Igneous Rocks Lab

Question 1

What are some diagnostic properties of quartz?

Answer 1

Very hard, fracture, not olive green or wine-red color, columnar form (if crystal is present)

Question 2

what are diagnostic properties of Plagioclase Feldspar (Albite and Labadorite)

Answer 2

Striations on one cleavage surface, schiller effect, prismatic cleavage @ 90, hard, for labradorite only—has a dark base color

Question 3

Diagnostic Properties of: Potassium Feldspar
(Microcline)

Answer 3

NO striations (the presence of exsolution lamellae can be described also as veinlets/veins/wispy intergrowths or lines), prismatic cleavage @ 90, hard

Question 4

Diagnostic properties of: Muscovite

Answer 4

Basal cleavage, very soft, elastic flakes, honey brown color, white streak, (the last two properties are useful to distinguish muscovite from biotite but the white streak conflicts with selenite gypsum or talc

Question 5

What are some diagnostic properties of biotite?

Answer 5

Basal cleavage, very soft, elastic flakes, light brown streak, black color (the last two properties are useful to distinguish biotite from muscovite, gypsum and talc)

Question 6

What are some diagnostic properties of hornblende?

Answer 6

Prismatic cleavage not @ 90, pale greenish-gray streak, black/dark color, Int to hard

Question 7

What are some diagnostic properties of pyroxene (diopside)?

Answer 7

Prismatic cleavage @ 90, pale greenish-gray streak, black/dark color, Int to hard

Question 8

What are some diagnostic properties of garnet (almandine variety?

Answer 8

Very hard, fracture, wine red color* (*refers the almandine variety only), equant form with a dodecahedral shape (if crystal is present)

Question 9

What are some diagnostic properties of olivine?

Answer 9

Very hard, fracture, olive green color, equant form (if crystal is present), granular texture like sandstone (if massive crystals are present) `

Question 10

The type of minerals that form in igneous rocks are dependent upon the composition of _____________.

Answer 10

the original magma.

This is because this composition determines how far the reaction process can continue before all of the magma/silica is used up.i.e., The original magma determines what minerals will form in the end….

Question 11

What is Zoning?

Answer 11

variation in the composition of a single crystal, due to a separation of the chemical components during crystal growth.

In cases where cooling happens relatively quickly, individual plagioclase crystals can be zoned

Question 12

How can igneous rocks be classified?

Answer 12

1. Composition
   Assemblage of minerals that compose rock

Elements that make up the rock. Inferred by color:
mafic
Intermediate
felsic

2. grain size
   Controlled by the rate in which the magma cooled

Cooling rate below Earth’s surface vs. at the surface

Slow cooling means it takes longer for the rock to cool and there is more time for minerals to grow resulting in larger grain sizes (i.e., a coarser texture)

Question 13

Study this igneous rock chart on composition

Answer 13

Question 14

Melting occurs deep within Earth (_________ km below the surface)

Answer 14

100-200

Question 15

Describe pegmatites

Answer 15

During the early stages of a magma’s crystallization, the melt usually contains a significant amount of dissolved water and other volatiles such as chlorine, fluorine, and carbon dioxide. Water is not removed from the melt during the early crystallization process, so its concentration in the melt grows as crystallization progresses. Eventually there is an overabundance of water, and pockets of water separate from the melt.

These pockets of superheated water are extremely rich in dissolved ions. The ions in the water are much more mobile than ions in the melt. This allows them to move about freely and form crystals rapidly. This is why crystals of a pegmatite grow so large.

The extreme conditions of crystallization sometimes produce crystals that are several meters in length and weigh over one ton.

Question 16

What are some Extrusive Pyroclastic Rocks?

Answer 16

scoria, pumice, and volcanic breccia.

Pyroclastic rocks do not readily fit into traditional classification schemes. They are the result of explosive volcanic eruptions and have both igneous and sedimentary affinities; the material making them up is mostly of igneous origin whereas the mode of emplacement is essentially sedimentary. In the dynamic environment of their deposition, they may be modified, eroded, redeposited, or intermixed by either igneous or sedimentary processes.

Question 17

Study this pic about where to find which kind of igneous rock

Answer 17

Question 18

Study chart on Common Igneous Rocks & Associated Minerals…

Answer 18

Question 19

Describe what happens with Porphyritic texture

Answer 19

Porphyritic: This type of texture develops when conditions during cooling of a magma change relatively quickly. Porphyritic basically means the mix of groundmass (microscopic crystals) with some larger crystals. These earlier formed crystals formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained (aphanitic) matrix. The result is an aphanitic rock with some larger crystals (phenocrysts) imbedded within its matrix. Examples of porphyritic rocks are: andesite porphyry and rhyolite porphyry.

Question 20

How do you distinguish between a granite and a diorite?

Answer 20

Granite is felsic in composition, and the
minerals quartz and feldspar are common; because of the larger percentage of felsic minerals,
granite is much lighter in color compared to diorite. Diorite is intermediate in composition
(felsic rocks contain about 15% mafic minerals whereas intermediate rocks contain roughly
equal amounts - around 50% of felsic and 50% of mafic minerals). Therefore, diorite is darker in
color than granite. Also, quartz and K-feldspar are rare in diorite (i.e., there is very little
present), however these are common minerals in granite.

Question 21

How do you distinguish between a gabbro and a diorite?

Answer 21

Gabbro is mafic in composition (mafic rocks
contain about ~ 65% mafic minerals) and it is composed of more dark-colored minerals. Gabbro
is therefore darker in color compared to diorite which is intermediate in composition
(intermediate rocks contain approximately 50% dark minerals).

Question 22

How do you distinguish between a basalt and a gabbro?

Answer 22

Basalt is aphanitic (fine-grained texture);
gabbro is phaneritic (coarse-grained texture). Both are mafic and dark in color, grain
size/texture is what is most helpful here.

Question 23

How do you distinguish between a rhyolite and andesite?

Answer 23

Rhyolite is felsic in composition, and the
sample is light pink in color, and andesite is intermediate in composition where the sample is
gray in color. Rhyolite is lighter in color compared to andesite. Note that both rocks can have a
porphyritic texture (i.e., larger crystals in a fine-grained background mass). So, the porphyritic
texture of andesite is not sufficient to distinguish it from rhyolite, as rhyolite can also be
porphyritic, and andesite and also be aphanitic (i.e., fine-grained) without any of the visible,
needle-like crystals of hornblende.

Question 24

How do you distinguish between a basalt and an andesite?

Answer 24

Basalt is mafic in composition (mafic
rocks contain about ~ 65% mafic minerals) and it is composed of more dark-colored minerals;
basalt is much darker in color compared to andesite. Also (as mentioned in the previous
answer) both rocks can have porphyritic textures, and so the porphyritic texture we see in
andesite is not sufficient enough to distinguish it from basalt (or any other fine-grained igneous
rock), as andesite can also be aphanitic. Color is what is most helpful here.

Question 25

Why do magmas move upward?

Answer 25

Magmas are less dense than surrounding rocks, so they move upward toward the surface

Question 26

What elements make up 99.9% of magma’s chemical composition?

Answer 26

Si, Al, Fe, Ca, Mg, K, Na, H, and O.

Question 27

What is the most abundant oxide in magma?

Answer 27

SiO2 (Silicon Dioxide).

Question 28

What is the silica content of mafic (basaltic) magma?

Answer 28

SiO2 content is 45-55%.

Question 29

What is the silica content of intermediate (andesitic) magma?

Answer 29

SiO2 content is 55-65%.

Question 30

What is felsic (rhyolitic) magma high in? What is the silica content of felsic (rhyolitic) magma?

Answer 30

Felsic magma is high in K (Potassium) and Na (Sodium), and low in Fe (Iron), Mg (Magnesium), and Ca (Calcium). SiO2 content is 65-75%.

Question 31

What gases are present in magmas?

Answer 31

Magmas contain H2O (Water), CO2 (Carbon Dioxide), and minor amounts of Sulfur, Cl (Chlorine), and F (Fluorine).

Question 32

Which type of magma usually has higher gas content?

Answer 32

Felsic magmas usually have higher gas contents than mafic magmas.

Question 33

What is the temperature range for mafic (basaltic) magma?

What is the temperature range for intermediate (andesitic) magma?

What is the temperature range for felsic (rhyolitic) magma?

Answer 33

A: 1000-1200°C.
A: 800-1000°C.
A: 650-800°C.

Question 34

How does the silica content affect magma’s viscosity? How about temp?

Answer 34

Higher SiO2 content results in higher viscosity (more resistance to flow). Lower temp is more viscous.