Mineral and Igneous Rocks Practicum

protolith

(parent rock), determine the grade (high, intermediate, low) of metamorphosis, and identify unknown samples of metamorphic rock.

Identifying the Minerals in an Igneous Rock Sample

1. Closely examine the sample, using a hand lens if needed, to locate multiple examples of the 2-3 most common minerals. 2. Start with the most common mineral in the sample 3. Look for cleavage, color, and shape. 4. Once you have identified the most common mineral, move on to the next most common.

Streak

1. Color in powdered form (less variable than crystal color) 2. Some are the same color as other minerals (congruent); some are different (incongruent)

The Process of Identifying an Unknown Igneous Rock Sample

1. Determine if it is igneous, sedimentary, or metamorphic 2. Identify the texture 3. You may have to make assumptions based on color, streak, hardness, and/or relative density

The Process of Identifying an Unknown Mineral Sample

1. Make a hypothesis 2. Make observations 3. Observe luster (metallic, submetallic, or non-metallic) 4. Determining whether fracture or cleavage 5. Then measuring the hardness 6. Physical properties can subsequently be used to reject many of the remaining choices

Minerals: Color

1. Most Obvious 2. Different colors may result from impurities

Luster

1. The way a mineral surface scatters light 2. Two subdivisions: Metallic/Non-metallic

Hardness

A measure of the ability of a mineral to resist scratching

Extrusive Igneous Rocks - Texture

Aphanitic Porphyritic Pyroclastic Glassy Vesicular

Pyroclastic

Can resemble a porphyritic texture in that larger particles are surrounded by fine-grained ones. Depending on the mineralogical composition, pyroclastic rocks may also contain chunks of pumice or scoria.

Mineralogic Composition of Igneous Rocks

Felsic Intermediate Mafic Ultramafic

While Identifying an Unknown Mineral Sample

Eliminate the common minerals before considering rare ones.

Extrusive (volcanic)

Fast cooling leads to a large number of crystals that are too small to see with the naked eye.

2.5 hardness

Fingernail

5.5 hardness

Glass

Special properties

Natural magnetism - magnetite. Feel - talc usually feels slippery, and halite may feel greasy. Smell - sulfur stinks, kaolinite smells like wet clay. Double refraction - when transparent calcite is placed on top of a picture, the viewer appears to see two separate images. Reaction to acid - many of the minerals in the carbonate group will fizz when dilute hydrochloric acid is applied to a fresh surface. The acid dissolves the mineral, leading to the release of CO2 gas, which causes bubbles to form in the liquid acid. Taste - some minerals that are soluble in water will have a recognizable taste. The mineral halite is what we commonly call table salt, so it tastes like salt; conversely, the mineral sylvite tastes bitter. Some minerals are harmful to human health, so tasting is used only in limited circumstances. Striations - plagioclase feldspar and a few other minerals often display shallow, parallel grooves on fresh cleavage faces

3.5 hardness

Penny/copper coin

Intrusive Igneous Rocks - Texture

Phaneritic and Pegmatitic

Calcite

Sample of a mineral with three direction cleavages (not right angles)

Texture

The shape, size, and arrangement of the component pieces is referred to as

Cleavage

The tendency of a mineral to break along flat surfaces

Fracture

The way a mineral looks when it breaks apart in an irregular way

Aphanitic

a fine-grained rock composed of crystals of similar size that are too small to see with the naked eye.

Vesicular

an aphanitic rock in which a small percentage of the total volume consists of roughly spherical, or partially rounded vesicles caused by gas bubbles that were trapped in the magma during rapid cooling

Waxy (Luster)

appears to be coated with a soft wax.

Physical properties of minerals

color, streak, luster, hardness, cleavage, fracture

Pegmatitic

contains crystals that are at least 1 inch in size; smaller, but still visible crystals may be mixed in with the large ones.

Intrusive (plutonic)

cool very slowly below the Earth's surface, producing mineral crystals that are large enough to be seen with the naked eye

Ultramafic

dark, dense rocks are composed almost entirely (> 90%) of mafic minerals. The most common ultramafic rock is peridotite; an intrusive rock. Extrusive ultramafic rocks are extremely rare.

Mafic

dark-colored rocks that contain at least 40% magnesium and ferric minerals. Ferric means that the minerals contain iron. The most common mafic rock is basalt, which is extrusive; gabbro is chemically equivalent to basalt, but it is intrusive.

Resinous (Luster)

deep coloration that appears smooth and creamy.

Pearly (Luster)

iridescent in a way similar to that seen in natural pearls or the inside of seashells.

Greasy (Luster)

dry samples appear as if they are covered in a thin layer of oil.

Earthy (Luster)

dull, nonreflective, seems to absorb light, reminiscent of dirt.

Glassy

extremely rapid cooling prevents the formation of mineral crystals.

Silky (Luster)

faint parallel features, like brushstrokes, may convey a soft appearance.

Fibrous (Luster)

fairly coarse parallel linear features that may resemble a bundle of strings.

Vitreous (Luster)

glassy, like glazed porcelain or window glass.

Intermediate

grey, greenish, or salt and pepper colored rocks that contain 60-80% felsic minerals, but less than 10% quartz. The most common intermediate rock is andesite, which is extrusive; diorite is the intrusive equivalent of andesite.

Phaneritic

individual crystals are visible to the naked eye and are typically of roughly uniform size. Tends to form in large intrusive bodies, such as batholiths, stocks, laccoliths, dikes, and sills.

Tenacity

is also known as toughness. How the mineral behaves in response to impact or bending

Felsic

light-colored rocks that are mostly (>80%) composed of feldspar and silica, with at least 10% quartz content. In addition to being light colored, felsic rocks usually have a density similar to quartz. Most common felsic rock is granite it is intrusive; rhyolite is the chemical equivalent of granite, but it is extrusive.

Heft

may be small, and you may end up comparing samples with vastly different sizes, so it is important to handle a lot of samples in order to develop a "feel" for the heft.

Submetallic (Luster)

opaque and reflects light poorly, like dull unpolished metal.

Metallic (Luster)

opaque and reflects light very well, shines like polished metal.

Foliated

rocks have platy or elongate minerals aligned in roughly parallel planes or in wavy bands or planes.

Nonfoliated

rocks may show colored bands that reflect minute impurities in the rock, but the dominant minerals show no visible alignment.

6.5 hardness

steel nail

Porphyritic

visible crystals are embedded in a fine-grained (aphanitic) background. This texture represents partial solidification below the Earth's surface (large crystals) followed by extrusion at the surface and rapid cooling (small crystals).