Orthoclase K-feldspar : Porcelain luster; commonly colored pink, white, or gray. Cleavage in 2 directions at right angles may be detected by a reflection of light when specimen is rotated. Plagioclase: Usually gray or white in granite, dark-bluish color in gabbro. Striations common. Biotite: Small black flakes with perfect cleavage in 1 direction layers , reflects light. Amphibole Hornblende : Long, black crystals in a light-colored matrix. Cleavage at 60 and degrees. Pyroxene Augite : Short, dull, greenish-black minerals in darker rocks.
Cleavage in two directions at 90 degrees. Privacy Policy. Skip to main content. Module 5 — Igneous Rocks. Search for:. There are three basic rock types: Igneous, sedimentary, and metamorphic. Igneous Rocks Igneous rocks fiery rocks are made when molten material inside or outside the earth cools and becomes solid. They cool gradually and slowly.
Individual crystals have time and space to grow large. Feldspar pink to white blocky mineral crystallizes at high temperatures early in the cooling process and, thus, has straight crystal sides. Quartz clear, glassy mineral crystallizes later and fills spaces. Characteristics: Coarse-grained — The grains, or crystals, are big enough to tell what mineral each one is.
The crystals are usually larger than 1 millimeter larger than the point of a pen or pencil. The edges of the crystals interlock. The crystals are shiny, flat surfaces which fit together like jigsaw puzzle pieces, with straight edges on some crystals. The minerals are very hard and will scratch glass. If the rock has been weathered by the elements, it may be crumbly.
The texture is generally uniform the same in all directions. They cool quickly with no time for large crystals to grow. They are, thus, very fine grained. Mineral composition can only be identified with a microscope unless phenocrysts are present see below.
Some lava flows, however, are not purely fine-grained. If some mineral crystals start growing while the magma is still underground and cooling slowly, those crystals grow to a large enough size to be easily seen, and the magma then erupts as a lava flow, the resulting texture will consist of coarse-grained crystals embedded in a fine-grained matrix.
This texture is called porphyritic. If so many bubbles are escaping from lava that it ends up containing more bubble holes than solid rock, the resulting texture is said to be frothy.
Pumice is the name of a type of volcanic rock with a frothy texture. If lava cools extremely quickly, and has very little water dissolved in it, it may freeze into glass, with no minerals glass by definition is not a mineral, because it does not have a crystal lattice.
Such a rock is said to have a glassy texture. Obsidian is the common rock that has a glassy texture, and is essentially volcanic glass. Obsidian is usually black.
Now let us briefly consider textures of tephra or pyroclastic rocks. Like lava flow rocks, these are also extrusive igneous rocks. A pyroclastic rock made of fine-grained volcanic ash may be said to have a fine-grained, fragmental texture. Volcanic ash consists mainly of fine shards of volcanic glass. It may be white, gray, pink, brown, beige, or black in color, and it may have some other fine crystals and rock debris mixed in.
An equivalent term that is less ambiguous is tuffaceous. Rocks made of volcanic ash are called tuff. A pyroclastic rock with many big chunks of material in it that were caught up in the explosive eruption is said to have a coarse-grained, fragmental texture.
However, a better word that will avoid confusion is to say it has a brecciated texture, and the rock is usually called a volcanic breccia. When magma cools slowly underground and solidifies there, it usually grows crystals big enough to be seen easily with the naked eye.
These visible crystals comprise the whole rock, not just part of it as in a porphyritic, fine-grained igneous rock. The texture of an igneous rock made up entirely of crystals big enough to be easily seen with the naked eye is phaneritic.
Phaneritic texture is sometimes referred to as coarse-grained igneous texture. Granite, the most well known example of an intrusive igneous rock, has a phaneritic texture. Sometimes an intrusion of magma that is crystallizing slowly underground releases large amounts of hot water. The water is released from the magma as extremely hot fluid with lots of chemical elements dissolved in it. A rock consisting of such large minerals is said to have a pegmatitic texture, which means the average mineral size is greater than 1 cm in diameter and sometimes is much larger.
The name of an igneous rock with a pegmatitic texture is pegmatite. Pegmatites are commonly found in or near the margins of bodies of granite. The most common igneous compositions can be summarized in three words: mafic basaltic , intermediate andesitic , and felsic granitic. Felsic composition is higher in silica SiO 2 and low in iron Fe and magnesium Mg.
Mafic composition is higher in iron and magnesium and lower in silica. Intermediate compositions contain silica, iron, and magnesium in amounts that are intermediate to felsic and mafic compositions. Composition influences the color of igneous rocks. Felsic rocks tend to be light in color white, pink, tan, light brown, light gray. Mafic rocks tend to be dark in color black, very dark brown, very dark gray, dark green mixed with black. The color distinction comes from the differences in iron and magnesium content.
Iron and, to a lessor extent, magnesium give minerals a darker color. Intermediate igneous rocks tend to have intermediate shades or colors green, gray, brown. 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. Above, we considered exsolution in alkali feldspars, K-Na feldspars.
But, what about ternary feldspars, K-Na-Ca feldspars? As noted, both alkali feldspar and plagioclase exist in a wide range of igneous rocks. But, feldspars with compositions in the middle of the feldspar ternary shown in white in Figure 6. There is a large miscibility gap in the ternary system. At o C all feldspar compositions, except those that plot in the white region, are stable.
But, as temperature decreases the miscibility gap grows wider show by the temperature contour lines , and if temperature gets as low as o C, feldspar is limited to compositions shown in the lightest pink in this figure.
The crystallization temperatures of the two plagioclase end members are different. Albite crystallizes at o C and anorthite crystallizes at o C Figure 6. For compositions between albite and anorthite, crystallization is complicated see Box because when an intermediate feldspar begins to crystallize, the first crystal produced is more calcium-rich than the original melt itself. In effect, the anorthite part of the feldspar crystallizes faster than the albite part. If an intermediate plagioclase is partially crystallized, the melt will become more albite-rich.
The feldspar that crystallized will therefore have to be more anorthite-rich than the original melt. So, end-member albite and anorthite crystallize congruently. This means they crystallize at specific temperatures and the crystals are the same composition as the melt. But, intermediate composition feldspars crystallize incongruently.
They crystallize over a range of temperature and the crystal produced is not the same composition as the melt until crystallization is complete. The temperature at which crystallization begins for a specific composition feldspar is shown by the liquidus curve in the diagram above.
Crystallization starts and then continues until temperature reaches the solidus. At temperatures between the liquidus and solidus, in the two-phase field , crystals and melt exist together and are of different compositions.
At the highest temperatures, above the liquidus, plagioclase of any composition will be melted. At the lowest temperatures, below the solidus, any plagioclase composition will be entirely crystalline.
In between, in the two-phase field, solid crystals and melt will coexist but be of different compositions. This is best described by example. Thus, when crystallization begins, the liquid still has its original composition, but the crystals that start to form are considerably more anorthite-rich than the melt. They have composition Ab 41 An As the melt continues cooling, crystals continue to form, but they change composition from D to C, following the solidus as temperature decreases.
Previously crystallized plagioclase reacts with the melt, so crystal composition changes with cooling and all crystals are homogeneous and have the same composition. A horizontal line can be drawn at any temperature. The intersection of the line with the solidus indicates plagioclase composition and the intersection of the line with the liquidus indicates the composition of the remaining melt.
Because the crystals are more anorthite-rich than the melt, the melt becomes more albite-rich as crystallization continues. As crystallization goes to completion, plagioclase crystals change in composition from Ab 41 An 59 Point D to Ab 80 An 20 the composition of the original melt.
Simultaneously, melt composition changes from Ab 80 An 20 to an extremely albite-rich composition Ab 96 An 04 , indicated by Point E on the diagram. And, if everything stayed in equilibrium, after complete crystallization, all plagioclase crystals would have the same composition, which must equal that of the original melt.
So, if the crystallization process always went to completion, plagioclase crystals would always end up the same composition as the original melt. However, several things can disrupt this equilibrium. Often crystallization occurs so fast that crystals do not have time to react with the melt as they form. In such cases, the first plagioclase to crystallize is preserved in the centers of large crystals. The crystals are compositionally zoned, the outer zones being more albite-rich.
Another complication arises if crystals and melt get separated during the crystallization process. In such cases, crystal-melt equilibrium is not maintained. Both processes are kinds of fractional crystallization that can create melts of different compositions from their parent melts. Feldspar Crystallization — K-feldspar. K-rich feldspars can crystallize from a magma, but they exhibit an interesting kind of incongruent crystallization.
Consider a melt with composition KAlSi 3 O 8. If it cools, it will eventually reach the liquidus temperature and crystallization will begin. But, as shown here Figure 6. So, as crystallization proceeds, the remaining melt will become more SiO 2 -rich. Finally, when the temperature reaches what is called the peritectic temperature , the leucite crystals will react with the remaining melt to produce K-feldspar and all will be solid.
If everything stays in equilibrium, we can ignore the temporary presence of leucite crystals. But, sometimes liquid and crystal become separated and leucite may be left behind. Feldspar Crystallization — Alkali Feldspars. The discussion above is directed at crystallization of K-feldspar, but most alkali feldspars are solid solutions between orthoclase and albite. This complicates crystallization relationships considerably. The phase diagram in Figure 6.
Assuming that leucite does not crystallize, or that we can ignore it, the alkali feldspar series involves a pair of two-phase fields, similar to the two-phase field on the plagioclase phase diagram Figure 6.
All other compositions are completely crystallized at this temperature. However, the solvus and miscibility gap at lower temperature mean that with more cooling, intermediate composition feldspars will exsolve. They will separate into an albite-rich feldspar and an orthoclase-rich feldspar.
The likely result is that perthite will form. See Box for further discussion. The feldspathoid minerals are framework silicates similar to feldspars in many of their properties. They are, however, much less common. Feldspathoids are a family of about a dozen minerals; they are all Na-, K-, and Ca-aluminosilicates. The family is not like other mineral groups, whose members are related by similar atomic arrangements and chemistries.
Instead these minerals are grouped together because of their relationship to feldspars. The blue box lists some examples of feldspathoids and their formulas. They have compositions that are, for the most part, equivalent to silica-deficient feldspars. These minerals commonly crystallize from magmas that are relatively low in SiO 2 or that contain more Na, K, and Al than can fit into feldspars. These minerals have large openings in their atomic arrangements that allow the minerals to contain significant amounts of large anions and molecular anions, including chlorine, carbonate, and sulfate.
They often occur with feldspars, and also with mafic minerals including amphiboles, olivine, and pyroxenes, but never with quartz. They are restricted to quartz-free rocks because they will react with quartz to produce feldspars by reactions such as:.
Analcime, a zeolite, is often considered to belong to the feldspathoid family. Other important feldspathoids include leucite, nepheline, sodalite, and lazurite. Leucite is a rare mineral found in K-rich volcanic rocks. It is unknown in plutonic, metamorphic, or sedimentary rocks. Nepheline is a common mineral in syenite and other silica-poor volcanic or plutonic igneous rocks.
Leucite and nepheline are usually associated with K-feldspar. Sodalite crystallizes from alkali-rich magmas and is also found in some metamorphosed carbonate rocks. Analcime also can crystallize from a magma, but it more commonly forms as a secondary mineral in vugs, cracks, or veins.
Occasionally it is found as a secondary mineral in sandstones or tuffs. When it crystallizes from a magma, it is typically associated with olivine, leucite, and perhaps sodalite. When it is secondary, other low-temperature minerals, including zeolites or prehnite, often accompany it. We just looked at framework silicates.
In those minerals, all four oxygen in SiO 4 and AlO 4 tetrahedra are shared with another tetrahedron. This is what creates the 3-dimensional framework. In sheet silicates, tetrahedra only share three out of four of their oxygen with adjacent tetrahedra. The most important groups of minerals within the sheet silicate subclass are micas, chlorites, and clays.
We will consider only the micas and, to a lesser extent, chlorites, in this chapter and defer discussion of clay minerals to the chapter on sedimentary rocks and minerals. Some specific formulas are given in the blue box above. The X atoms are most often K and, less commonly Na or Ca, or rarer elements. If the X atom is K or Na, we call the mica a common mica , if it is Ca e. Z atoms are mostly Si, with lesser amounts of Al, and even lesser amounts of Fe or Ti.
Additionally, some Cl or F may substitute for OH. So, micas have highly variable chemistry. Structurally, micas are classified as dioctahedral if the number of Y atoms is 2, and trioctahedral if the number of Y atoms is 3.
Intermediate micas, part way between dioctahedral and trioctahedral, do not exist. Of the micas listed in the box, muscovite, margarite, paragonite, and lepidolite are all dioctahedral; annite and phlogopite are trioctahedral. The photos below show examples common micas.
The specimen is 10cm tall. The specimen is about 6cm tall. The specimen is 10 cm tall. By far, the most abundant micas are muscovite and biotite.
Biotite is the name we use for micas that are mosty solid solutions of phlogopite and annite. Muscovite is more common than biotite, but both occur in a wide variety of igneous and metamorphic rocks. Muscovite Figure 6. Without chemical analysis, it is sometimes hard to distinguish it from other silvery dioctahedral micas such as margarite Figure 6. Biotite Figure 6.
Biotite sometimes incorporates a very small amount of muscovite as well. Micas react to form clays and other minerals when exposed to prolonged weathering, and are therefore absent from most sedimentary rocks. Biotite, and especially muscovite, are relatively Al- and Si-rich compared with many other igneous minerals.
Muscovite, therefore, is found in silicic igneous rocks such as granite, but rarely in rocks of intermediate or mafic composition. Biotite is found in rocks ranging from granitic to mafic composition.
Phlogopite is occasionally found in ultramafic rocks. The name annite refers only to the ideal iron-rich mica end member, but the name phlogopite is often used in a more general sense to describe any magnesium-rich biotite. Phlogopite is often brown, compared to the more common black color of biotite, so many geologists use the names muscovite, phlogopite, and biotite in a generic sense to refer to silvery, brownish, and black micas, respectively.
Besides Al-Fe-Mg substitutions, other substitutions, such as F — substituting for OH — , occur in micas, but they are generally minor. Lepidolite, an especially lithium-rich mica similar to muscovite, is an exception. It is a common large and euhedral mineral in pegmatites, and is often associated with the lithium-aluminum pyroxene, spodumene. The photo of lepidolite in Figure 6. Micas contain sheets layers of Si,Al O 4 tetrahedra with alkalis and other metals between. Consequently, well-developed basal cleavage planar and sheet-like characterizes mica and most other sheet silicates.
See the photos above in Figures 6. The name chlorite is often used in a general way to refer to any greenish mica. But, strictly speaking, chlorite is a group of minerals consisting of a number of different species that are difficult to distinguish from each other. Chlorite group minerals are stable over a wide range of conditions, generally occurring in low to medium-grade metamorphic rocks.
They are also common as alteration products in igneous rocks, where they form from biotite, pyroxene, amphibole, and other mafic minerals.
In micas, and other sheet silicates, silicon tetrahedra are polymerized to form sheets. In chain silicates, the tetrahedra polymerize to form chains. This is done in two ways, shown in Figure 6. Single chain silicates have tetrahedra that zig-zag back and forth. Double chain silicates also involve tetrahedra that zig-zag back and forth, but the chains come in pairs, and an oxygen anion, called a bridging oxygen, links chains together.
A few relatively obscure minerals have chains that are wider and more complex than double chains — the are part way to being sheet silicates. Pyroxenes and amphiboles are the two most important groups of minerals in the chain silicate subclass. In fact, they are almost the only minerals in the subclass. All chain silicates contain single or double chains of tetrahedra linked by cations.
Pyroxenes, and their cousins pyroxenoids , are both single chain silicates and have closely related atomic arrangements. Amphiboles, such as tremolite or hornblende, are double chain silicates. Because two oxygen are shared with other tetrahedra, pyroxenes have formulas that contain SiO 3 or Si 2 O 6. Less obvious, but equally valid, is that amphibole formulas contain Si 8 O In both pyroxenes and amphiboles, however, Al may substitute for significant amounts of Si.
Diopside is a Ca-Mg pyroxene single chain , and tremolite is a Ca-Mg amphibole double chain. In both figures, the view is down the chain direction; the yellow tetrahedra are the SiO 4 units that form the chains.
In pyroxenes, the linking cations blue and red in these figures occupy two distinct kinds of sites between the chains; in amphiboles, they occupy three kinds of sites between the chains. In both pyroxenes and amphiboles, two interchain sites blue are significantly larger than the others. In diopside and tremolite, the large sites contain Ca and the smaller sites contain Mg. The large light blue spheres in the tremolite structure are hydroxyl OH groups.
Compare these figures with the one above Figure 6. Pyroxenes are anhydrous minerals having simple formulas compared with amphiboles, all of which are hydrous.
Still, the similarity between the structures of pyroxenes and amphiboles results in some similarity in physical properties. For example, unless cleavage is visible, it can be difficult to distinguish dark-colored pyroxenes from dark-colored amphiboles.
Pyroxenes contain many different elements, but all pyroxenes have the general formula shown in the blue box. So, we can describe the chemistries of most pyroxenes in terms of the five end members listed in the box.
The formulas for wollastonite, ferrosilite and enstatite have been written with six oxygen, but they could be written with three. We generally use six to keep formulas consistent with other pyroxenes such as diopside and hedenbergite and also because Ca, Fe, and Mg occupy two distinctly different atomic sites in pyroxenes. Distinguishing the different pyroxene species can be challenging. In the absence of a chemical analysis, black colored pyroxenes are often called augite.
Some pyroxenes contain minor to appreciable amounts of Na and Al. Na may substitute for Ca, Mg, or Fe. Al can substitute for Ca, Mg, or Fe, and also for Si.
An end-member pyroxene, jadeite , has the formula NaAlSi 2 O 6. It is found in high-pressure metamorphic rocks and is one of two types of jade that are sometimes prized as gemstones. The other is an amphibole. It is about 5 cm across. Li, Cr, and Ti also are sometimes in pyroxene, but are normally minor elements.
However, spodumene LiAlSi 2 O 6 , is a major mineral in some pegmatites. Like jadeite, spodumene, especially the lilac-colored variety called kunzite , can be a valuable gemstone.
The ternary diagram in Figure 6. The mineral wollastonite , although used as an end member to describe pyroxene compositions, is not a pyroxene. It has a slightly different atomic arrangement and belongs to the pyroxenoid group. In pyroxenoids, the tetrahedral chains do not zig-zag back and forth as regularly as is shown in Figure 6. As with the feldspars, we use abbreviations to describe pyroxene compositions. It has the formula Ca 0. This composition is plotted as a dot near the ferrosilite corner of the triangle in Figure 6.
We call the lines between end members on ternary diagrams joins. The four-sided polyhedron bounded on the top by the diopside-hedenbergite join, and on the bottom by the enstatite-ferrosilite join, is the pyroxene quadrilateral. It encompasses the compositions of all natural Ca-Mg-Fe pyroxenes. Natural pyroxenes fall into two main series, distinguished by slightly different atomic arrangements and different crystal shapes: the orthopyroxene series Opx and the clinopyroxene series Cpx. Orthopyroxenes, predominantly solid solutions of the end members ferrosilite and enstatite, have the general formula Mg,Fe 2 Si 2 O 6.
The photo on the left below Figure 6. Natural orthopyroxenes often contain small amounts of CaSiO 3. So, their compositions do not plot on the bottom of the quadrilateral, but slightly above. The photo on the right above Figure 6. Most clinopyroxene, is predominantly a solution of diopside and hedenbergite, and so has the general formula Ca Mg,Fe Si 2 O 6. But natural pyroxenes may be somewhat deficient in CaSiO 3.
This give them a composition that does not plot on the diopside-hedenbergite join but, instead, plots within the pyroxene quadrilateral. A less common kind of clinopyroxene, pigeonite , has composition close to orthopyroxene. No pyroxenes have compositions more calcic than diopside or hedenbergite because Ca is limited to only the larger two of the four sites between the tetrahedral chains in pyroxenes.
We call pyroxenes with compositions that plot within the quadrilateral augite , subcalcic augite , or pigeonite see Figure 6. Pigeonites, first found at Pigeon Point, Minnesota, are very high-temperature pyroxenes that only form in quickly cooled volcanic rocks. Because orthopyroxenes and clinopyroxenes have slightly different atomic arrangements there is a complex solvus between them.
At high temperatures, intermediate composition pyroxenes — those that plot within the quadrilateral can be stable. At low temperatures most of the pyroxene quadrilateral is taken up by a large miscibility gap, so intermediate composition pyroxenes are unstable. Because of this gap, homogeneous pyroxenes with intermediate compositions are only found in some rare high-temperature rocks.
This sometimes leads to exsolution and textures similar to perthitic feldspars. The colors are artifacts and not the true color of the minerals. An original pyroxene subcalcic augite unmixed upon cooling, producing exsolution lamellae the stripes and the texture we see here. The blue-red lamellae are clinopyroxene and the light colored lamellae are orthopyroxene. The compositions of both lamellae depend on the temperature at which exsolution occurred Box So, the compositions of exsolved pyroxene and feldspar lamellae are sometimes used as geothermometers to learn the temperature at which a rock equilibrated.
The miscibility gap between orthopyroxene Opx and clinopyroxene Cpx is sometimes used to calculate the temperature at which igneous or metamorphic rocks formed. Petrologists call such mineral systems geothermometers. Geothermometers are based on a fundamental consequence of thermodynamics: at high temperatures, solid-solution minerals may have intermediate compositions, but at low temperatures many solid solutions unmix so that compositions are relatively close to end members.
The gap narrows at high temperature and does not extend to the very high temperatures where magmas crystallize and pyroxenes first form. Consequently, pyroxenes of any composition can crystallize from a magma.
But, with cooling, many pyroxenes will unmix, producing compositions closer to end member enstatite and diopside than the original pyroxene. A pyroxene of composition X will be stable at temperatures above T 0 , but if it crystallizes or equilibrates at lower temperature, it will unmix to form two pyroxenes. So, as temperature decreases from supersolvus to subsolvus at T 0 , unmixing will produce separate grains of Opx and Cpx in the same rock, or it may produce single grains of pyroxene containing blebs small inclusions with exsolved compositions in a larger host or exsolution lamellae of different compositions.
As temperature decreases from T 0 to T 1 to T 2 , the coexisting pyroxenes will become closer to end member enstatite and diopside. By analyzing the compositions of coexisting Opx and Cpx, petrologists can estimate the temperature of equilibration. Pyroxenes are not the only minerals that can be used as geothermometers.
Feldspars, carbonates, and others can serve the same purpose. Geothermometry is not, however, always simple or straightforward. Many things besides temperature affect the compositions of coexisting minerals. Among others, petrologists must be concerned with the effects of pressure, of minor elements in minerals, and of disequilibrium. Both are chain silicates, but the atomic arrangement in amphiboles is more complex than in pyroxenes.
Like pyroxenes, amphibole chemistry is highly variable and yields many different end member formulas. Just a few are listed in the blue box.
Also, like the pyroxenes, amphiboles fall into two main series: the orthoamphibole series and the clinoamphibole series. Anthophyllite is an orthoamphibole and cummingtonite is a clinoamphibole; the two are polymorphs. The lengthy generic formula at the top of the blue box describes hornblende , the most common kind of amphibole. The formula is complicated, but really does not reflect all possible compositional variations.
The quadrilateral contains a large miscibility gap between the calcic amphiboles and the calcium-poor amphiboles, analogous to the one between clinopyroxene and orthopyroxene.
The quadrilateral includes the compositions of all Ca-Mg-Fe amphiboles. Along the base of the diagram, the figure is complicated because Ca-free amphiboles form two distinct series, having different atomic arrangements: the anthophyllite series and the cummingtonite-grunerite series.
We call the most Mg-rich amphiboles anthophyllite or, if they contain appreciable amounts of iron, ferroanthophyllite. The cummingtonite- grunerite series contains other intermediate and Fe-rich amphiboles that are poor in calcium.
We give aluminous anthophyllite not shown in the amphibole quadrilateral the name gedrite. The largest piece is about 4 cm wide. Calcic amphiboles can have any composition between an Mg end member tremolite and an Fe end member ferroactinolite.
We call compositions between actinolite. The left and center photos above, Figures 6. Actinolite is generally green, and tremolite is white or light colored. The bladed crystals form because of the chain nature of atomic arrangements. The amphibole quadrilateral depicts variations in Ca, Mg, and Fe content well, but many amphiboles, especially hornblendes, contain K, Na, Al, Ti, and other elements in significant amounts.
Additionally, hornblende, the most common amphibole, is a complex mineral that contains many possible elements. It does not have the same number of atoms in its formula as quadrilateral amphiboles. Although many amphibole varieties have specific names, without chemical analyses, telling different amphiboles apart is difficult. So, the name hornblende is commonly used to refer to any black amphibole. This photo Figure 6.
All the dark mineral grains are hornblende. Many amphibole end members have analogs in the pyroxene group; we compare some of the more important end members in the tables below.
For example, hornblende is widespread. It is common in granitic rocks but is even more common in rocks of intermediate to mafic composition. Amphiboles also exist in many metamorphic rocks, including marbles and metamorphosed mafic igneous rocks. They are especially common in high-temperature amphibolites like the one seen above in Figure 6. Amphibolites always contain amphibole and plagioclase, and generally other minerals including biotite, epidote, or garnet.
Its presence is often associated with rocks formed in subduction zones under high pressures and moderate temperatures. Other Ca-free amphiboles anthophyllite, gedrite, cummingtonite, and grunerite are found in metamorphic rocks and occasionally in extrusive igneous rocks. Tremolite is common in high-temperature marbles.
As with some other silicates found in marbles, identifying it may be difficult because of its inconspicuous white color. See Figure 6. Ring silicates, generally containing rings of six silicon tetrahedra see Figure 6. Beryl, tourmaline and cordierite are the only common examples.
A few obscure and uncommon minerals have rings of three tetrahedra. Beryl, tourmaline, and cordierite are sometimes beautiful gemstones. But, the more common varieties are often rather drab. Figures 6. Because these mineral have atoms arranged in hexagonal rings, their crystals often exhibit hexagonal symmetry, as can be seen in Figure 6.
It is similar to one we saw in Figure 4. Epidote, zoisite, and lawsonite are the most common examples. But, epidote and zoisite contain paired tetrahedra and tetrahedra in other configurations as well. The photos, in Figures 6. More common specimens are not so pretty.
The specimen is 5. The crystal is 7. The largest crystals are about 5 mm across. Beryl and tourmaline contain beryllium and boron, respectively — these elements are generally quite rare.
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