Mafic Igneous Minerals

1. Mafic Igneous Minerals Olivine, Pyroxene, Amphiboles, Micas

2. So far… • What are the most important mineral (groups) that we have viewed so far • Quartz (not a group but hey its everywhere) • Feldspar • Plagioclase • Alkali Feldspar • 3 of the 6 most important minerals you will ever see

3. What are the other 3 • Pyroxene • Amphiboles • Micas • Olivine, we had to stick it somewhere even though it is not as common as the ones above

4. Olivine • It is actually a group of minerals • Lumped together to be called Olivine • End members • Fayalite (Iron Silicate) • Generally darker because it has iron • Heavier, higher index of refraction • Forsterite (Magnesium Silicate) • In general these minerals are hard to differentiate, so we lump them into Olivine

5. Olivine • Nesosilicate • Stronger bonds • Close packing of ions • The Silicate Tetrahedrons are unbound to other tetrahedrons • The Tetrahedrons are bound by interstitial ions What does that mean Higher index of refraction Higher hardness

6. Micas • Phyllosilicates • Most of these are flat and platy • Lots of smooth and flat ones, also powders

7. Muscovite and Biotite • Thin platy minerals that we have seen before (even on the quiz) • Biotite is generally brown to black • Muscovite is white, silver, yellow, green… • Glauconite is one of the important ones. • Come back next week and tell me why glauconite is important. I’ll ask you at the start of class. • Think of formation conditions.

8. Amphiboles • Chain silicates (single or double) • Inosilicates • Dark minerals in hand sample • Oblique cleavage 120º/60º • Generally have strong pleochroism • Hornblende • Glaucophane tetrahedrons share two oxygens with two other tetrahedrons two single chains lie side by side so that all the right sided tetrahedrons of the left chain are linked by an oxygen to the left sided tetrahedrons of the right chain

9. Amphiboles • Constituent of igneous rocks • Often making up the dark colored fraction of minerals in an igneous rock (sounds familiar) • Hornblende is one common variety • You don't generally find amphiboles and pyroxenes together • The presence of amphiboles indicates high pressure and the presence of fluorine and gaseous water (hence the hydroxyls) • High pressure conditions suitable for the formation of amphiboles occur at great depth or under a tectonic load.

10. Pyroxenes • Chain Silicates • Tell them apart in hand sample from amphibole by their 90º Cleavage • Generally form in high temperature magmas with a lack of water. • If there is water amphiboles would most likely form

11. Pyroxenes SiO6 • Pyroxenes are an important constituent in igneous rocks • Most often confused with amphiboles which are similar in color and occurrence • Distinguished by their cleavage angles. A mnemonic to use: the "x" in pyroxene indicates cross-wise (square) cleavage. • Pyroxenes are readily altered to other minerals such as calcite or limonite

12. Pyroxene • Augite • Wollastonite • Diopside • Enstatite • Hypersthene • This contains both ortho And clino

13. Microscope

14. Parallel and inclined extinction • Inclined--mineral is extinct when the crystal face of cleavage face forms an angle with the crosshairs of the microscope. The extinction angle is a diagnostic property of minerals. Hornblende is an example. All biaxial minerals excluding orthorhombic minerals have inclined extinction (clinopyroxenes). • Parallel--mineral is extinct when the crystal face or cleavage face is parallel to the crosshairs of microscope. An example is biotite. All uniaxial minerals have parallel extinction, but so do orthorhombic biaxial minerals (olivine, orth-pyroxenes).

15. Olivine • Moderately high relief • Clear, occasionally very light yellowish or greenish • No cleavage • Commonly rimmed with greenish alteration products (A) • Internal fracturing of grains common (B) • Never occurs with quartz • Skittles-taste the rainbow

16. Orthopyroxene • All have low birefringence • (first order red maximum) • parallel extinction, • 87 degree cleavage. • Pale green, pale red, or pale purple pleochroism occurs in some grains. • Distinguished from clinopyroxene by low order interference colors and • parallel extinction • Notice the extinction

17. Clinopyroxene • Generally difficult to distinguish between individuals in this group optically. • All are • moderate relief • up to second order birefringence colors, • 87 degree cleavage • and inclined extinction

18. Amphiboles • Cleave angle ~120/60 • Green brown pleochroism • Usually 2nd Order

19. Muscovite • Birds eye extinction • Gives the mineral a pebbly appearance as it passes into extinction. This is caused when the grinding tools used to create petrographic slides of precise widths alter the alignment of the previously perfect basal cleavage planes which split micas up into its characteristic thin sheets. The resulting, slightly roughened surface alters the extinction angle of various parts of the crystal lattice, leading to this type of extinction • high birefringence • parallel extinction • and excellent basal cleavage

20. Biotite • strong pleochroism in brown, reddish brown, and green • Hornblende has similar pleochroic colors and can be confused with biotite, hornblende has inclined extinction, not parallel extinction(Hornblende also has internal cleavage ~120/60). • Little black dots are radiation damage

21. Bowens Reaction Series

22. Goldich Stability Series