Mineralogy

Prof. Stephen A. Nelson

Tectosilicates, Carbonates, Oxides, & Accessory Minerals

Tectosilicates (Framework Silicates)

Since the Si - O bonds are strong covalent bonds and since the structure is interlocking, the tectosilicate minerals tend to have a high hardness. 

There are nine known polymorphs of SiO₂, one of which does not occur naturally.  These are:

Name	Crystal System	Density (g/cm3)	Refractive Index  (mean)
Stishovite	Tetragonal	4.35	1.81
Coesite	Monoclinic	3.01	1.59
Low (α) Quartz	Hexagonal	2.65	1.55
High (β) Quartz	Hexagonal	2.53	1.54
Kaetite (synthetic)	Tetragonal	2.50	1.52
Low (α) Tridymite	Monoclinic or Orthorhombic	2.26	1.47
High (β) Tridymite	Hexagonal	2.22	1.47
Low (α) Cristobalite	Tetragonal	2.32	1.48
High (β) Cristobalite	Isometric	2.20	1.48

  

Transformations between α Cristobalite, α Tridymite, and α Quartz, however, as well as between the high pressure polymorphs and Quartz, are reconstructive transformations.  Since reconstructive transformations require significant structural rearrangement and significant changes in energy, they occur slowly, and the high temperature and high pressure polymorphs can occur as metastable minerals at the Earth's surface. 

Quartz is hexagonal and commonly occurs as crystals ranging in size form microscopic to crystals weighing several tons.  Where it crystallizes unhindered by other crystals, such as in cavities in rock or in a liquid containing few other crystals, it shows well-developed hexagonal prisms and sometimes showing apparent hexagonal pyramids or dipyramid.  When it crystallizes in an environment where growth is inhibited by the surroundings, it rarely show crystal faces.  It is also found as microcrystalline masses, such as in the rock chert, and as fibrous masses, such as in chalcedony.

As visible crystals, Quartz is one of the more common rock forming minerals.  It occurs in siliceous igneous rocks such as volcanic rhyolite and plutonic granitic rocks.  It is common in metamorphic rocks at all grades of metamorphism, and is the chief constituent of sand.  Because it is highly resistant to chemical weathering, it is found in a wide variety of sedimentary rocks.

Several varieties of Quartz can be found, but these are usually only distinguishable in hand specimen.

A fibrous variety of Quartz is called Chalcedony.  It is usually brown to gray to translucent with a waxy luster.  It is found lining or filling cavities in rock where it was apparently precipitated from an aqueous solution.  When it shows bands of color, it is commonly called by the following names:

Very fined grained aggregates of cryptocrystalline quartz makes up rock like Flint and Chert.  Flint occurs as nodules in limestone, whereas chert is a layered rock deposited on the ocean floor.  The red variety of flint is called Jasper, where the color results from inclusions of hematite.  

Optical Properties

Quartz is uniaxial positive with a low relief and low birefringence, thus exhibited only 1^o gray to 1^o white interference colors. In thin section it is almost always colorless when viewed without the analyzer inserted.  One of its most distinguishing properties in thin section is that it usually has a smooth, almost polished-like surface texture.  Quartz is easily distinguished from the Feldspars by the biaxial nature of feldspars, and from Nepheline which is uniaxial negative.  Apatite, has similar birefringence to quartz, but is uniaxial negative and has a very high relief.

In Chalcedony, the fibers are usually elongated perpendicular to the c-crystallographic axis and thus are length fast.  Normal quartz, when it show an elongated habit, is elongated parallel to the c axis, and is thus length slow.

Tridymite is the high temperature polymorph of SiO₂.  Thus, it is only commonly found in igneous rocks that have been cooled rapidly to surface temperatures, preventing the slow transformation to quartz, the stable form of SiO₂ at surface temperatures.  Because of this, we only expect to find Tridymite in siliceous volcanic rocks like rhyolites, where it commonly occurs as wedge shaped crystals in cavities in the rock. In volcanic rocks, Tridymite is commonly associated with Cristobalite and Sanidine.

Optical Properties

Tridymite usually occurs as orthorhombic or monoclinic wedge shaped crystals with a positive 2V between 40 and 90^o.  The wedge shape of the crystals is the result of twinning on {110}, and usually as 2 to 3 twinned individuals.  Although it has similar birefringence to quartz and feldspar, it has lower refractive indices, and thus shows negative relief compared to quartz and feldspars.  

 
Cristobalite

Cristobalite is also a high temperature SiO₂ polymorph, and thus has a similar occurrence to Tridymite. It also occurs in thermally metamorphosed sandstones.  In volcanic rocks it can occur both as a lining in open cavities, and as fine grained crystals in the groundmass of the rock.  

Optical Properties

Cristobalite is tetragonal and thus uniaxial.  It has a negative optic sign and shows lower relief than quartz, but has similar birefringence.  

Opal

Opal is amorphous, and thus a mineraloid, with a formula  - SiO₂^.nH₂O.

The feldspars are the most common minerals in the Earth's crust.  They consist of three end-members:

KAlSi₃O₈ - Orthoclase (or),  NaAlSi₃O₈ - Albite (ab), and CaAl₂Si₂O₈ -Anorthite (an)

KAlSi₃O₈ and NaAlSi₃O₈ form a complete solid solution series, known as the alkali feldspars and NaAlSi₃O₈ and CaAl₂Si₂O₈ form a complete solid solution series known as the plagioclase feldspars.

The feldspars have a framework structure, consisting of SiO₄ tetrahedra sharing all of the corner oxygens.  However, in the alkali feldspars 1/4 of the Si⁺⁴ ions are replaced by Al⁺³ and in the plagioclase feldspars 1/4 to 1/2 of the Si⁺⁴ ions are replaced by Al⁺³.  This allows for the cations K⁺, Na⁺, and Ca⁺² to be substituted into void spaces to maintain charge balance.  

Albite - ab₉₀ to ab₁₀₀
Oligoclase - ab₇₀ to ab₉₀
Andesine - ab₅₀ to ab₇₀
Labradorite - ab₃₀ to ab₅₀
Bytownite - ab₁₀ - ab₃₀
Anorthite - ab₀ to an₁₀

Plagioclase is the most common feldspar. It forms initially by crystallization from magma.  The plagioclase solid solution series is coupled solid solution where the substitution is:

Na⁺¹Si⁺⁴ <=> Ca⁺²Al⁺³

Thus, the general chemical formula for plagioclase can be written as:

Ca_xNa_1-xAl_1+xSi_3-xO₈

where x is between 0 and 1.

Plagioclase occurs in basalts, andesites, dacites, rhyolites, gabbros, diorites, granodiorites, and granites. In most of these igneous rocks, it always shows the characteristic albite twinning. Plagioclase also occurs in a wide variety of metamorphic rocks, where it is usually not twinned. In such rocks where the plagioclase is not twinned, it is difficult to distinguish from the alkali feldspars. Plagioclase can be a component of clastic sedimentary rocks, although it is less stable near the Earth's surface than alkali feldspar and quartz, and usually breaks down to clay minerals during weathering.

Properties

In hand specimen, plagioclase is most commonly white colored and shows perfect {100} and good {010} cleavage.  It is most easily identified and distinguished from quartz, sanidine, orthoclase, and microcline, by its common polysynthetic twinning on {010}. If this twinning is not present, plagioclase can still be distinguished from quartz by its cleavage, but cannot easily be distinguished from the alkali feldspars. If both plagioclase and alkali feldspar occur in the same rock, the two can usually be distinguished by differences in color or differences in the extent of weathering.  

In metamorphic rocks plagioclase may not show twinning making it difficult to distinguish from orthoclase.  The two can be distinguished by staining the thin section with stains that make the K-feldspars one color and the more Ca-rich feldspars another color. In this class, we will not have time to look at these staining techniques. You should, however, be aware, that such staining techniques exist, so that if you need them in the future, you can use them.

The optical properties of the plagioclase series vary widely as a function of composition of the plagioclase. In general, all plagioclases show low order interference colors, and thus, low birefringence. Optic sign and 2V vary widely, and are thus, not very distinguishing features of plagioclase. Although, as you have seen in lab, it is possible to estimate the composition of plagioclase from a combination of extinction angle and twinning.

As an alkali feldspar cools from high temperature to lower temperature, the crystal structure changes from that of sanidine, which is monoclinic, through orthoclase, also monoclinic, but with a different crystal structure than sanidine, to microcline, which is triclinic. These transformations are order-disorder transformations, and thus require large amounts of time.  Furthermore, if the feldspar is allowed to cool very slowly, then exsolution will occur, and the solid solution will separate into a Na-rich phase and a K-rich phase. Thus, one expects to find sanidine in rocks that were cooled very rapidly from high temperature, i.e. volcanic rocks. Orthoclase and microcline will be found in plutonic igneous rocks (cooled slowly at depth in the earth) and in metamorphic rocks. In addition, in the plutonic rock types if the cooling takes place slowly enough, then perthitic exsolution lamellae may also form.

All of the alkali feldspars have low relief and low birefringence. Thus the interference colors may range up to 1^o white. Since this is the same interference color we expect for quartz, care must be taken to avoid confusing feldspars and quartz.

Sanidine

Sanidine generally occurs with an equant habit (almost square) and shows perfect {001} and {010} cleavages, which readily distinguish it from quartz. Rarely does sanidine show twinning, but when it does, it is usually simple twinning. Optic axis figures will only be found on sections showing both cleavages. Sanidine is optically negative with a 2V of 20 - 50^o. This distinguishes it from quartz, which is uniaxial positive, and from the other alkali feldspars which show larger values of 2V.  

Orthoclase

Orthoclase is a common alkali feldspar in granitic rocks and K - Al rich metamorphic rocks. It often shows perfect {001} and {010} cleavages which will distinguish it from quartz. Also, quartz usually shows a smooth surface texture, while orthoclase appears much rougher. Orthoclase is also biaxial, which further distinguishes it from quartz. The 2V of orthoclase varies from 60 to 105^o, and thus it may be either positive or negative. The 2V angle distinguishes orthoclase from sanidine, but is otherwise not very useful because of the its wide range.

Microcline

Microcline is the lowest temperature form of alkali feldspar. Upon cooling, orthoclase must rearrange its structure from monoclinic to triclinic. When this happens, twinning usually results. The twinning characteristic of microcline is a combination of albite twinning and pericline twinning. This results in a cross-hatched pattern (often called tartan twinning) that is the most distinguishing characteristic of microcline.

Anorthoclase

Anorthoclase is a Na - rich feldspar with approximately equal amounts of the Anorthite (Ca) and orthoclase (K) components. Generally anorthoclase occurs in Na - rich volcanic rocks. Like the other alkali feldspars, it has perfect {001} and {010} cleavages. Sections showing both of the cleavages are best for determining the optic sign and 2V. Anorthoclase sometimes shows twinning, but generally not the multiple twinning seen in the plagioclase feldspars, but a cross-hatched twinning similar to that seen in microcline, but on a very fine scale. Anorthoclase, like sanidine shows a low 2V of 5 to 20^o, and is optically negative. Anorthoclase can sometimes be distinguished from sanidine by the fact that anorthoclase usually forms crystals with a tabular, elongated habit, while sanidine forms crystals with a more equant habit.

The feldspathoid group of minerals are SiO₂ poor, alkali rich minerals that occur in low SiO₂, high Na₂O - K₂O igneous rocks. In general, these minerals are not compatible with quartz, and therefore, are rarely, if ever, seen in rocks that contain quartz. They do, however, often occur with feldspars. Because of the alkalic nature of the rocks that contain feldspathoids, associated pyroxenes and amphiboles are of the sodic variety, i.e. aegerine or riebeckite.

The main feldspathoids are Nepheline (Na,K)AlSiO₄, Kalsilite KAlSi₂O₆, and Leucite KAlSi₂O₆. At high temperature there is complete solid solution between Nepheline and Kalsilite, but at low temperature Nepheline can contain only about 12 wt% K₂O.   

Other similar members of the feldspathoid group are:

Sodalite 3NaAlSiO₄.NaCl
Nosean 3NaAlSiO₄.NaSO₄
Haüyne 3NaAlSiO₄.Ca(Cl,SO₄)

Nepheline

Nepheline occurs in both volcanic and plutonic alkaline igneous rocks.  In hand specimen, Nepheline is difficult to distinguish from the feldspars, and thus must usually be identified by its association with other alkalic minerals.  Nepheline has a yellowish colored alteration product, called cancrinite.  Nepheline is hexagonal, and thus uniaxial, making it easy to distinguish from the feldspars. Furthermore, it is optically negative, making it distinguishable from quartz. It usually shows no cleavage, has low birefringence, and low relief (refractive indices are smaller than the feldspars). The only other common mineral with which nepheline could be confused is apatite, which is also uniaxial negative. Apatite, however, shows much higher relief than does nepheline.

Sodalite
Sodalite occurs predominantly in alkali-rich plutonic igneous rocks, like syenites, but can also be found in volcanic rocks.  It is essentially 3 nepheline molecules with an added NaCl molecule. It is a clear colored isometric mineral with low relief. Thus, the only thing sodalite might be confused with is a hole in the thin section.  The blue color of sodalite in hand specimen and its association with other alkali-rich minerals is usually necessary to detect its presence in a rock.

Leucite
Leucite is found in alkalic volcanic rocks, and is rarely found in plutonic rocks.  It is a tetragonal mineral, however, its refractive indices e and w are so close together that it almost always appears isometric. It usually occurs as small, slightly rounded, low relief grains that go extinct upon insertion of the analyzer. Commonly, leucite contains tiny inclusions within the mineral, and sometimes shows a slight twinning, barely visible with the analyzer inserted. 

Oxides

The oxide minerals are very common and usually occur as accessory minerals in all kinds of rocks.  The most common oxide minerals are the following:

Corundum - Al₂O₃
Corundum is hexagonal and optically negative.  It occurs in Al-rich igneous and metamorphic rocks.  If transparent blue, it is the gemstone sapphire, if transparent red, it is the gemstone ruby.  When it occurs as an accessory mineral it usually shows its hexagon shaped outline when looking down the c-axis.  It has high refractive indices, thus shows very high relief in thin section.  But it has low birefringence and commonly shows lamellar twinning.  

Spinel - MgAl₂O₄
Spinel is an isometric mineral that occurs ultrabasic rocks like peridotite, and in many low silica ignoeous rocks like basalts, where it contains high concentrations of Cr.  It is also found in Al-rich contact metamorphic rocks.  It shows a wide variety of colors depending on trace amounts of other ions substituting for both Mg and Al.  Because of the isometric nature, Spinel is difficult to distinguish from garnet, although spinel tends to occur as much smaller crystals.  

Chromite - Fe⁺²Cr₂O₄ 
Chromite is a major ore of Cr.  It is found in in low silica, Mg-rich igneous rocks, usually associated with Olivine.  Often it is seen as small inclusions in Olivine, indicating that it is an early crystallizing phase in basaltic and gabbroic magmas.  Chromite is isometric, and usually opaque in thin section.  Electron Microprobe analysis is usually necessary to distinguish it from other opaque oxide minerals.

Magnetite - Fe₃O₄
Magnetite is one of the most common oxide minerals.  It is a major ore of Fe, and is found as an accessory mineral in all rock types.  It is isometric and commonly crystallizes with an octahedral habit. In hand specimen it is most easily identified by its strongly magnetic nature, black color, and hardness of 6.  In thin section it is opaque and thus difficult to distinguish from the other opaque oxide minerals.  As discussed below, it forms a solid solution with Ulvospinel - Fe₂TiO₄.

Ilmenite - FeTiO₃  
Ilmenite is a major ore of Ti.  It is found as a common accessory mineral in a wide range of igneous volcanic and plutonic rocks, as well as metamorphic and clastic sedimentary rocks.  It forms a solid solution series with Hematite, as will be discussed below, and commonly occurs along with Magnetite.  Ilmenite is hexagonal, but is usually opaque which makes its distinction from other oxide minerals difficult.  Ilmenite, however, often shows an elongated or acicular habit, whereas Magnetite usually crystallizes as more equant crystals with an octahedral habit. 

Hematite - Fe₂O₃

Hematite is one of the most important ores of Fe.  It is more oxidized than Magnetite, and thus forms as an alteration product of magnetite as well as other Fe bearing minerals.  In most unaltered igneous rocks, hematite occurs as a component of Ilmenite in solid solution.  Hematite is hexagonal, but rarely occurs in crystals where its symmetry can be determined.  It is found in a variety of forms, ranging from oolitic spherules, to massive fine grained aggregates, to botryoidal masses.  It is most easily distinguished by its black to dark red color and reddish brown streak.  In thin section it is not easily distinguished from other opaque oxide minerals.

Under magmatic conditions, Ilmenite and Hematite form a complete solid solution series, often called the rhombohedral series since both minerals crystallize in the hexagonal system.  Similarly Magnetite and Ulvospinel form a complete solid solution series, called the spinel series. 

Carbonates

The carbonates are an important group of minerals near the Earth's surface.  Carbonate minerals make up the bulk of limestones and dolostones.  Are found as cementing agents in clastic sedimentary rocks, and make up the shells of many organisms.  The carbonates are based on the CO₃^-2 structural unit, which has carbon surrounded by 3 oxygens in triangular coordination.  Thus each Oxygen has a residual charge of -2/3.  In the carbonate structure, no two triangles share the corner oxygens and the C-O bonds are highly covalent.

There are three structural types of carbonates:

The Calcite Group

The calcite group minerals are all hexagonal.  They have Ca, Mg, Fe, Mn, or Zn divalent cations in 6-fold coordination with the CO₃^-2 groups, in a structure that is similar to that of NaCl.  All members of this group show rhombohedral cleavage {012}, thus breaking into rhomb-shaped cleavage blocks.  

Calcite CaCO₃- The most common carbonate mineral is calcite.  It is the principal constituent of limestone and its metamorphic equivalent - marble.  Deposits of fine grained calcite in powder form are referred to as chalk.  It forms the cementing agent in many sandstones, and is one of the more common minerals precipitated by living organisms to form their skeletal structures.  

Calcite is also precipitated from groundwater where it form veins, or in open cavities like caves and caverns can form the cave decorations - like stalactites and stalagmites, and encrustations.  It is also precipitated from hot springs where it is called travertine.

Calcite does occur in rare igneous rocks called carbonatites.  These form from carbonate magmas.  Calcite is also precipitated from hydrothermal fluids to form veins associated with sulfide bearing ores.

Properties
In hand specimen, calcite is distinguished by its rhombohedral cleavage, its hardness of 3, and by its effervescence in dilute HCl.  It can range in color from white, to slightly pink, to clear, but dark colored crystals can also occur.  In thin section it is most readily distinguished by its high birefringence, showing high order white interference colors, by its rhombohedral cleavage and its uniaxial negative character.  Because of its high birefringence, it shows a large change in relief on  rotation of the stage.  Furthermore, its ε refractive index direction (low RI direction) when parallel to the polarizer shows a negative relief when compared to the mounting medium of the thin section. Calcite can be distinguished from Aragonite by the lack of rhombohedral cleavage and biaxial nature of Aragonite.

Magnesite  MgCO₃
Magnesite is a common alteration product of Mg-rich minerals on altered igneous and metamorphic rocks.  Like calcite, it shows perfect rhombohedral cleavage, but unlike calcite, it does not readily effervesce in dilute HCl.  It does, however, effervesce in hot HCl.  These properties and its association with Mg-rich minerals and rocks make it distinguishable from Calcite.

Siderite FeCO₃
Siderite forms complete solid solution series with Magnesite, although the environment in which the two minerals occur usually determines that either Mg-rich Magnesite or Fe-rich Siderite will form, and one rarely sees intermediate end members.  In hand specimen, siderite is usually brown colored and effervesces only in hot HCl.  In thin section it resembles Calcite, but has a much higher ε refractive index than Calcite and is commonly pale yellow to yellow brown in color without the analyzer inserted.

Rhodochrosite MnCO₃
Rhodochrosite is the Mn bearing carbonate, and is thus found only in environments where there is an abundance of Manganese.  It is relatively rare and occurs as hydrothermal veins and as an alteration product of Mn rich deposits.  In hand specimen it show a distinctive pink color along with the rhombohedral cleavage common to the Calcite group minerals.  Hot HCl is required to make the mineral effervesce.   

The Aragonite group of minerals are all orthorhombic, and can thus be distinguished from minerals of the calcite group by their lack of rhombohedral cleavage.  Aragonite (CaCO₃) is the most common mineral in this group.

Properties
In hand specimen, Aragonite, like calcite effervesces in cold HCl.  But, unlike Calcite, Aragonite does not show a rhombohedral cleavage. Instead it has single good {010} cleavage. It is usually transparent to white in color and forms in long bladed crystals.  Twinning is common on {110}, and this can produce both cyclical twins, which, when present, make it look pseudohexagonal, and single twins. In thin section Aragonite is distinguished by its high birefringence, showing high order white interference colors, its biaxial character with a 2V of about 18^o, and extinction parallel to the {010} cleavage.  

In thin section it is more difficult to distinguish from calcite, unless it is twined.  In order to facilitate its identification in thin section, the sections are often stained with alizarin red S.  This turns calcite pink, but leaves the dolomite unstained.

Apatite Ca₅(PO₄)₃(OH,F)

Apatite is another very common and almost ubiquitous (always present) accessory mineral in igneous rocks and many metamorphic rocks. If the rock contains any phosphorous it is usually found in apatite. Apatite is hexagonal, hence uniaxial with a negative optic sign. Its refractive indices ω = 1.624 to 1.666 and ε = 1.629 to 1.667 are higher than both quartz and nepheline, giving apatite a higher relief than these minerals. Its birefringence, expressed as 1^o gray interference colors is similar to that of quartz and nepheline. Quartz, however, is optically positive. Nepheline, while optically negative, shows much lower relief than does apatite. The crystal form of apatite is usually distinctive. If cut parallel to {0001}, it usually has a hexagonal outline. If cut parallel to the C axis, it appears as doubly terminated prisms.

Examples of questions on this material that could be asked on an exam

1. What are the eight polymorphs of SiO₂? Briefly describe where each of these polymorphs are stable and which, if any do not have unstable polymorphs at the earth's surface conditions.
   
   
2. For the SiO₂ polymorophs, is their a correlation between density and temperature or pressure of stability?  If so, can you explain why this is?
   
   
3. State whether each of the following minerals would be most commonly found in igneous rocks, metamorphic rocks, or both igneous and metamorphic rocks: (a) quartz, (b) plagioclase, (c) sanidine, (d) microcline, (e) nepheline, (f) sodalite, (g) leucite, (h) apatite.
   
   
4. Which minerals would be good ores for Cr, Al, Fe, Ti, or Mn?
   
   
5. What is the major difference between the calcite group and the aragonite group of carbonate minerals?

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