GEOS 254: INTRODUCTION  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 and infer the history of the rock (e.g. has the rock been deformed? Did the deformation occur before, during or after the growth of the minerals?)

Rock Microstructure  Microsturucture is the term used to cover the shape of the components of the rock (minerals, volcanic glass, fossils). In older books “texture” is used to mean the same thing.  The microstructure of igneous, metamorphic and sedimentary rocks are very different and are generally more diagnostic than the list of minerals present in defining the rock type.

What do you need to identify rocks and minerals? 1. A “drivers license” for the microscope 2. Access to a list of mineral properties 3. Information on which rock types each mineral occurs. 4. A knowledge of rock microstructures and their significance.

Sample preparation  Thin sections are slices of rock polished on one side, then stuck to a glass slide with araldite and then ground down to 30 microns or 3 thou of an inch.  So the light is not diffused by the frosted upper surface, a very thin cover-glass is glued onto the upper surface. A much more expensive alternative is to polish the upper surface and this is the method that must be used for mineral analysis using the Electron Microprobe.

Remember the Rules  Always start holding the slide up to the light. How many minerals/ What grainsize/ Is the slide homogeneous?  Then go to LOW POWER (the slide must be right- way-up) in plane polarised light (natural colours)  Then go to crossed polars where the “interference colours” show twinning, zoning etc and generally distinguish quartz and feldspars having “low” colours from most grains of olivine and pyroxene that have “high” colours.  High power is generally only used for “conoscopic optics” which are tests that can only be applied to very special grains of any mineral.

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

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

Graphic or runic intergrowth of quartz and orthoclase

Chromite-rich layer in Bushveldt Layered Gabbro

Fusilinid fossils in limestone

Quench olivine spikes in slag. They also occur in komatiites

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

Another high temperature metamorphic rock (near the melting temp). It is glacier ice!

Quartz-biotite schist with shape of quartz controlled by the aligned mica.

Quartz crystals in a vein showing crystal faces and growth zones.

Spectacular compositional zoning in igneous plagioclase

Zoning and multiple twinning in plagioclase phenocrysts in tuff.

Sanidine phenocrysts in trachyte

Simple twin in orthoclase which also shows perthite exsolution lamellae

Microcline with perthite exsolution lamellae

Myrmekite (intergrowth of plagioclase and quartz) common in slightly deformed rocks.

Rim of plagioclase around orthoclase phenocryst (rapakivi)

Ophitic microstructure (dolerite) plagioclase is included in large pyroxene crystals.

Close up of plagioclase partly included in pyroxene in ophitic dolerite. Top is PPL, bottom is XP

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

Radiating aggregate of pyroxene in granite with hornblende forming a rim around the pyroxene.