Name:

Name: Grade:

Physical Geology Laboratory

IGNEOUS MINERALS AND ROCKS IDENTIFICATION - PRELAB #4

Introduction & Purpose:

In this lab you will learn to identify igneous rocks in hand samples from their physical properties. You will become familiar with the most common assemblages of igneous-rock forming minerals. The nature and origin of magmas, important aspects of mineral crystallization, the major types of intrusive and extrusive igneous rock structures, and the connection between plate tectonics and the rock cycle will also be explored. The purpose of this laboratory experience is to become familiar with both identifying common igneous rocks and understanding their origin.

PRELAB SECTION – To be completed before the lab meeting (due at start of lab #4)

I. General Classification of Igneous Rocks

A. Overview: The classification of igneous is based upon two major criteria:

1. Texture (crystal grain size and grain size distribution)

2. Composition (mineralogy/geochemistry)

B. Igneous Textures: Igneous rocks are divided into four major textural catgories based on

differences in crystal grain size and distribution:

1) Pegmatitic = extremely coarse-grained (plutonic)

2) Phaneritic = coarse- to medium-grained (plutonic)

3) Porphyritic = mixed-grained = coarse-grained surrounded by fine-grained (volcanic)

4) Aphanitic = very fine-grained (volcanic)

There are three additional textural classifications:

1) Glassy = the absence of minerals (volcanic)

2) Vesicular = the presence of vesicles (volcanic)

3) Pyroclastic = composed of volcanic rock fragments (volcanic)

An igneous rock’s texture is controlled primarily by the 1) rate of cooling of the magma as it

crystallizes into a solid rock, and also its 2) dissolved gasses content:

ü Igneous rocks that are pegmatitic or phaneritic have an intrusive (plutonic) origin and thereby cooled very slowly into a course-grained rock

ü Igneous rocks that have a aphanitic, glassy, vesicular, or of fragmental texture, have an extrusive (volcanic) origin and thereby, cooled very quickly into a fine-grained rock.

ü Igneous rocks that are porphyritic have a complex cooling history: first cooling slowly underground (partially crystallizing the magma), followed by transport to the surface, where the remaining molten material is cooled very rapidly into a mixed-grain rock.

ü Glassy = extremely fast cooling – magma is literally quenched with no crystals forming.

ü Vesicular = degassing (effervescing) of lava while it undergoes very rapid cooling.

C. Igneous Compositions: Igneous rocks are divided into four major groups based upon

their mineralogy, which reflects the rocks’ geochemical make-up:

1. Silicic or Felsic = silica, sodium and potassium-rich; lots of quartz and feldspar minerals

2. Intermediate = plagioclase and amphibole minerals most abundant;

3. Mafic = iron, magnesium, and calcium-rich; plagioclase and pyroxene most abundant;

4. Ultramafic = silica-poor; very rich in iron and magnesium; mostly pyroxene and olivine

An igneous rock’s composition is controlled primarily by the 1) composition of its parent magma, 2)

crystallization fractionation of magma, 3) magma mixing, or 4) assimilation of wall rock into magma.

The classification and naming of igneous rocks is based in part on their composition:

1) Igneous rocks with a felsic-silica-rich composition are classified as either granite

(intrusive) or rhyolite (extrusive).

2) Igneous rocks with an intermediate composition are classified as either diorite (intrusive)

or andesite (extrusive).

3) Igneous rocks with a mafic-rich composition are classified as either gabbro (intrusive) or

basalt (extrusive).

4) Igneous rocks with an ultramafic-rich composition are classified as either peridotite

(intrusive) or komatiite (extrusive).

D. Common Igneous Rock-Forming Minerals: There are eight common igneous rock-forming

minerals that you should be familiar with by now (studied and identified in mineral lab). Each of the

four type-pairs of igneous rocks (listed above) has a unique assemblage of these minerals. For

example, the granite-rhyolite pair is rich in the light-colored feldspars and quartz, poor in the dark-

colored biotite and amphibole, and totally lacking pyroxene and olivine. On the other side of the

compositional spectrum, the mafic gabbro-basalt pair is rich in dark-colored amphibole, pyroxene,

and olivine, including calcium-rich plagioclase, but totally lacks light-colored minerals such as

potassium feldspar and quartz. The reason for the unique mineral assemblage for each igneous

rock pair is explained by Bowen’s Reaction Series, which involves the systematic crystallization of

specific minerals in a given composition of magma as the temperature falls during cooling.

Directions: Write down the names of the nine minerals in the appropriate column, either as light-colored or dark-colored. Knowing and understanding the relationship between each igneous rock pair (discussed above) and their respective mineral assemblage will make classifying and identifying igneous rocks much easier. You will identify and carefully (re)examine the nine igneous mineral samples in lab.

Light-colored minerals Dark-colored minerals

1. _____________________ 1. _________________________

2. _____________________ 2. _________________________

3. _____________________ 3. _________________________

4. _____________________ 4. _________________________

5. _________________________

E. Igneous Rock Names: Review the four igneous rock intrusive/extrusive pairs (subgroups) and

the common minerals associated with each igneous rock sub-group. Intensely study Figure 5.3 in

your lab manual. Make sure you understand all the textural and compositional terms before

analyzing this lab’s rock samples. Carefully examine the color index, mineralogical assemblage,

and rock nomenclature charts of Figure 5.3. Note: These three charts are lined up vertically to

illustrate the compositional relationship between color index, mineralogy, and rock nomenclature.

Directions: List the names of the four paired igneous rock groups and their associated minerals

as discussed in sections C. and D. above.

Rock Pair Felsic Intermediate Mafic Ultramafic

Intrusive: ____________ ______________ ______________ ______________

Extrusive: _____________ ______________ _______________ ______________

Mineral _____________ ______________ _______________ ______________

Assemblage _____________ ______________ _______________ ______________

_____________ ______________ _______________ ______________

v Question 1: Comparison of the igneous rock pair in terms of composition and texture?

Name(s): Grade:

Physical Geology Laboratory

IGNEOUS MINERALS AND ROCKS IDENTIFICATION - LAB #4

Introduction & Purpose:

In this lab you will learn to identify igneous rocks in hand samples from their physical properties. You will become familiar with the most common assemblages of igneous-rock forming minerals. The nature and origin of magmas, important aspects of mineral crystallization, the major types of intrusive and extrusive igneous rock structures, and the connection between plate tectonics and the rock cycle will also be explored. The purpose of this laboratory experience is to become familiar with both, identifying common igneous rocks, and understanding their origin.

IN-LAB SECTION –To be completed during lab

I. Review ID of Igneous Rock-forming Minerals:

Directions: Carefully examine each of the following unknown igneous mineral samples. Write the

sample number next to the correct mineral name.

Sample Number Sample Number

1. Quartz ____ 6. Hornblende (amphibole) ____

2. Plagioclase Feldspar ____ 7. Augite (pyroxene) ____

3. Orthoclase Feldspar ____ 8. Olivine ____

4. Muscovite (mica) ____ 9. Tourmaline ____

5. Biotite (mica) ____ 10. Garnet ____

II. The Intrusive/Plutonic Igneous Rocks:

A. Introduction: Intrusive rocks have textures that you can clearly see nearly ALL the crystal

grains by eye = “megascopic”. In contrast, most crystals in a volcanic rock are invisible to the eye.

B. Plutonic Textures: The two basic plutonic rock textures are: Pegmatitic (very coarse-grained)

and Phaneritic (coarse-grained). (see Figure 5.3 and page 80). Both types form by very slow

cooling of magma at depth.

C. Color Index (CI) is a quantitative feature of phaneritic igneous rocks that expresses the rock’s

mineral composition in terms of the volume percentage of dark minerals found in the rock.

Color index is used exclusively for classifying only intrusive igneous rocks (see page 81 in lab book)

D. Intrusive Sample Collection Analysis: Eight samples are found in Collection A for close study

and comparison. Texture, mineralogy and color index of each sample will be discussed in class.

Rock Name CI ID’d Minerals Texture Sample# A1 Granite ___ _________________________________ ________________

Sample# A2 Granite ___ ________________________________ ________________

Sample# A3 Granite ___ ________________________________ ________________

Sample# A4 Diorite ___ ________________________________ ________________

Sample# A5 Diorite ___ ________________________________ ________________

Sample# A6 Gabbro ___ ________________________________ ________________

Sample# A7 Gabbro ___ ________________________________ ________________

Sample# A8 Dunite ___ ________________________________ ________________

III. The Extrusive/Volcanic Igneous Rocks:

A. Introduction: Extrusive or volcanic igneous rocks are distinguished from their intrusive or plutonic

compositional twin by their texture: extrusive rocks have textures where by you cannot distinguish

most, or all, of the crystal grains by eye (microscopic); the grain size is so small, that you need a

microscope to view the minerals. Color index does not apply to the naming of volcanic rocks.

However, the groundmass color of a volcanic rock can be used as a rough first guess for

identifying the rock’s composition, e.g. dark = basalt; light = rhyolite.

B. Volcanic Textures: The five basic volcanic rock textures are Porphyritic, Aphanitic, Glassy,

Vesicular, and Volcanoclastic (see Figure 5.3 and page 80). Each textural type is based on both

grain size and rock “fabric”, e.g. vesicles and/or rock fragments. These rocks cooled very fast.

C. Volcanic Sample Collection Analysis: Eight samples are found in Collection B for study and

comparison. Texture, mineralogy and color index of each sample will be discussed in class.

Rock Name Rock Color Visible Minerals Texture Sample# B1 Rhyolite __________ ______________________ __________________

Sample# B2 Rhyolite __________ ______________________ __________________

Sample# B3 Andesite __________ ______________________ __________________

Sample# B4 Andesite __________ ______________________ ___________________

Sample# B5 Basalt __________ ______________________ ___________________

Sample# B6 Basalt __________ ______________________ ___________________

Sample# B7 Obsidian __________ ______________________ __________________

Sample# B8 Volcanic Tuff __________ ______________________ __________________

IV. Identifying Igneous Rocks – Detailed Description and Identification

A. Identifying the Various Igneous Rock Types: Identification of unknown igneous rock samples

are done utilizing a simple step-by-step procedure that is outlined in Figure 5.2 in your Lab manual.

Igneous Rock Identification Procedure:

Step 1: Estimate the rock’s Color Index (if coarse-grained) or Overall Rock Color (if fine-grained)

Note that color index is applicable for phaneritic rocks ONLY! IF Aphanitic or Porphyritic

where there is little to no observable minerals, then estimate the composition by the Overall Rock

Color (“light-colored” = felsic/silisic, “medium-colored” = intermediate, or “dark-colored” = mafic).

Step 2: Identify all visibly discernable minerals. Estimate relative abundances of each mineral type.

Step 3: Observe and record the rock’s TEXTURE (Use seven texture terms in part III.B. above)

Step 4: Use the igneous rock flowchart in your lab manual to NAME the ROCK

B. Identifying Unknown Igneous Rock Samples

Directions: Identify the twelve (12) unknown igneous rock samples found in sample Collection C.

Circle all the appropriate attributes for each unknown sample and then list the rock’s name.

a) Color Index (plutonic) OR Overall Rock Color (volcanic);

b) Identifiable minerals; if none observable than write “None Observed”

c) Texture (pegmatitic, phaneritic; porphyritic, aphanitic, aphanitic vesicular, glassy, volcano-clastic)

d) Cooling Origin: 1) Solidified deep underground = (Slow); 2) Partially crystallized deep

underground followed by eruption (Slow-then-Fast); 3) Crystallized almost entirely at or near

surface (Fast); or 4) crystallized in the air after eruption (Super Rapid / Air-Cooled)

e) Write down the Name of the rock.

UNKNOWN IGNEOUS ROCK WORKSHEET

Sample# C1

a) Color Index ____% (if pegmatitic or phaneritic); OR Overall Rock Color (if aphanitic or porphyritic) ________

b) Observed Minerals Quartz; K-spar; Plag; Muscovite; Biotite; Tourmaline; Hornblende; Pyroxene; Olivine