lamellae are sometimes visible with the naked eye, but frequently require a microscope to detect. The figure below Figure 6. If the original feldspar was K-rich, most of the layers will be K-rich feldspar. After it exsolves, it is technically called perthite.

If the original feldspar was Na-rich, most of the layers will be Na-rich, and after exsolution, it is technically called antiperthite. For practical purposes, most mineralogists call any exsolved alkali feldspar perthite , because they need compositional information to distinguish perthite from antiperthite. This is a polished slab of rock; the view is 4. Because they exsolve, alkali feldspars form what is called a limited solid solution at lower temperatures.

Intermediate compositions between orthoclase and albite do not exist. In contrast, intermediate plagioclase between albite and anorthite is common. At high temperatures all compositions are stable and, with cooling, there is generally no significant exsolution.

But, several small solvi exist at very low temperature which — if cooling is slow enough — can result in microscopic unmixing. The exsolution may give the feldspars an iridescence or a play of colors that helps to identify them. Labradorite, for example, is identified by its labradorescence , a bluish schiller, caused by exsolution.

Because exsolution, if present in plagioclase, is on a very fine scale, mineral properties are relatively homogeneous. So, for most purposes, we can ignore the presence or absence of exsolution in plagioclase. This figure 6. Consider a feldspar of composition Ab 60 Or 40 marked with an X on the diagram. At high temperature, it will exist as one alkali feldspar anorthoclase. As it cools to low temperature it separates into two feldspars beginning at about o C. At o C, for example, the compositions of the two feldspars are shown as red dots.

At o C, the two compositions are shown as orange dots. And at o C, the compositions are shown by light blue dots. The compositions start at Ab 60 Or 40 and then follow the solvus toward orthoclase and albite gray arrows as temperature decreases. The K-feldspar polymorph stable at low temperature is microcline.

So, if the feldspar cools completely, the final result will be a mix of microcline-rich feldspar and nearly pure albite, and because the overall composition is closer to albite than orthoclase, there will be more albite than microcline.

The likely result will be anitperthite — a single feldspar crystal that contains exsolution lamellae of different compositions Exsolution and Ternary Feldspars. Biology 10th class guide of chapter Life Processes? What is the difference between light and dark guppy fry? What is the difference between black coloured object and a dark shadow? What is the difference between dark-colored honey and lighter-colored honey? What are rocks called with alternating bands of light and dark silicate minerals?

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