Minerals and Igneous Rocks

Quiz Instructions

• Device Requirement: Participants must take out a device to complete the quiz.

• Password: igneous!

Minerals Overview

Mica Types

• Muscovite:

  • Characteristics:

  • Basal cleavage

  • Thin clear sheets

• Biotite:

  • Characteristics:

  • Basal cleavage

  • Thin greenish/black sheets

Plagioclase Feldspar

• Characteristics:

  • Shiller (pearlescent luster)

  • Straight laminae

Pyrite

• Characteristics:

  • Brassy color

  • Cubic crystals

  • Striations on cubic faces

Orthoclase Feldspar (Potassium Feldspar)

• Characteristics:

  • Salmon color (typically)

  • Wavy lamellae

Amphibole (Hornblende)

• Characteristics:

  • Sheet-like, elongate but NOT prismatic

  • Appears fibrous

Garnet

• Characteristics:

  • Typically deep red color

  • Forms dodecahedral crystals

Graphite

• Characteristics:

  • Soft texture

  • Gray streak

  • Metallic luster

Olivine

• Characteristics:

  • Green color

  • Glassy and granular texture

Gypsum

• Characteristics:

  • Fibrous texture

  • Often clear or white

  • One distinctive cleavage plane (3 cleavage planes total)

Halite

• Characteristics:

  • Salty taste (not recommended to lick)

  • Perfectly cubic crystals

  • Edges are often rounded due to reaction with water in air.

Calcite

• Characteristics:

  • Rhombohedral crystals

  • Exhibits double refraction

  • Fizzes when exposed to hydrochloric acid (HCl)

Copper

• Characteristics:

  • Metallic luster

  • Tarnishes blue/green

Galena

• Characteristics:

  • Metallic luster

  • Formed in cubic structure

  • High density

Talc

• Characteristics:

  • Pearly luster

  • Very soft texture

Hematite Specular

• Characteristics:

  • Glittery appearance

  • Metallic luster

  • Produces a red streak

Hematite Oolitic

• Characteristics:

  • Reddish color

  • Also produces a red streak

Quartz

• Characteristics:

  • Must answer 'yes' to all three questions:

  • Is it hard? (Yes)

  • Is it glassy? (Yes)

  • Can it be used to make a tool (via conchoidal fracture)? (Yes)

Augite (Pyroxene)

• Characteristics:

  • Green tint

  • 90-degree cleavage angles

  • Crystals tend to be blocky

Igneous Rocks Overview

Igneous Rock Formation

• Formation Process:

  • Formed when magma cools.

  • Involves a phase change from liquid rock to solid rock.

  • Characterized by differences in composition and texture.

Composition of Igneous Rocks

• Depends on: The composition of the magma.

• Types:

  • Felsic: Rich in feldspars and silicon (especially quartz); poor in magnesium and iron.

  • Mafic: Rich in magnesium and iron; poor in silicon and feldspars.

Felsic vs. Mafic

• Felsic Rocks:

  • Typically light-colored.

  • Composed mainly of quartz and feldspars.

  • Common mafic mineral: biotite mica.

• Mafic Rocks:

  • Typically dark-colored.

  • Composed mainly of pyroxene.

  • Ultramafic rocks contain olivine.

Intermediate Igneous Rocks

• Composition:

  • Contains a mix of at least 35% both felsic and mafic components.

  • Most common felsic mineral: plagioclase.

  • Common mafic minerals: amphiboles.

Volcanic Glass

• Characteristics:

  • Non-crystalline (amorphous).

  • Cannot be identified using the typical dark-mafic/light-felsic classification criteria.

Igneous Rock Textures

Cooling History

• Influence on Texture:

  • Determines texture without affecting composition.

Intrusive Rocks

• Formation:

  • Cools slowly at depth.

  • Magma is not erupted, allowing crystals time to grow, leading to coarse-grained texture.

Intrusive Textures

• Phaneritic Texture:

  • Crystals visible to the naked eye.

  • Crystals are roughly of similar size and randomly arranged.

• Pegmatitic Texture:

  • Crystals larger than 2.5 cm.

  • Often felsic, with greater variety of minerals like feldspar, quartz, and muscovite.

Extrusive Rocks

• Formation:

  • Cools quickly at the surface after eruption.

  • Crystals do not have sufficient time to grow, resulting in a fine-grained texture.

Extrusive Textures

• Aphanitic Texture:

  • Crystals not visible to the naked eye.

  • May feel gritty to touch.

• Porphyritic Texture:

  • Exhibits a dual cooling history:

  • Some crystals visible (phenocrystals from slow cooling).

  • Embedded in a fine crystalline or glassy matrix (groundmass from fast cooling).

• Glassy Texture:

  • Result of extremely rapid cooling preventing crystal formation.

  • Characterized by conchoidal fracture.

  • Cannot be classified by color.

• Vesicular Texture:

  • Contains numerous holes left by escaping gases.

  • Presence of vesicles typical of the upper part of a lava flow.

• Pyroclastic Texture:

  • A wide variety of types produced by explosive volcanic eruptions.

  • Not typically included in basic identification exercises due to complexity in hand samples.

Rock Composition and Color Chart

• Color Indicators:

  • Light vs. Dark colored minerals affecting specific gravity.

  • Proportions dictate classification into Felsic (<100) and Mafic (>100).

Rock Texture and Composition Table

• Various igneous rocks characterized by mineral percentages:

  • Granite Family: High silica content (Granitic pegmatite, Granite, Diorite).

  • Mafic Family: Low silica content (Mafic pegmatite, Gabbro).

  • Fine-grained Rocks: Rhyolite, Andesite.

  • Porphyritic Types: Different varieties based on groundmass grain size (e.g., granite porphyry).

Class Instructions

• In-Class Exercise:

  • Complete Question 1 using PowerPoint to describe mineral characteristics in the provided chart.

  • Fill out all columns for each hand sample in the appropriate charts.

• Homework:

  • Complete Question 3.

  • Submit combined document (pages 2-5 and parts 2-7 through 2-9) as a single PDF before the next lab session.

• Upcoming Topics:

  • Next week will focus on the Sedimentary Rocks quiz.