CEE 437 – Rock and Mineral Lab

© Thomas W. Doe, 2009

Part 1. Minerals

The minerals section of this laboratory includes both a collection of the rock forming minerals as well as

some other minerals that illustrate concepts of crystal form, cleavage, and other mineral properties.

There are several minerals here just for general inspection. I will give you some general characteristics

and see if you can pick out which is which.

Calcite- the minerals that makes up limestones and marbles and is a very important vein and fracture

filling as it is very mobile in groundwater. In large crystals calcite shows it rhombic cleavage, however

it's crystal growth form is a hexagonal pyramidal with a form commonly called “dogtooth spar”.

Take a calcite rhomb and set it on a piece of paper with a pencil dot. Rotate the crystal. Sketch what

you see.

What properties of the crystal does this illustrate? How does the demonstrate anisotropy?

Fluorite is a cubic mineral that cleaves along an octahedron plane, that is, it cuts off the corners of the

cubes. It can have many different colors but is commonly violet or purple.

Sulfides -- I have included some sulfides including a massive sample of sphalerite ore (ZnS), galena (PbS),

which is cubic and silver-metallic in color, and pyrite, Fe2S). Pyrite is a very important and common

mineral that can occur in both igneous and sedimentary environments. It particularly occurs whenever

there are reducing conditions and the iron bonds with sulfur under anoxic conditions.

Evaporite Minerals – Halite, Sylvite, Gypsum

Halite (NaCl salt) is here as a core sample. It has a cubic crystal structure as does sylvite with very good

cleavages on the cubic faces. Note the fluid inclusions (left over water) in the salt. What does their

form tell you about the salt crystal structure?

Note the clear crystal of gypsum. Note the cleavages. What is the angle between the cleavages? What

does this tell you about the crystal symmetry compared with halite?

Silicates - Rock-forming Minerals

Among the rock forming minerals, I have the following samples:

Quartz - I have several prisms of quartz showing their hexagonal form and well formed pyramid

terminations.

Do you see a cleavage plane?

How would the lack of cleavage relate to quartz’s crystal structure?

Feldspars – Start with the single crystal of milky-white potassium feldspar (orthoclase). This is the

feldspar typical of granitic rocks. The crystal is a bit rough (it weathered out of a coarse-grained rock

and has broken on cleavage planes.

How many directions of cleavage do you see?

Next there is a polished sample of a Ca-plagioclase. This is more typical of mafic, oceanic rocks like

gabbro or basalt. Try to identify the individual crystals present.

Note the striations. These are “twinning”. Crystal twinning occurs when two separate crystals share

some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two

separate crystals in a variety of specific configurations

Left – Plagioclase twin, right staurolite twin

Ferro-magnesian or Mafic Minerals

Olivine - small green crystals, isolated silica tetrahedra structure (no bonds between tetrahedra)

important component of mantle and oceanic rocks that are high in Fe and Mg. This mineral has a cubic

structure

Amphibole and Pyroxene – There are no good samples of pyroxene in this lab. The amphibole here is in

the form of amphibolite, Note the form of the crystals -- black, elongate prisms. Pyroxene looks similar

but is less elongate. Look for amphibole crystals especially in the volcanic rocks. Recall these are the

main ferro-magnesian minerals in igneous rocks. Pyroxene has single chains of silica tetrahedral and is

higher temperature and higher Fe-Mg content. It will be in more oceanic rocks. Amphibole is a double

chain, lower in melting point and FeMg content and appears in intermediate to more silicic (also acidic)

igneous rocks of continents and island arcs.

Mica

Note the samples of muscovite mica. Biotite is the more ferro-magnesian form of the mineral . These

samples show clearly the sheet silicate structure that micas and clays have in common. It is closely

related (maybe even same family) as clays, but potassium bonds between the sheets make it a bit

stronger. Micas appear as important components in igneous rocks, but they are the key minerals in

rocks formed by metamorphism of clay-rich rocks – shales and claystones. Not surprisingly micas form

from recrystallization of clays and degenerate to clays during weathering. Consider what effect having

aligned micas, as occurs in schists, slates, and phyllites does to rock property anisotropy!

Note silica tetrahedral layer (silica sheet) and octahedral (Al or Mg and O) layers.

Look at the figure of silicate structures. What symmetry group would you expect for these minerals?

Recognizing Minerals in a Rock

Included in this minerals section I have put some samples of pegmatite. Pegmatite is a rock that forms

from the very last portions of the granite melt to freeze. At this stage in the freezing process the melt is

very rich in water and the ions in the melt are very mobile resulting in unusually large crystals. In these

pegmatite samples you can see very clearly the quartz, feldspar, and micas that make up the rock.

Note the difference in appearance between the feldspar and the quartz. The quartz is glassy and has no

cleavage, while the feldspar is milky white and has a very clear cleavage which presents itself through

planar surfaces that reflect light readily. The micas are the flaky, light-colored mineral in the rock.

Part 2. Igneous Rocks

The classification of igneous rocks is based on crystal size and mineral composition.

Aphanitic and Phaneritic are simply fancy terms for visible by naked eye or not. This largely divides

slow-cooling rocks, where the crystals have time to grow from magma, from fast cooling rocks which are

quenched in lavas. Magma a term for a molten rock below the surface, while lava is what we call it

when it on the surface. You can also get phaneritic rocks in the magma-filled fractures that feed (or try

to if they don’t reach the surface) called dikes.

More commonly seen

altered to serpentine

Acidic of felsic Intermediate Basic or Mafic Ultramafic

Continents Intermediate Oceanic Crust Mantle

When a magma comes out as a lava, it may have started crystallizing, so one often sees “phanereritic”

crystals in a mass that is “aphanitic”. The extrusive origin of the rock is determined by the fine-grained,

aphanitic mass.

In addition to these rocks on this chart, and as described in Waltham, note that there is an important

group of volcanic rocks that are deposited from material that “blows” rather than “flows” out of

volcanoes. These rocks are called “pyroclastic” or broken up stuff from a fiery source. High silica

magma has much lower viscosity than low silica magma. Basalt flows travel long distances as liquid. The

Miocene flood basalts (10-16 My ago) of eastern Washington traveled from vents near Walla Walla to

the Pacific, and Hawaiian volcanoes have low cone angles. The high viscosity high silica magmas also

have high gas and water contents, and tend to explode into ash (volcanic ash, non-welded tuff) and rock

fragments or flow as glowing, glassy clouds of gas charges melt (nuées ardentes or glowing clouds –

welded tuff and pumice). Hence rhyolite is not a very common rock and much of what a volcano like

Composite volcanoes (like the Cascade volcanoes) are mixes of usually andesitic lava, which are quite

competent, with pyroclastics that have a very big range of properties from competent to very weak.

These can pose big engineering challenges.

Basalts have their own internal structures – some oceanic basalts form blobs or pillows. Surface-based

lavas have a layered structure of an upper and lower chill zone that can be very rubbly and scoriaceous

(scoria is a frothy material the freezes while it is flowing) and a central zone with various cooling

fractures. The rubbly flow tops are prolific aquifers in the eastern part of the state with big porosities

and permeabilities.

Pick out the following:

Scoriaceous flow top

Consider how permeable this could be!

Basalt with olivine crystals

Compare to olivine in mineral section – why might this be more brown than green?

Basalt Dike Cutting Andesite

Note chill zone!

Basalt Cobble

This is from glacial till, and probably came from Canada. Note zonations of light colored plagioclase

crystals. This sometimes indicates changing melt composition while these crystals were growing.

Pick out the Andesite Samples

These are porphyritic (big crystals in smaller matrix) and from Mt. Rainier. Make a sketch showing

plagioclase and amphibole crystals.

Pyroclastics

Identify pumice and welded tuff samples (one with funny channels on the fracture face). Look carefully

and note flow layering and compressed glassy blobs. This is from the proposed Yucca Mountain,

Nevada, radioactive waste disposal site.

Plutonic Rocks

The plutonic rock classification above is rough field approximation. For both plutonics and volcanics

there are many further subdivisions which can be scary! Most of these reflect variations in the quartz

content and plagioclase type. I am showing them only because field geology reports and maps may use

them. For the most part these distinctions don’t affect engineering properties.

I have a range of rocks here. I will describe them briefly.

The ultramafics are a medium to light green peridotite from France and a serpentine. Peridotite is

olivine and garnet rich – the name peridotite comes from the name for the gem form of olivine –

peridot.

These are rare to see but we will have some like them on the field trip. More often, because they

weather and alter relatively easily, you see a rock called serpentine, which is a hydrothermal alteration.

Serpentine is the green, slightly greasy feeling rock.

Compare likely engineering properties of peridotite and serpentine. Serpentine is found a few places in

the Cascades (it causes stability problems on Blewett Pass along US-97). It is also the state rock of

California.

The diorites are actually rather dark with reddish feldspars. They are from an underground lab in

Sweden. Don’t spend much time on these, but note if they have quartz.

There is also a fine grained gabbro. Pick it out.

I have several granites. Compare the minerals you can see to the pegmatite in the mineral section. One

of these is a thin sheet. This does not reflect the rock structure but is a spall of the intersection of two

underground galleries in highly stressed rock. Most CE problems won’t involve failure of intact hard

rock, but this is in an exception. Why it spall there?

For reference interest, I have added two interesting samples. I have two bags of decomposed granite.

These were intact when I first collected them and have fallen apart since carrying them around. What is

the likely mineral change going on here?

There is also a breccia (rock made of angular broken pieces) made from granite. This is from a fault zone

underground at a hydroelectric development site – the pressure tunnel. This is a piece of what was

producing mode of the water flow in the tunnel. Note the high porosity and consider how permeable it

could be. Also, note the crystals growing in the voids. What is the crystal form? Is it calcite (usually a

good guess)? If not, why not. What else could it be?

Part 3: Sedimentary Rocks

Sedimentary rock are those deposited by/from water (and wind). The big groups are siliciclastics,

carbonates, and evaporites. Siliciclastic simply means broken up stuff made of mainly silica minerals.

Siliciclastics

Classification is rather simple:

Take some time to identify the samples of shale/claystone, conglomerate, sandstone and siltstones.

Note the two samples that have two rock types – sandstone and silty claystone. Look for cross bedding

and make a sketch when you find it. Cross bedding is the formed from the angle-of-repose foreslopes of

ripples and dunes. How does this tell you which side the rock is up? See picture below for help.

Cross beds from ancient sand dunes

There are also some “flames” and small slumps show sediment instability. Make a little sketch. We see

more of these on the field trip in larger scale.

Carbonates

Carbonate rocks are sedimentary rocks made of dominantly calcite and sometimes dolomite (Mg

replaces one of the calciums in the lattice – this happens after deposition). They are formed mainly in

the tropics from shells, coral debris, and other organic material. They are also “clastic” and one can

have lime sands, lime muds, lime breccias, and lime conglomerates.

From the samples of carbonates pick out and admire:

1. A limestone that is a layer of lime mud and well-cemented lime sand – sketch

2. A dark oil saturated limestone that is high porosity and composed of sand size grains

3. An oolite (highly rounded grains that start as sand and take on coatings as they are agitated in

by wave activity

4. A limestone made mainly of oyster shells.

Evaporites

I have a single core of anhydrite here. Not much to talk about but you saw the salt core in the mineral

section. Salt is a relatively low density and very low viscosity rock (all rocks have viscosity but they need

time, temperature, and pressure to flow). Salt is well known for flowing under gravity instability to form

domes and other flow features. Salt domes are common on the Gulf coast and active flow structures

are known in the Rockies. They are important as oil traps, for hosting gas storage caverns, and

radioactive waste disposal in New Mexico.

What kind of permeability would salt have? What does salt being there tell you about access to fresh

water?

Part 4: Metamorphic Rocks

Foliated Rocks – Slate, Phyllite, Schist, Gniess

Metamorphic rocks are those that recrystallize under conditions of temperature and pressure, usually

due to burial. They are classified by the types of mineral/chemistry and the inferred degree of

recrystallization.

• Original Material – sandstone, limestone, shale, basalt)

• Metamorphic Grade (Temperature, Pressure)

• Source of Metamorphism (Regional, Contact)

The sources for metamorphic rocks are mostly sediments/sedimentary rocks. As we discussed in

lecture, these tend to be made of quartz-rich sand and silts and clays. Clay metamorphism mainly

proceeds by recrystallization of clay minerals into their cousins, micas.

These rocks have strong foliation due to preferred orientation of the mica. “Foliation” comes from

“folia” which is Latin for leaves.

What does foliation do to property anisotropy?

The rock types are based on mica crystal size:

Slate –dull color, strong foliation

Phyllite (thin sheets -- the root is the Greek for Leaves -- like phyllo dough) – some mica sheen but not

visible crystals

Schist – visible micas, strong “schistosity”.

Identify these in the metamorphic group.

The slate is from Home Depot. The light colored schist is Manhattan Schist from Central Park in New

York, and the phyllite is from near Cle Elum.

Normal clays will produce a mainly light-colored muscovite (light mica) schist. Metamorphism of more

mafic (Fe-Mg rich) materials will produce biotite schists with various accessory minerals. The dark schist

here is from Icicle Creek near Leavenworth. It represents the metamorphism of a variety of basalts and

serpentines by the granite body that forms Mount Stuart (and the Enchantments). Note the garnet

crystals.

Eventually at the highest grades (and approaching melting) the mineralogy goes to something granite-

like – quartz, feldspar, micas, and some ferromagnesians. These are called gneiss. They can look like

granite but the foliation strongly suggests a metamorphic rather than igneous origin.

Quartzite and Marble

The metamorphic progression for quartz sandstones and limestone is straightforward with no major

changes in mineral content as the quartz recrystallizes and remains quartz, and the calcite recrystallizes

and remains calcite. Dolomites, due to the addition of Mg to the calcite structure, can also recrystallize

out an Mg-olivine if there is appropriate silica present.

Pick out and note the samples of quartzite and marble.

On the core sample, look at both ends. Are the materials the same? Note

There is a core that is whitish colored, but closer inspection shows two rock types present, marble and

quartzite. Look carefully at both ends, and using what you learned about cleavage in the mineral

sections, determine which is which.

The last two rocks are from the contact zone of the Snoqualmie Granite batholith/pluton from Denny

Creek, Snoqualmie Pass. One, which is looks a lot like basalt is actually a hornfels (see below) and the

other is a very fine grained granite (white) with broken bits of the host rock (turned to hornfels?)

embedded in it.