Petrology

Tulane University

Olivines, Pyroxenes, Garnets, and Aluminosilicates

Nesosilicates (Island Silicates)

We now turn our discussion to a systematic look at the most common rock forming minerals, starting with the common nesosilicates.  Among these are the olivines, garnets, Al₂SiO₅ minerals, staurolite,  and sphene.

Olivines

The olivines consist of a complete solid solution between  Mg₂SiO₄ (forsterite, Fo) and Fe₂SiO₄ (fayalite, Fa).  There is limited substitution of the following end members:

Also found substituting in octahedral sites are Ni⁺² and Cr⁺³, particularly in Mg-rich olivines.

• Occurrence
  
  Pure forsterite is limited to metamorphosed Mg-rich limestones and dolomitic metamorphic rocks.
  
  Fo_{90 - 95} is found in ultrabasic igneous rocks, particularly dunites (>90% by volume olivine), and peridotites (Olivine + Cpx + Opx).
  
  Fo_{60 - 90} is found in basic igneous rocks likes basalts and gabbros, and sometimes in andesites, where it occurs with plagioclase and pyroxene.
  
  Fa_{100 - 40} is found in Fe-rich siliceous igneous rocks like rhyolites and granites.
  
  Mg-rich olivines rarely occur in quartz bearing rocks and quartz rarely occurs with Mg-rich olivine because the reaction shown below runs to the right for most pressures and temperatures.
  

Mg₂SiO₄ + SiO₂  <=> 2MgSiO₃
      Fo          Qtz                En

Note however, that Fe-rich olivines can occur with quartz.

• Structure
  
  The structure of the olivines is illustrated on pages 114-115 of Sen.  Note that 2 different kinds of octahedral sites occur.  One is a regular octahedron, labeled M2, and the other is a distorted octahedron, labeled M1.  Fe⁺² and Mg⁺² have no particular preference for either site, but if Ca⁺² is present it prefers the M2 site.
  
  
  
• Identifying Properties
  
   The olivines are orthorhombic (2/m2/m2/m) and usually green colored in hand specimen.  
  
  The most characteristic property in thin section is their surface texture that kind of looks like a piece of sandpaper (see photo on the back wall of the Mineralogy lab).  
  
  Because of their good {010} cleavage and common {100} parting, they show parallel extinction relative to the cleavage or parting.  
  
  Maximum birefringence as seen in the interference colors in thin section varies between 3^rd order blue (for Fo rich varieties) and 3^rd order yellow (for Fa-rich varieties), but remember that this is the maximum birefringence that will only be seen for grains with a and g parallel to the microscope stage.  
  
  Fo-rich olivines are usually clear in thin section, but Fa-rich olivines show pale yellow, greenish yellow, or yellow amber absorption colors and sometimes show pleochroism with a = g = pale yellow, b = orange, yellow, or reddish brown.
  
  Because optical properties vary with composition of the olivine, 2V is useful in distinguishing olivine compositions.  Look at the graph on page 11 of Deer, Howie, and Zussman.  From the graph you can see that very Fo-rich olivines(>Fo₉₀) are optically positive with a 2V between 82 and 90^o.  Between Fo₉₀ and Fa₁₀₀ the olivine is optically negative with 2V_g  between 90 and 130 (2V between 90^o and  50^o.  Thus, by estimating the 2V, you should be able to estimate the composition of the olivine.
  
  Olivines are distinguished from orthorhombic pyroxenes (opx) easily because olivines show higher maximum birefringence and do not show the characteristic {110} cleavage of the pyroxenes.  They are distinguished from the clinopyroxenes (Cpx) which show inclined extinction relative their {110} cleavage and show a biaxial positive character with a 2V of 50 to 60^o.

Garnets are isometric minerals and thus isotropic in thin section, although sometimes they are seen to be weakly birefringent (slightly anisotropic).  They are also nesosilicates, and therefore based on the SiO₄ structural unit.  The general formula for garnets is:

A₃B₂(Si₃O₁₂)

where the A sites are cubic sites containing large divalent cations, usually Ca, Fe, Mg, or Mn, and the B sites are octahedral sites occupied by smaller trivalent cations, like Al and Fe⁺³.

Garnets with no Ca in the A site and Al in the B site are called the pyralspite series.  These consist of the end members:

Pyrope - Mg₃Al₂Si₃O₁₂

Almandine - Fe₃Al₂Si₃O₁₂ 

Spessartine - Mn₃Al₂Si₃O₁₂ 

Garnets with Ca in the A site are called the ugrandite series and consist of the end members:

Uvarovite - Ca₃Cr₂Si₃O₁₂ 

Grossularite - Ca₃Al₂Si₃O₁₂ 

Andradite - Ca₃Fe⁺³₂Si₃O₁₂ 

 Limited solid solution exists between end members of each series.

• Occurrence
  
  The garnets occur mostly in metamorphic rocks where they are often seen to form euhedral (well-formed) crystals. 
  
  The Mg-rich garnet, pyrope, is found in metamorphic rocks formed at high pressure and in eclogites (basalts metamorphosed at high pressure) and peridotites (ultrabasic rocks containing olivine, Opx, Cpx, and garnet).

The Fe-rich garnet, almandine, is the most common garnet and is found in metamorphic aluminous schists.

The Mn-rich variety, spessartine, is limited to Mn-rich metamorphic rocks like meta-cherts.

• Identifying Properties
  
  Garnets are generally isotropic although some may be weakly birefringent.  In hand specimen they exhibit a wide range of colors and these are sometimes seen in thin section. Color is controlled by the amounts of Fe⁺², Fe⁺³, Mg⁺², and Cr⁺³ present.
  
  Pyrope is usually pinkish red to purplish in hand specimen and is usually clear in thin section.
  
  Almandine is usually deep red to brownish black in hand specimen and pink in thin section.
  
  Spessartine ranges from black to red to brown and orange and is usually pink in thin section.
  
  Grossularite has a color in hand specimen that reflects the amount of Fe and Mn present and thus ranges from brown to yellow to pink. If Cr is present, the color is usually green.  In thin section grossularite varies in color from clear to brown or green in Cr-rich varieties.
  
  Uvarovite, with high Cr concentration is usually deep green in hand specimen and green in thin section.
  
  Andradite ranges from yellow to dark brown, but if appreciable amounts of Ti are present, the color could be black in hand specimen and brown in thin section.
  
  The composition and identity of the garnets is best determined either by association with other minerals or by more sophisticated techniques such as electron microprobe or XRD.
  
  Garnets are easily distinguished from other minerals by their high relief, isotropic character, and common euhedral habit.

The Al₂SiO₅ minerals are common in aluminous metamorphic rocks (meta-shales and meta-mudstones) and sometimes found in aluminous igneous rocks.  

• Sillimanite
  
  Sillimanite is orthorhombic with a good {010} cleavage.  It generally occurs in long fibrous crystals that are length slow, with extinction parallel to the {010} cleavage.  In sections lying on {001}that show well-developed {110} forms, the cleavage is usually seen to cut across the crystal as shown here.  Maximum birefringence is generally seen to be between 2^o yellow to 2^o red. Sillimanite is biaxial positive with a 2V of 21 - 31^o.

• Andalusite
  
  Andalusite is also orthorhombic , but shows a length fast character.  It generally tends to occur as euhedral blocky crystals with a maximum birefringence in thin section between 1^o yellow and 1^o red.  It sometimes shows weak pleochroism with a = rose-pink, b = g = greenish yellow.  Some varieties show a cross, termed the chiastolite cross, which is made up of tiny carbonaceous inclusions oriented along crystallographic directions (see illustration on page 499 of Klein).  Andalusite generally occurs as euhedral crystals with an almost square prism.  It is biaxial  negative with 2V = 73 - 86^o.
  
  
• Kyanite
  
  Kyanite is triclinic and thus shows inclined extinction relative to its good {100}and {010}cleavages and {001} parting.  In hand specimen kyanite is commonly pale blue in color, but is clear to pale blue in thin section.  Because of its good cleavages and parting, two cleavages or partings are seen in any orientation of the crystal in thin section.  These cleavages intersect at angles other than 90^o and thus look like parallelograms in two dimensions.  Because Kyanite has high relief relative to other minerals with which it commonly occurs, it stands out in thin section and sometimes appears to have a brownish color.  This color is more due to its high relief and numerous cleavages rather than due to selective absorption.
  
  Kyanite is biaxial negative with 2V = 78 -83^o

Zircon ZrSiO4

Zircon is a common accessory mineral in nearly all kinds of rocks, particularly the more siliceous igneous rocks, like granites, granodiorites, and syenites.   Still, it is not often found in thin section because it is so hard that it gets plucked out during the grinding of the section. Zircon usually contains high amounts of radioactive elements like U and Th.  Thus, when it is found as inclusions in minerals like biotite, it produces pleochroic haloes in the biotite as seen in thin section.  Because it contains high concentrations of U and Th, it is very useful in obtaining U-Pb and Th-Pb radiometric dates on old rocks.  It is very resistant to weathering and may also survives during metamorphism, allowing for dates to be obtained on the original rock prior to metamorphism (often called the protolith).

In hand specimen Zircon usually occurs as tiny reddish colored crystals.  In thin section, it shows extremely high relief, with w = 1.923 to 1.960 and e = 1.968 to 2.015. and is uniaxial positive. Zircon has high birefringence, with interference colors in the higher orders (lots of reds, pinks and light greens). It is commonly colorless to pale brown or pinkish brown in polarized light without the analyzer. Generally it occurs as small crystals with relief higher than almost anything else in the thin section. This latter property should tip you off to its presence.

Sphene (Titanite) CaTiSiO₄(OH)

Sphene is another common accessory mineral in  plutonic igneous rocks like granites, granodiorites, and syenites.  It is also found as larger crystals in metamorphic gneisses and chlorite bearing schists.

In hand specimen as an accessory mineral, it is usually seen as small wedge-shaped crystals with a resinous to adamantine luster and brown to yellow brown color.  In thin section, Sphene,  has a relief similar to that of zircon, and is usually found in small crystals with an elongated diamond shape. It is generally brownish in color, shows a well developed {110} cleavage, and high order interference colors.

Staurolite (Mg,Fe)₂Al₉Si₄O₂₂(OH)₂

In hand specimen and in thin section it characteristically is seen to show staurolite twinning, either the right-angle cross, twinned on {031} or the oblique cross, twinned on {231}

Sorosilicates

Sorosilicates are the double island silicates.  Only one important mineral group, the epidote group, has this structure.

Epidote, Clinozoisite, Zoisite

The important minerals in the epidote group are epidote, clinozoisite, and zoisite.  Since the sorosilicates are based on the Si₂O₇ ^-6 group, the structural formula can be written as:

Ca₂(Al,Fe⁺³)Al₂O(SiO₄)(Si₂O₇)(OH)

Thus, the epidote group contains both the double tetrahedra and the single tetrahedron, separated by groups of AlO₆ octahedra and Ca in nine to 10 fold coordination with Oxygen or OH.

The formula can be rewritten as:

Ca₂(Al,Fe⁺³)Al₂Si₃O₁₂(OH)

Epidote is the Fe-rich variety and has the above general formula.  Clinozoisite is the Fe-free variety with the chemical formula:

Ca₂Al₃Si₃O₁₂(OH)

Both clinozoisite and epidote are monoclinic (2/m).  Zoisite has the same chemical formula as clinozoisite, but is orthorhombic.

Epidote is usually pistachio green in color with perfect {001} cleavage and imperfect {100} cleavage.  It is optically negative with a 2V of 64 - 90^o.  It usually shows pleochroism with a - colorless to pale yellow, b - greenish yellow, and g - yellowish green, and shows high relief relative to feldspars and quartz.  It's birefringence is high enough to show 3^rd order interference colors.  It usually shows an anomalous blue extinction.

Clinozoisite shows similar relief and cleavage to epidote, but it is optically negative with a 2V of 14 to 90^o, shows no pleochroism, and lower birefringence (1^st to 2^nd order interference colors).  Zoisite is similar to clinozoisite, except it will show parallel extinction relative to faces parallel to the crystallographic axes.  

Epidote is a common mineral in low grade metamorphic rocks, particularly metamorphosed volcanic rocks and Fe-Al rich meta shales.   Both Clinozoisite and epidote occur as alteration products of  plagioclase and as veins in granitic rocks.

Cyclosilicates

Beryl

Be₃Al₂Si₆O₁₈ is hexagonal (6/m2/m2/m) with a strong prismatic habit with the form {100} usually the only form present.  It is usually deep green to yellowish green in color.  Beryl forms different gemstones depending on color - Aquamarine when it is pale greenish-blue, Morganite if pink, and emerald if deep green and transparent.  Beryl is a common constituent of coarse grained granitic rocks and pegmatites and is found in aluminous mica schists. 

In thin section, Beryl shows higher relief than quartz, and is distinguished from quartz by its negative optic sign and length-fast character.  The only other mineral that it can be confused with is apatite, but apatite shows even higher relief than Beryl.

Cordierite

Cordierite is (Mg,Fe)₂Al₄Si₅O₁₈^.nH₂O.  It is orthorhombic (2/m2/m2/m), but shows a pseudohexagonal character due to its common cyclical twinning on {110}.  In thin section it may show a twinning that looks like albite twinning, which makes it hard to distinguish from plagioclase.  But, cordierite is usually dusted with tiny opaque inclusions.  In thick sections it shows a pale -yellow, violet, pale blue  pleochroism.  It can be distinguished from quartz by its biaxial character.

Cordierite is a common constituent of aluminous metamorphic rocks.  It is common in contact metamorphic rocks where it is commonly associated with sillimanite or andalusite, feldspars and micas.

Tourmaline

Tourmaline - Na(Mg,Fe,Mn,Li,Al)₃Al₆Si₆O₁₈(BO₃)₃(OH)₄ is hexagonal (3m) and is commonly found as well-formed prismatic crystals, with a rounded triangular cross section perpendicular to the c crystallographic axis.

Tourmaline is a common mineral in pegmatites (SiO₂ - rich igneous rocks with large grain size), where it is associated with quartz and alkali feldspar. It is also found in metasomatized rocks of all types, where it is precipitated from a Boron and Silica - rich fluid phase. 

It's most distinguishing properties are its uniaxial negative optical character and its pleochroism with w = dark green or dark blue and e = yellow or violet. Tourmaline usually forms in euhedral crystals with well developed prism faces and extinction parallel to the prism faces.

Inosilicates (Single Chain Silicates)

XYZ₂O₆

where X = Na⁺, Ca⁺², Mn⁺², Fe⁺², or Mg⁺² filling octahedral sites called M2

Y = Mn⁺², Fe⁺², Mg⁺² , Al⁺³, Cr⁺³, or Ti⁺⁴ filling smaller octahedral sites called M1

Z = Si⁺⁴ or Al⁺³ in tetrahedral coordination.

The pyroxenes can be divided into several groups based on chemistry and crystallography:

Orthorhombic Pyroxenes (Orthopyroxenes - Opx)

These consist of a range of compositions between enstatite - MgSiO₃ and ferrosilite - FeSiO₃

Monoclinic Pyroxenes (Clinopyroxenes - Cpx)