The purpose of this laboratory is to further acquaint you with the minerals that comprise igneous rocks and to examine the diversity of igneous rock types.

Task 1: Description of minerals that commonly occur in igneous rocks

This exercise will reacquaint you with several minerals that you learned to recognize in the previous laboratory session,, while introducing these in the context of Bowen’s reaction series (see page 4 of this handout and/or the poster to be shown by your GSI). In Table 1 below, provide a summary of the diagnostic properties that help you identify this mineral. This will include: color, cleavage, hardness, etc. Note that these specimens are much larger than the ones that typically comprise igneous rocks, and as such, it is important to learn to recognize the diagnostic features in naturally occurring, typical crystals within a rock. This will be aided if you spend some time re-examining and describing these specimens.

TABLE 1: ROCK FORMING MINERALS (1 point each = 24 points total)
Mineral Color Cleavage Hardness
Olivine
Pyroxene
Amphibole
Plagioclase

Biotite
K-Feldspar
Muscovite
Quartz

Task 2: Igneous Rock – Textures and Composition

Now that you know how to identify the most common igneous minerals, we are going to look at rocks – aggregates of minerals that reflect the temperature and composition of a magma. This exercise will test your ability to identify these minerals in various rock specimens and then to classify these into subdivisions of the Igneous Rock types.

For each specimen, record the following information:

Rock Color: As a first step, rock composition is reflected by its color. For this exercise, classify each rock as: dark/mafic, grey-green/intermediate, or pink-white/felsic. Note that this will provide an effective means of separating your rock types into 3 basic groups that are helpful later on, when you further subdivide them into specific rock names.

Rock Textures: Describe the texture of the rock (coarse/phaneritic; fine/aphanitic (vesicular?); glassy; porphyritic, etc.) or other special textural features.

APHANITIC – igneous rocks that are so fine-grained that their component mineral crystals are not detectable by the unaided eye. This geological texture results from rapid cooling in volcanic or hypabyssal environments.

PORPHYRITIC – a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group.
● FINE-GRAINED MATRIX – large crystals are surrounded by matrix crystals that are distinctly smaller yet still visible to the unaided eye.
● GRAINS NOT VISIBLE – the matrix is so fine-grained that crystals are not detectable by the unaided eye.
PHANERITIC – an igneous rock in which the crystals are so coarse that individual minerals can be distinguished with the naked eye. Phaneritic rocks are intrusive rocks that cooled slowly enough to allow significant crystal growth.

Minerals Present: This will require that you focus on the individual crystals that comprise the rock as these represent discrete minerals. Identify and tabulate all of the minerals that are present within each specimen. For this you will rely on the knowledge you developed in last week’s lab and in Task 1. For each mineral that you observe, make some estimate of its abundance (abundant, common, sparse, rare). The relative abundance of different minerals will be important in your final designation of the rock type (see the diagram on page 3).

Extrusive versus Intrusive: On the basis of the textural features that you observed above, for each rock specimen, do you think that it was extrusive or intrusive?

Felsic, Mafic or Intermediate: Based on your description of rock color and the constituent minerals, for each rock specimen do you think that it is felsic, mafic or intermediate?

Rock Name: Based on all of the above observations, name the specimens provided by your instructor (see attached table).

Terms to know:
OBSIDIAN – a hard, dark, glasslike volcanic rock formed by the rapid solidification of lava without crystallization.

VESICULAR – a small cavity in an aphanitic or glassy igneous rock, formed by expansion of a bubble of gas or steam during solidification of the rock.” Such a rock is said to be vesicular.

PUMICE – a very light and porous volcanic rock formed when a gas-rich froth of glassy lava solidifies rapidly.

RHYOLITE – an extrusive igneous rock with a very high silica content. It is usually pink or gray in color with grains so small that they are difficult to observe without a hand lens.

TUFF – Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock containing 25% to 75% ash is described as tuffaceous.

GRANITE – a coarse-grained, light-colored igneous rock composed mainly of feldspars and quartz; it also contains minor amounts of mica and amphibole minerals.

PEGMATITE – A pegmatite is an igneous rock, formed by slow crystallization at high temperature and pressure at depth, and exhibiting large interlocking crystals usually greater in size than 2.5 cm.

ANDESITE – Andesite is an extrusive volcanic rock of intermediate composition. In a general sense, it is the intermediate type between basalt and rhyolite. It is fine-grained to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.
BASALT – a mafic extrusive igneous rock formed from the rapid cooling of lava rich in magnesium and iron exposed at or very near the surface of the Earth.

SCORIA – a cindery, vesicular basaltic lava, typically having a frothy texture.

DIORITE – a group of coarse-grained igneous rocks with a characteristic “salt and pepper” appearance composed of white plagioclase and black andesite and biotite.

GABBRO – Gabbro is a phaneritic, mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth’s surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt.

PERIDOTITE – Peridotite is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica.

Figures:
The following figure shows the relative mineral abundance in igneous rocks ranging from most felsic (left) to ultramafic (right). You can find such a figure also in the textbook or in the powerpoints.

The following figure is the well-known reaction series of Bowen. It is well-illustrated in many textbooks (in all its complexities),.