Igneous Rocks Identification Sheet Minerals and Percentages % Felsic Minerals %’Maflc Minerals Composition/Color Intrusive/Extrusive Rock Name 13B 158…

Igneous Rocks(Lab 5)

For this lab you (I) will need:

·       Rock and Minerals Quick Study Guide

·       Mineral and Rock Identification Chart (in Bb)

·       Rock and Mineral Set – Blue River 

·       Hand lens

In this lab you (I) will identify 12 igneous rocks.  You will use continue to use the skills acquired in the previous labs to identify these rocks.  You will use the Rock and Minerals Quick Study Guideand the Classification of Igneous Rocks Table in the Minerals and Rocks Description Guideto identify the unknown igneous rocks.  

Igneous Rock Composition and Texture Identification

Rocks are aggregates of one or more minerals and are classified into three different rock types: igneous, sedimentary, and metamorphic.  These three rock types serve as a tool to tell us about the history of the earth and the processes that occurred in the past.  These processes are often associated with plate tectonics, volcanic eruptions, erosion, and different depositional environments.  

This lab focuses on igneous rocks. Igneous rocks form when molten rock cools and solidifies to form an aggregate of minerals.  The term magmais used to describe molten rock underneath the surface of the Earth. When magma is erupted on the surface during volcanism, it is called lava. During cooling, minerals crystallize from the melt in a particular sequence until all of the melt has become solid.  This cooling sequence is called Bowen’s Reaction Series.  Igneous rocks that crystallize from magma inside the curst of the Earth are called plutonic(intrusive) igneous rocks while those that crystallize from lava at the surface of the Earth are called volcanic(extrusive) igneous rocks.

Igneous rocks are classified by their composition and texture.  Composition refers to the chemical composition of the magma or lava that the igneous rock crystallized from.  The magma composition is determined by the mineral content of the rock.  The general color of the rock often is related to the mineral content and can also be used as an indicator of chemical composition. Below is a description of magma compositions for igneous rocks.  See the Classification of Igneous Rock Table in the Minerals and Rocks Study Guidefor percentages of minerals associated with the different mineral compositions.

Magma Composition

Felsic:              Light gray to pink color.  Commonly contain quartz, orthoclase feldspar, and plagioclase feldspar with minor ferromagnesian minerals such as biotite and hornblende.

Intermediate:  Neither light nor dark, often a medium shade of gray.  Commonly contain plagioclase and some orthoclase feldspar, minor amounts of quartz, ferromagnesian minerals such as biotite and hornblende.

Mafic:               Dark color.  Commonly contain plagioclase feldspar, ferromagnesian minerals such as biotite, hornblende, augite, and olivine.

Ultramafic:       Green to black color. Usually composed entirely of olivine and augite.

When speaking about the texture of a rock, usually what is described is the size of the minerals or grains comprising the rock.  In igneous rocks, the rate of cooling is what determines the size of the mineral grains. Intrusive igneous rocks – or those that are cooling inside the Earth – cool slowly.  This slow cooling allows enough time for the crystallizing minerals to grow to larger sizes and are visible to the unaided eye.  We describe this texture as coarse grained or phaneritic.  Extrusive igneous rocks – or those that cool at the surface of the Earth – cool very rapidly and the minerals do not have enough time to grow.  Typically these then have a very fine grained texture and mineral grains are too small to see.  This fine grained texture is described as aphanitic.  Sometimes a melt can go through two or more different phases of cooling such as a very slow rate of cooling followed by a very rapid rate of cooling.  This typically results in two very different mineral grain sizes – very large minerals in a much finer grained matrix. This texture of two different grain sizes is called porphyritic.  The following is a list of possible textures describing igneous rocks:

Igneous Textures:

Phaneritic:       Coarse grained (>1mm), mineral grains visible with unaided eye.

Aphanitic:        Fine grained (<1mm), mineral grains are too small to be seen.  There may be a few small (~1mm) minerals present in the matrix.  Aphanitic textures usually make mineral identification impossible without the aid of special equipment.

Porphyritic:      Two very different mineral grain sizes.  Large grains are called phenocrystsand the smaller grains are collectively called the matrix or groundmass. The matrix for porphyritic rocks can be either fine or coarse grained.

Pegmatitic       Exceptionally coarse-grained, most mineral grains are larger than 1 cm in diameter.

Vesicular:         Voids called vesicles in the rock that form from trapped gas bubbles escaping lava during an eruption.  Vesicles can range in size from large to almost imperceptibly small.

Glassy:              No crystals at all, only glass.  Cooling was so rapid that minerals could not grow.

Pyroclastic:      Fragments of various sizes consisting of minerals and glass shards.

Igneous Rock Identification

Complete the Igneous Rock Identification sheet for each sample.  Fill in the appropriate terminology describing texture (i.e. phaneritic, aphanitic, etc.), and if applicable, identify (using your hand lens included in the Mineral and Rock kit) and name minerals with their relative percentages, and composition (i.e. felsic or mafic).  Use the texture to determine whether the sample is intrusive or extrusive, and name the rock using the Rock and Minerals Quick Study Guideand the Classification of Igneous Rocks Table in the Minerals and Rocks Description Guide.  The 12 igneous rocks in the lab are:

Gabbro

White Granite

Diorite

Pink Granite

Peridotite

Scoria

Pumice

Obsidian

Rhyolite

Andesite

Porphyritic Andesite

Basalt

Igneous Rocks – Identification Sheet

Igneous Rocks — Identification Sheet Minerals and Percentages % Felsic Minerals %’MaflcMinerals Composition/Color Intrusive/Extrusive Rock Name 13B 158

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