GEOS 254: INTRODUCTIONGEOS 254: INTRODUCTION

Petrological microscopes are used to:Petrological microscopes are used to: (a) identify the minerals present(a) identify the minerals present (b) determine the microstructure(b) determine the microstructure

These data are then used to classify the rock These data are then used to classify the rock and infer the history of the rock (e.g. has and infer the history of the rock (e.g. has the rock been deformed? Did the the rock been deformed? Did the deformation occur before, during or after deformation occur before, during or after the growth of the minerals?) the growth of the minerals?)

Rock MicrostructureRock Microstructure

Microsturucture is the term used to cover the Microsturucture is the term used to cover the shape of the components of the rock (minerals, shape of the components of the rock (minerals, volcanic glass, fossils). In older books “texture” volcanic glass, fossils). In older books “texture” is used to mean the same thing. is used to mean the same thing.

The microstructure of igneous, metamorphic and The microstructure of igneous, metamorphic and sedimentary rocks are very different and are sedimentary rocks are very different and are generally more diagnostic than the list of generally more diagnostic than the list of minerals present in defining the rock type.minerals present in defining the rock type.

What do you need to identify What do you need to identify rocks and minerals?rocks and minerals?

1.1. A “drivers license” for the microscopeA “drivers license” for the microscope

2.2. Access to a list of mineral propertiesAccess to a list of mineral properties

3.3. Information on which rock types each Information on which rock types each mineral occurs.mineral occurs.

4.4. A knowledge of rock microstructures and A knowledge of rock microstructures and their significance.their significance.

Sample preparationSample preparation Thin sections are slices of rock polished on Thin sections are slices of rock polished on

one side, then stuck to a glass slide with one side, then stuck to a glass slide with araldite and then ground down to 30 microns araldite and then ground down to 30 microns or 3 thou of an inch.or 3 thou of an inch.

So the light is not diffused by the frosted So the light is not diffused by the frosted upper surface, a very thin cover-glass is upper surface, a very thin cover-glass is glued onto the upper surface. A much more glued onto the upper surface. A much more expensive alternative is to polish the upper expensive alternative is to polish the upper surface and this is the method that must be surface and this is the method that must be used for mineral analysis using the Electron used for mineral analysis using the Electron Microprobe.Microprobe.

Remember the RulesRemember the Rules Always start holding the slide up to the light. How Always start holding the slide up to the light. How

many minerals/ What grainsize/ Is the slide many minerals/ What grainsize/ Is the slide homogeneous?homogeneous?

Then go to LOW POWER (the slide must be right-Then go to LOW POWER (the slide must be right-way-up) in plane polarised light (natural colours)way-up) in plane polarised light (natural colours)

Then go to crossed polars where the “interference Then go to crossed polars where the “interference colours” show twinning, zoning etc and generally colours” show twinning, zoning etc and generally distinguish quartz and feldspars having “low” distinguish quartz and feldspars having “low” colours from most grains of olivine and pyroxene colours from most grains of olivine and pyroxene that have “high” colours.that have “high” colours.

High power is generally only used for “conoscopic High power is generally only used for “conoscopic optics” which are tests that can only be applied to optics” which are tests that can only be applied to very special grains of any mineral.very special grains of any mineral.

Details are easy to see in thin Details are easy to see in thin section. Cleavages at 60/120 in section. Cleavages at 60/120 in

hornblendehornblende

Minor alteration of cordierite Minor alteration of cordierite around the edge (S-type around the edge (S-type

granite)granite)

Graphic or runic intergrowth Graphic or runic intergrowth of quartz and orthoclaseof quartz and orthoclase

Chromite-rich layer in Chromite-rich layer in Bushveldt Layered GabbroBushveldt Layered Gabbro

Fusilinid fossils in limestoneFusilinid fossils in limestone

Quench olivine spikes in slag. Quench olivine spikes in slag. They also occur in komatiitesThey also occur in komatiites

Olivine in mantle rock (high Olivine in mantle rock (high temperature metamorphic rock) temperature metamorphic rock)

is polygonalis polygonal

Another high temperature Another high temperature metamorphic rock (near the metamorphic rock (near the

melting temp). It is glacier ice!melting temp). It is glacier ice!

Quartz-biotite schist with shape Quartz-biotite schist with shape of quartz controlled by the of quartz controlled by the

aligned mica.aligned mica.

Quartz crystals in a vein Quartz crystals in a vein showing crystal faces and showing crystal faces and

growth zones.growth zones.

Spectacular compositional Spectacular compositional zoning in igneous plagioclasezoning in igneous plagioclase

Zoning and multiple twinning Zoning and multiple twinning in plagioclase phenocrysts in in plagioclase phenocrysts in

tuff.tuff.

Sanidine phenocrysts in Sanidine phenocrysts in trachytetrachyte

Simple twin in orthoclase which Simple twin in orthoclase which also shows perthite exsolution also shows perthite exsolution

lamellaelamellae

Microcline with perthite Microcline with perthite exsolution lamellaeexsolution lamellae

Myrmekite (intergrowth of Myrmekite (intergrowth of plagioclase and quartz) common plagioclase and quartz) common

in slightly deformed rocks.in slightly deformed rocks.

Rim of plagioclase around Rim of plagioclase around orthoclase phenocryst (rapakivi)orthoclase phenocryst (rapakivi)

Ophitic microstructure (dolerite) Ophitic microstructure (dolerite) plagioclase is included in large plagioclase is included in large

pyroxene crystals.pyroxene crystals.

Close up of Close up of plagioclase plagioclase

partly partly included in included in pyroxene in pyroxene in

ophitic ophitic dolerite.dolerite.

Top is PPL,Top is PPL,bottom is XPbottom is XP

Eclogite (ultra high grade meta-Eclogite (ultra high grade meta-basalt) with pyroxene and garnet basalt) with pyroxene and garnet

(black)(black)

Radiating Radiating aggregate of aggregate of pyroxene in pyroxene in granite with granite with hornblende hornblende forming a forming a

rim around rim around the the

pyroxene.pyroxene.