GeologyRocks

I am so sorry. I don't know what just happened with the post. Please disregard the first paragraph. it is simply this: "How can I tell whether something that appears 'black' in hand sample of igneous or metamorphic rock is either hornblende, biotite mica or pyroxene?"

If you take it to your science lab they could get multiple xrays and could most likely the machine could tell you what is what. I know SFA has a radiaton room where they keep theres

Hello Calebt36, and welcome to GR.

Your question could form the basis of a several hour lecture since it is not terribly specific.  Also you have picked some difficult ones.

HORNBLENDE. The name referes to a series of minerals which are notoriously difficult to identify.   They is usually black to dark green. Monoclinic.  Crystals  tend to be short, 'chunky' prismatic, as well as long and thin (acicular [needle like]), and can have a hexagonal cross section, although they are rarely symmetrical.    They rarely have good termination, but they appear to be two faces of adome, but in reality are two of a four faced prism.  However it can also be granular or massive.  The streak is brown to grey and the SG 2.9 to 3.4.  Really the best field indicators are the cross sectional crystal habit, the colour and the cleavage which is imperfect at 56 and 124 degrees.

PYROXENE. Again this is complex and unspecific. The name pyroxene is a group of inosilicate minerals, usually tetrahedrons (augite, aegerine, clinoenstatite, diopside, essenite, hedenburgite, hypersthene, jadite, jervisite, johannesite, omphrasite and spodumene -  plus some others).  They are simply early forming crystals before a magma erupts.  the chemical signature of the magma dictate which minerals will form of course.   No definite field indicators - you simply have to know.

MICA.  Again there are a number of these ( some varieties are - biotite, muscovite, fuscite, phogotite etc.) They form 'books' - that is a number of very thin sheets, often in a rough hexagonal shape. They all fall into the monoclinic system with a pearly, metallic lustre (makes rock sparkle!).  The colour is dependant on the variety, but all have a hardness of around 3 and have one plane of cleavage.  As they are flexible they can bend a long way before they fracture.  The streak is always white no matter which variety. Easily identified in the field, and only a relatively few varieties to learn.

Does that help?

 John

“Civilisation exists by geological consent, subject to change without notice.” -  Will Durant

It does!
This post was really humbling.
I have a lot of knowledge to look forward.

Also, in the field you wouldn't be able to identify most black minerals (apart from mica) in, say, granite, which is why geologists tend to sidestep the issue and just refer to them all as "mafic", which means black.  It's quite a good cop-out and it makes it sound as if you know what you're talking about.

Nice one Gus!  Beautifully simplified.

John

“Civilisation exists by geological consent, subject to change without notice.” -  Will Durant

It is merely experience as well as a deep interest.  Just keep banging the rocks together.  One day it will gel.  Took me 74 1/2 years, a lot of field trips and many many discussions in pubs....which is the real secret.

John

“Civilisation exists by geological consent, subject to change without notice.” -  Will Durant

Oh Gus how I wish you were my field work lecturer.... We were MADE to identify each of these minerals in the field we'd loose marks by saying "mafic minerals" as we weren't putting the effort in . It can actually be pretty easy if you know what rock you;re looking at, but it did take until my masters year field trip before I could. Now  I wonder why I had such a problem in the first years.

That being said I did pick up some beautiful single pyroxene crystals half way Mt Etna last year, I'll see if I can dig them out and post some pics.

My tips for hand specimen would be harndess/cleavage/colour/streak (if you have a scratch plate available) 

Leah 

 This is the difference between having had a formal education in geology and (like me) virtually none whatsoever.  I've never used hardness/streak/ lab tests to identify minerals but it's rare I can't identify hand specimens in the field.  When you're in the situation where you have to assess the economic value of a lode in situ by eye alone you soon tend to ignore whether the rock around it contains augite or hornblende as it really doesn't matter in those circumstances.

Distinguishing biotite is easy - 1 perfect cleavage.  Amphibole and pyroxene both have 2 cleavages.  In pyroxenes, the 2 cleavages intersect at right angles so the surface of the minerals usually look like microscopic lego blocks stuck together.  The exception is important - clinopyroxenes from mantle rocks that sometimes contain diamonds!

There are many circumstances where knowing whether one has biotite, amphibiole or pyroxene is essential to determining the economic (or scientific) worth of a rock. Ignoring data is an easy way to lose your money.