Identifying Minerals and Igneous Rocks

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Questions and Answers

What are the seven ways to identify a mineral?

Color/clarity, crystal form, luster, streak, hardness, cleavage, fractures

How do you know when a mineral is harder than a streak plate?

The mineral will scratch the streak plate and make a white streak of powder from the streak plate.

What does a mineral look like when it has one cleavage?

Looks like pages of a book, that split apart along flat sheets.

What does a mineral look like that has two cleavages that intersect at or near 90 degrees?

Elongated prisms that fracture along short rectangular cross sections. Signup and view all the answers

What does a mineral look like that has three cleavages that intersect at or near 90 degrees?

Shapes made of cubes and parts of cubes; if not at 90, shapes are made of rhombohedrons and parts of rhombohedrons. Signup and view all the answers

What are some of the special characteristics a mineral can have?

Tenacity, reaction to acid, striations, magnetism, specific gravity. Signup and view all the answers

What is the definition of magma?

Rock (heated until liquefied) that exists in isolated bodies below Earth's surface. Signup and view all the answers

What is the definition of intrusion?

A body of magma that pushes its way into Earth's crust. Signup and view all the answers

How does intrusive igneous rock form?

When magma intrudes into Earth's crust, it will eventually cool to form coarse-grained intrusive igneous rock comprised of visible mineral crystals. Signup and view all the answers

What is another name for intrusive rock?

Plutonic. Signup and view all the answers

What is the definition of lava?

Bubbly magma which explodes out onto Earth's surface at volcanoes. Signup and view all the answers

How does extrusive igneous rock form?

When lava cools on Earth's surface, forming a fine-grained extrusive igneous rock which is comprised of tiny crystals and/or glass. Signup and view all the answers

What is another name for extrusive rocks?

Volcanic. Signup and view all the answers

What is the definition of felsic minerals?

Light colored minerals that usually refer to feldspars and other silica materials. Signup and view all the answers

What is the definition of mafic minerals?

Generally dark colored, usually refer to magnesium and iron in their chemical formulations. Signup and view all the answers

Describe the color index of igneous rocks from lightest color to darkest.

Felsic, Intermediate, Mafic, Ultramafic. Signup and view all the answers

What is the order of texture from intrusive to extrusive?

Pegmatitic, phaneritic, porphyritic, aphanitic, glassy, vesicular, pyroclastic/Fragmental. Signup and view all the answers

What are the color index minerals from felsic to mafic?

Quartz, Plagioclase Feldspar, Potassium Feldspar, Muscovite Mica, Biotite Mica, Amphibole, Pyroxene, Olivine. Signup and view all the answers

Describe aphanitic texture.

Fine grained texture; rapid cooling. Signup and view all the answers

Describe phaneritic texture.

Comprised of visible crystals (coarse-grained); magma cooled slowly which let crystals grow. Signup and view all the answers

Describe pegmatite texture.

Very coarse grained formed by intrusive event; very slow cooling rate. Signup and view all the answers

Describe porphyritic texture.

Igneous rocks that have two distinct sizes of crystals; large crystals are called phenocrysts, and the smaller more numerous crystals are called the matrix; slow cooling followed by rapid cooling. Signup and view all the answers

Describe vesicular texture.

When gas bubbles get trapped in cooling lava they are called vesicles; created extrusively. Signup and view all the answers

Describe pyroclastic texture.

Composed of rocky materials that have been fragmented and/or ejected by explosive volcanic eruptions. Signup and view all the answers

What does a porphyritic texture indicate?

Indicates that a body of magma cooled slowly at first (to form the large crystals) and more rapidly later (to form the small crystals). Signup and view all the answers

What is glassy texture?

Formed by rapid cooling and/or very poor nucleation. Signup and view all the answers

What are the five characteristics of minerals?

<ol> <li>Naturally occurring 2) Usually inorganic 3) Definite chemical composition 4) Solid substance with specific crystalline structure 5) Distinctive physical properties.</li> </ol> Signup and view all the answers

How are minerals formed? (4 ways)

<ol> <li>From molten rock 2) From solution 3) Within living cells 4) By recrystallization.</li> </ol> Signup and view all the answers

What are the three degrees of crystallinity?

<ol> <li>Euhedral = well developed faces, edges, corners 2) Subhedral = partially developed 3) Anhedral = no faces, edges, or corners.</li> </ol> Signup and view all the answers

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Study Notes

Identifying Minerals

Seven ways to identify a mineral: color/clarity, crystal form, luster, streak, hardness, cleavage, fractures
A mineral is harder than a streak plate if it scratches the streak plate and leaves behind a streak of powdered mineral
Cleavage: Flat sheets, like pages of a book
Two Cleavages that intersect at or near 90 degrees: Elongated prisms, fractured along short rectangular cross-sections
Two Cleavages that do not intersect at 90 degrees: The mineral will not have a rectangular shape, or a shape of square/cubed
Three Cleavages that intersect at or near 90 degrees: Shapes made of cubes and parts of cubes
Three Cleavages that do not intersect at 90 degrees: Shapes made of rhombohedrons and parts of rhombohedrons
Special characteristics of minerals: tenacity, reaction to acid, striations, magnetism, specific gravity

Igneous Rocks

Magma: Rock that is heated until liquefied, found in isolated bodies beneath Earth's surface
Intrusive igneous rocks: Form when magma intrudes into Earth's crust
Intrusive igneous rock form: Formed by slowly cooling magma, comprised of visible mineral crystals
Intrusive igneous rocks are also known as: Plutonic
Lava: Bubbly magma that explodes out onto the Earth's surface
Extrusive igneous rocks: Form when lava cools on Earth's surface
Extrusive igneous rock form: Formed by rapidly cooling lava, comprised of tiny crystals and/or glass
Extrusive igneous rocks are also known as: Volcanic
Felsic Minerals: Light colored minerals, typically comprised of feldspars and silica
Mafic Minerals: Dark colored minerals, typically comprised of magnesium and iron
Color Index of Igneous Rocks from Lightest to Darkest: Felsic, Intermediate, Mafic, Ultramafic
**Order of Texture from Intrusive to Extrusive **: Pegmatitic, Phaneritic, Porphyritic, Aphanitic, Glassy, Vesicular, Pyroclastic/Fragmental

Textures of Igneous Rocks

Aphanitic texture: Fine-grained, rapid cooling
Phaneritic texture: Comprised of visible crystals (coarse-grained), slow cooling
Pegmatitic texture: Very coarse-grained, very slow cooling
Porphyritic texture: Two distinct sizes of crystals, large crystals are called phenocrysts, small crystals are called the matrix, slow cooling followed by rapid cooling
Vesicular texture: Gas bubbles trapped in cooling lava, vesicles, created extrusively
Pyroclastic texture: Composed of fragmented rocky materials, ejected by explosive eruptions
Glassy texture: Rapid cooling, very poor nucleation
Porphyritic Texture indicates: That magma cooled slowly at first, then more rapidly

Minerals

Five Characteristics of Minerals:
- Naturally occurring
- Usually inorganic
- Definite chemical composition
- Solid substance with specific crystalline structure
- Distinctive physical properties
Four ways Minerals form:
- From molten rock
- From solution
- Within living cells
- By recrystallization
Three Degrees of Crystallinity:
- Euhedral: Well-developed faces, edges, and corners
- Subhedral: Partially developed
- Anhedral: No faces, edges, or corners