AboutC. Robert Reszka, Jr. Expertise I can answer any general geology question (rocks, minerals, stratigraphy, geomorphology etc.). My expertise is in the geology of the Michigan Basin, PreCambrian, Paleozoic and Recent. I can answer questions concerning mining and petroleum exploration and production and the laws concerning those activities. I can also answer questions concerning stratigraphy of the Michigan Basin. I will also answer questions about mineral and rock collecting in the Basin. I won`t be able to answer many specific questions on hydrology, geophysics or geochemistry. I may be able to answer very general questions in those venues.
Experience I have been working for the State of Michigan for 24 years as a Geologist and a Resource Analyst. I have experience with Subsurface Geology and Petroleum Geology, mining in Michigan, and Sand Dune Mining and Protection issues.
Organizations Michigan Basin Geological Society
Publications Decade of North American Geology.
Bedrock Geology of Michigan
Question I'm always difficult to tell the difference between biotite amphibole and pyroxene from a rock sample in the field, even with the help of hand lens. Would you give me some tips on that, please?
Answer Hi Ben,
This is a difficult question and, unfortunately the best way to tell the difference is just through looking at as many as you can find. But that doesn't help to start with.
They are all very common, silicate, rock-forming minerals. Biotite is a phyllosilicate and belongs to the mica group. While amphiboles and pyroxenes are both inosilicates and are both mineral groups themselves.
Biotite should be the easiest to tell apart. It is usually black with some variation; brownish-black, greenish-black and so on. It has perfect cleavage in one direction so it forms perfect thin plates. The plates are brittle but can be split off thin enough to be translucent. It can form monoclinic crystals but you will see striations on the crystal edges. Those striations are the cleavage planes. So... biotite will be shiny, black, have flat faces, split into very thin sheets that can be translucent, and the sides of the crystals will have striations that are parallel to the crystal face.
There are many members of the amphibole and pyroxene groups but I will use common examples of each. I'll start with hornblende (amphibole).
It's dark and can be black, but there is almost always a greenish hue to the black. It is noticeable. It also has perfect cleavage, but in two directions. It will look like a diamond if it's cleaved and will be "pointy". The crystals are usually long and look platy but mostly they will look fibrous rather than glassy or metallic. Since the cleavage planes run more along the length of the crystals, a specimen will shatter more often if you try to free it from it's parent rock with a chisel. You have to get the whole crystal or you risk destroying it.
Augite is a very common pyroxene. It can be dark green but is brighter than hornblende. It has vitreous or almost metallic luster. It has a greenish streak. Both the others have a colorless streak. It is monoclinic as well but it cleaves at nearly right angles; so no diamond shape. It usually forms prismatic crystals and are rather short.
So....Biotite; thick, wide, flat topped, black crystals that form perfect flat sheets when cleaved, and have distinct striations parallel to cleavage.
Hornblende (amphibole); taller crystals and flatter, fibrous looking, definite greenish cast to the black color, cleaves in two directions (not one like biotite), and when it cleaves it will look like a diamond. Fractures more easily than biotite and can look less lustrous and more amorphous because of the fracture characteristics.
Augite (pyroxene); shiny, vitreous, green to dark green, cleavage at almost right angles (not one as in biotite or sharply as hornblende), has a greenish streak while hornblende and biotite have a colorless streak, generally the crystal will be smaller than biotite or hornblende.
Finally then; black and splits into thin translucent sheets it's biotite. Amorphous, fibrous, black, fractured it's probably hornblende. Shiny, glassy, green it's probably augite.
Of course this all presupposes they are in the correct rocks and geology for these minerals.
