E. AMPHIBOLES:

BH250-8, BH250-24, BH-250-23, BH250-
The amphiboles, like the pyroxenes, are chain silicates. But in amphiboles a double chain, rather than a single chain, of silica tetrahedran form the basic structural unit. The cleavage planes that break across the edges of these wider double chains intersect at angles of 56 and 124 degrees. This characteristic amphibole cleavage shows up well in thin sections. They are also a complex group of Na Ca, Mg, Fe, Al, silicates and have a general formula that is expressed as follows:

W0 -1X2-3 Y5 Z8 O 22( OH)
Where
W= Na
X= Ca, Na, K, Mn,
Y= Mg, Fe+2, Fe+3, Al, Ti, Mn, Cr, Li, Zn,
Z= Si, Al,

In addition the (OH)- ion can be replaced by F-, Cl-, and sometimes O-2
The amphiboles can be placed into three main subgroups based on Na and Ca content as follows:
a). Anthophyllite-cummingtonite subgroup (Ca + Na = 0 approx.)
b). Calcium amphibole subgroup (Ca >> Na)
c) Alkali amphibole subgroup (Na << Ca)

When distinguishing the amphiboles from the pyroxenes in thin section keep the following in mind:
a). Amphiboles have cleavage planes at 56 and 124 degrees.
b). Amphibole crystals tend to be long and narrow while pyroxene crystals tend to be short and stubby.
c). Amphiboles are often strongly colored in contrast to generally weakly colored pyroxenes.
d). In the monoclinic amphiboles the extinction angle is generally 10-20o while in the monoclinic pyroxenes it is generally 30 - 50o.

1. Common Hornblende BH-250-8
hornblende is the commonest of amphiboles and is present in volcanic rocks, plutonic rocks, medium grades metamorphic rocks, and detrital sedimentary rocks. A number of mineral end-member are included under the umbrella term "hornblende". This mineral is distinguished by its light to darker green ( and sometimes brown) colors, low to moderate extinction angles, and variable, but generally high 2V (negative). Actinolite (in the Tremolite- Ferroactinolite series ) is similar but mostly of paler shades of green.

A number of slides show good example of hornblende in various types of rocks. Look at these slides and become familiar with the appearance of this mineral in thin sections. While you are looking at these slides be sure to note the other types of minerals present, many of which you are now familiar with.

Many slides show good Euhederal hornblende. Draw the following three sketches of this hornblende as you observe it in this slide.

a). Perpendicular to c-axis. (as best as you can find)

b). Centered Bxa.

Label the principal optic directions in each of these views, label their color, show the angle between cleavages and between optic directions and cleavages and finally give Miller indices for all cleavages and crystal faces shown.

Next, pick one of these slides and work out the optical properties of the hornblende in detail.
a) 2V,
b) Extinction angle
c) Pleochroic formula (i.e., colors of the three optic directions).
d) Birefringence
d) Sign of elongation
f) Cleavage angles

Explain what sections you used to determine the pleochroic formula and tell what section you used to measure extinction angles.

2.Basaltic hornblende
Basaltic hornblende (also termed as Oxyhornblende) seems no more than oxidized common hornblende produced either naturally in volcanic rocks, or by artificial heating. It may represent virtually any hornblende composition where appreciable Fe+2 has been oxidized to Fe+3 with accompanying replacement of O2- for (OH)-. Slide 83 shows basaltic hornblende, look at this mineral and other minerals on the slide and answer the following questions:

a). What property of the basaltic hornblende best indicates it is an amphibole?

b). What color do you see for the X vibration direction (i.e alpha).

3. Tremolite. BH250-31
In the tremolite slides, tremolite is the colorless mineral with second order interference colors. Look at this slide and answer the following :

a). What is the 2V and extinction angle of tremolite in this slides?

b). what kind of rock is this (Igneous, metamorphic or sedimentary)? your answer may vary with the slides that you are examining.

4. Glaucophane. BH250-23
Glaucophane is a distinctive blue amphibole that occurs in low temperature, high pressure metamorphic rocks of the blue schist facies. Look at the glaucophane slides and become familiar with this mineral. In addition look at the other minerals in these slides, many of which are characteristic of blue schist rocks.
a). Is glaucophane length fast or length slow?

b). What is the color of the fast ray in sections that show maximum extinction angles?

5. Anthophyllite BH250-
There are five slides labeled " Anthophyllite ". They are all cut from the same rock. Look at one of these slides and answer the following:

a) What are the maximum interference colors, extinction angle, 2V and pleochroic formula for the anthophyllite?

b). What other minerals are abundant in this slides?

c). What minor minerals can you identify in this slides?