I’m going to aim for getting this done while its still Tuesday locally, but things around here have been a bit crazy due to the rather high snow amounts that fell Sunday / Monday.
This week is also a bit different in that we’re going to be looking at two minerals instead of one and both minerals are opaques. Opaque minerals absorb all of the light that is coming up from the light source below and appear black both in PPL and XPL. Sometimes, we can tell what they are (or at least have a good guess) based on crystal habit. But usually, its not obvious and we need to use either reflected light microscopy or some more serious analytical technique (SEM, EMP, etc.).
PPL of a hornblende gabbro; http://www.jeffreycreid.com/petrography/pet_igneous.html
same hornblende gabbro in XPL; http://www.jeffreycreid.com/petrography/pet_igneous.html
I haven’t gotten to reflected light microscopy yet in my series on how to use the microscope (how light behaves & PPL are up), but its coming eventually. If you’re not up on your reflected light techniques, don’t worry – you’re not alone. Reflected light is frequently not even taught in normal mineralogy courses (sorry to out you Kim, but I don’t remember it when I took min & no professor I’ve worked with as a TA taught it). So unless your prof had a love of economic mineralogy, you may not have spent time looking at opaques to discern what they are.
Magnetite in reflected light: pale yellow to light-brown to grey in color (color variation is very minute in reflected light).
Chalcopyrite (yellow, centre) is intergrown with euhedral pyrite (light yellow-white, high reflectance, bottom left) and magnetite (brown-grey, top centre) which carries chalcopyrite inclusions. Curved laths of molybdenite (right) show strong bireflectance and reflection pleochroism (light grey to dark brown-grey) and also carry chalcopyrite inclusions (top right). http://www.smenet.org/opaque-ore/15f.jpg
A euhedral chromite (green-grey, centre) is fractured (black), the fractures are less well-developed in the magnetite (light brown) overgrowth. http://www.smenet.org/opaque-ore/07f.jpg
Hematite in reflected light: white to white-blue to blue in color. Difficult to differentiate from magnetite. Frequently present as an alteration product, so present as rims or within fractures that may be narrow & difficult to ID.
The ilmenite host (pink-brown) has fine- and coarse-grained haematite (white, bottom left) exsolution bodies oriented along the (0001) direction of the ilmenite. The coarse-grained ilmenohaematite has fine ilmenite exsolution within it. Rutile (lilac-grey, left) is oriented perpendicularly to the crystallographic orientation of the main exsolution bodies. http://www.smenet.org/opaque-ore/08f.jpg
Tabular haematite (blue) and subhedral pyrite (pale yellow-white, centre bottom) are intergrown with chalcopyrite (yellow). The relief difference between the harder haematite and softer chalcopyrite can be seen clearly (centre right). http://www.smenet.org/opaque-ore/21b.jpg
Two detrital grains of ilmenohaematite (centre, right centre) comprise exsolution bodies of ilmenite (brown) in a haematite host (white). The right-hand ilmenohaematite has an ilmenite-free haematite overgrowth (left margin). Very poorly crystalline haematite (green-white, top centre) is poorly polished and is a pseudomorph after an original ferromagnesium mineral, whereas a single crystal of haematite (top left) is well polished.http://www.smenet.org/opaque-ore/42f.jpg
Magnetite & hematite together:
- An andesite contains subhedral to euhedral magnetite crystals (light brown, centre) that are partially oxidized to haematite (blue-white, centre left). The alteration proceeds along crystal edges and along curved fractures. (Very subtle color difference.) http://www.smenet.org/opaque-ore/14a.jpg
- Magnetite (pink-brown, right) is coarse-grained and is intergrown with haematite (white, centre), which encloses an euhedral basal section of quartz (dark grey, centre bottom). http://www.smenet.org/opaque-ore/64d.jpg
I’ll hit the other properties of hematite and magnetite later this week, which will include the environments that we tend to find them in.
Life in Plane Light 
