Rocks Under a Microscope: A Web Tour and Exercise, Part One

Diabase (magnification 40X) is an Intrusive Igneous Rock, which means a magma formed deep in the Earth was emplaced in the crust and cooled slowly. Evidence for slow cooling can be seen in the crossed polars picture, in which Plagioclase grains (white grains with dark striping due to Crystal Twinning) are partly to completely enclosed by Pyroxene grains (all the colored grains: yellow, orange, red, and blue). This is called an Ophitic texture, and indicates that the minerals crystallized out of the magma to in a certain order: first plagioclase, then pyroxene.

Question 1: Relief is a term used to describe how prominent a mineral appears in a thin section in plain light. Generally, a mineral which looks grayer in plane polarized light has higher relief. Look at the plane polarized light image of the Diabase on the left, and compare it to the crossed polars image (same sample!) on the right. Then tell me which mineral in the rock (pyroxene or plagioclase) has higher relief?

These are photographs of a thin section of Granite (magnification 10X), perhaps the most common Igneous Rock we encounter. By comparing the plane polarized light (Left) and crossed polarizer (Right) images, we can see that there are three minerals in this granite: Biotite (brown grains in plain light and pinkish brown with crossed polars - the brown color is due to a property called Pleochroism, typical of minerals that are rich in iron); Quartz (gray to white, featureless grains in crossed polars); and Feldspar (gray to white grains under crossed polars, with banded or plaid-like striping (Twinning), and sometimes flecked with fine sparkly grains of Sericite (an alteration mineral due to weathering).

Question 2: By looking at these pictures, try and estimate which of the three major minerals named above - Biotite, Quartz, or Feldspar - is the most abundant in this Granite. (hint - look at BOTH images to make your "guesstimate"!)

Question 3: Both Granite and Diabase are Intrusive igneous rocks, which means they both cooled slowly deep in the crust. By looking at the pictures, can you tell which mineral in the Granite crystallized first? (If you can't, what might that say about the way that a Granite crystallizes?).

This is a 10X magnification picture of a Basalt, a common Volcanic igneous rock. Volcanic rocks form from Lavas which erupt at the surface and cool rapidly. The lava that erupted to make this basalt is of about the same composition as the magma that cooled to make the Diabase, so the crystallizing minerals are similar. This texture, consisting of randomly oriented, small grains, is called Felty, because it sorta looks like felt.

Question 4: Can you identify the Plagioclase feldspar and Pyroxene grains in this Basalt? If you have problems, refer to the higher power pictures below.

The black grains visible in the 50x plain light image of Basalt are opaque oxide grains, probably Magnetite. Other parts of the thin section, which are clear in plain light but black with the polarizers crossed, are natural volcanic glass (what's left of the lava, now frozen solid like windowpane glass)

All of the above rocks display different varieties of Crystalline textures, which is to say that these rocks formed via the growth of crystals. Igneous and Metamorphic rocks all typically preserve Crystalline textures. However, Sedimentary Rocks form at the Earth's surface from the weathered fragments of other rocks. These rocks show Clastic textures, in which the fragments of minerals and rocks are either squashed together, or held together by some sort of cement.

This 10x magnification photograph is a thin section of an Arkosic Sandstone. Note that it is made of of rounded grains, cemented together by a fine matrix of opaque minerals (the red-brown color of the rock suggests these opaques may be some sort of iron oxide mineral). Arkosic Sandstones contain high abundances of Feldspar as well as Quartz. Because of this, they are believed to represent the weathered products of a Granite.

Question 5: Try to distinguish and describe the quartz and feldspar grains (This may be hard, because like all sediments these grains are heavily weathered, but give it a go! Look back at the Granite slide - what features distinguish Quartz and Feldspar there?).

Whe quartz sand is transported into the oceans and lithified as a sedimentary rock, it can become a Glauconitic Sandstone, such as is pictured in these 10X photomicrographs. Glauconite is a green, mica-like mineral that only forms in marine sediments. Another hint that this rock was made in an ocean is the fact that it is cemented together by Calcite, which can precipitate from seawater. Calcite has such high birefringence that its color with crossed polars is just a sparkle of pink and green flashes.

In this 10X thin section photograph, we have a Calcareous Sandstone, in which angular quartz grains are cemented together by calcite (here showing its distinctive rainbow hues in crossed polars), along with various pieces of calcite of different origins.

Question 6: Describe the shapes of the grains in the three sandstone thin sections above. Given that grains are rounded by tumbling on a river bottom (or like processes), a process called Maturation which rock would you say is made of more "mature" grains?

Question 7: What do you think the long, arcuate grains of calcite in the Calcareous Sandstone slide might be? (Hint: what does a clam shell look like edge-on?)

Link back to Rocks and Meteorites Introductory Page.

Link to Page on Meteorites under a Microscope.

Link back to the Moons, Planets... Homepage

Link back to Jeff Ryan's Homepage