Download Unit 3 Rocks and Minerals

Glossary for Rocks and Minerals
Mineral – is a naturally occurring solid crystalline substance with a distinct chemical
composition and is usually inorganic.
Rock – is an assemblage of minerals usually cemented together. It may contain only
one type of mineral or many.
Rock Type Classification – this places each rock into a category based on how it was
formed. It includes igneous, metamorphic and sedimentary rocks.
Igneous rock – this is a rock formed from molten rock (a magma or a lava).
Magma – molten rock that remains below the surface in magma chambers or other
structures such as; batholiths (large scale magma chamber), stocks (smaller magma
chambers), dikes (magma cutting across rock material) and sills (magma moving along
the layers or surfaces of rock bodies that are subsurface).
Lava – molten rock that runs along the surface of the earth from vents and volcanoes.
Plutonic rock – this is an igneous rock formed beneath the earth’s surface from molten
rock. It is called intrusive because it did not make it to the surface. The molten material
from which it formed is called magma because the molten rock did not make it to the
earth’s surface.
Volcanic rock – is a rock that has formed on the earth’s surface from molten rock. It is
called extrusive because it made it to the surface. The molten material from which it
formed is called a lava because the molten rock made it to the earth’s surface.
Intrusive rock – is a rock formed from magma that remains below the earth’s surface.
Extrusive rock – is a rock formed from lava that is on the earth’s surface.
Metamorphic rock – is a rock that has undergone temperature and pressure changes
that have caused the minerals in the original rock material to change. These minerals
may melt and form larger crystals than previously existed, or new minerals will form
because the heat and pressure have created melted rock material. There are several
metamorphic facies that relate temperature and pressure to the amount of changes that
the original rock material has experienced.
Metamorphic facies – a collection of metamorphic rocks that have formed over the
same range of physical conditions which include variations in pressure and temperature
forming new minerals from the original rock material. The facies are: contact
metamorphic faces – low pressure (P) and low to high temperature (T); Zeolite facies
with low T and low to intermediate P; Greenschist facies low to intermediate P and T;
Amphibolite facies intermediate to high P and T; Galucophane schist facies low T and
very high P; Granulite facies high P and T; and Eclogite facies high P and T.
Sedimentary rocks –a rock formed by lithification (consolidation and compression) of
sediments laid down by wind or water. They are divided into three basic groups. One is
clastic sedimentary rocks, another is organic and the last is chemical.
Clastic sedimentary rocks – formed from land sediments including clay, silt, sand and
gravel, usually forming claystones, siltstones, sandstones, and conglomerates.
Organic sedimentary rocks – usually lack abundant land material and are mostly composed
from sea creatures. Fossiliferous limestone will contain fossil shells, limestone from broken
shelly material and diatomite (formed from single-cell siliceous algae known as diatoms,
radiolara and other sea life remains), coal (formed from plant material) and coralline (from
coral).
Chemical sedimentary rocks – are rocks formed from chemical processes including the
evaporation of solution to leave salts and other precipitates yielding evaporites and
ironstones.
Feldspar – is a tectosilicate mineral group that forms an alkali (potassium, K) group, a
plagioclase (calcium, Ca and sodium, Na) group and a barium (Ba) group. These are the
most abundant minerals in igneous rocks with the Si,AlO4 tetrahedra linked together with
the Barium variety being rare.
Feldspathoid – is a tectosilicate mineral group that forms with Si,AlO4 and failed to form
feldspars because the molten solution is undersaturated in silica.
Quartz – is one of the most important rock forming minerals that is silicon dioxide SiO 2. It
is the major constituent in sandstone (the sand grains are silicon dioxide). It also is and
important mineral in defining plutonic and volcanic rocks.
Felsic – this describes rocks which have a combination of feldspar and silica or the light
colored minerals.
Mafic – this describes the rocks which have a combination of magnesium and iron or the
dark colored minerals.
Texture – the small-scale structures recognized in hand specimens. To describe grain size
we use: coarse, medium and fine including aphanitic and porphyritic.
Vesicles – these are holes in rocks. A volcanic rock often displays vesicles where gases
escaped from the magma as the lava was cooling.
Aphanitic – the rock crystals cannot be seen with the naked eye.
Porphyritic- the rock crystals have a fine texture where most crystals are not visible with the
naked eye and has large visible crystals set in the fine groundmass. These large visible
crystals are called phenocrysts.
Country rock – this is the original rock existing where a magma intrudes.
Xenolith – this is a piece of country rock broken off and carried into the magma. Some of
the original chemistry and structure of the country rock remains and is inconsistent with the
surrounding rock.
Introduction to Some Common Rocks
The rocks below are those initially introduced in Geology 103 Lab - pages vii to xviii.
Sedimentary rocks
Mudstone – clastic sedimentary rock with grains smaller than .06 mm and is less fissile
along bedding planes than shale
Sandstone – clastic sedimentary rock with cemented sand grains between 1/16 mm and 2
mm in diameter.
Limestone – a rock formed from carbonated minerals, principally calcite but may also
include dolomite. Organic limestones are formed from the calcareous skeletons of living
organisms. Clastic limestones are those derived from preexisting calcareous rocks.
Conglomerate - clastic sedimentary rock with grains greater than 2mm with rounded or
smoothed pieces (clasts) that represent some wearing and rounding of the clasts.
Chert – an extremely hard, white, gray or black type of chalcedony that occurs as masses
or layers in limestone. The nodular form is called flint. Silica-rich fluids can have nodules
form by the silica rolling upon itself or can move through rocks and precipitate out in holes
and cavities.
Sedimentary Breccia – clastic sedimentary rock with grains greater than 2 mm and very
sharp and angular pieces (clasts) that represent rock formation near the source material.
Shale – a well laminated sedimentary rock that is fissile and splits easily along bedding
planes. The fissile nature of the rock is attributed to the clay minerals which cause rocks to
have a tendency split along these flat clay minerals.
Metamorphic rocks
Slate – is a metamorphic rock formed from sedimentary shales, mudstones, siltstones and
other fine-grained argillaceous sediments, which have undergone low-grade regional
metamorphism. It appears to be a fine-grained rock in which the parallel orientation of platy
crystals of mica and chlorite results in the production of “slaty” cleavage. The original
bedding planes are where the minerals were aligned and grew during metamorphism,
retaining the layers after the metamorphism. A slate is harder with more rigid cleavage than
a shale.
Schist – is a strongly foliated coarse-grained metamorphic rock formed from shales or other
rocks rich in mica minerals, and these minerals produce a subparallel orientation that
crystallize perpendicular to the direction of maximum stress. A schist has undergone greater
pressure and/or temperature than a slate, and a schist has lost all original bedding planes.
Gneiss – is a coarse-grained metamorphic rock in which quartz and feldspar are
predominant over micas. Coarse, granular bands of quartz and feldspar alternate with thin,
dark bands rich in micas and amphiboles. These rocks are formed during high-grade
regional metamorphism. Orthogneisses are metamorphosed igneous rocks, like granites.
Marble – is a metamorphic rock formed from limestones, rich in calcite or dolomite. It is
produced by both regional and contact metamorphism.
Quartzite – is a metamorphic rock produced by pure sandstone where the quartz crystals
interlock, making it a tough, massive rock consisting almost entirely of quartz and being pale
in color.
Amphibolite – is metamorphic rock that is formed from regional metamorphism and is rich in
amphiboles, making the rock mafic. It is derived from basaltic lava and tuff, or from gabbros
and diabases or intermediate plutonic rocks like tonalites and diorites.
Igneous rocks
Plutonic igneous rocks
Granite – is a plutonic or intrusive igneous rock that is defined by the percentages of quartz,
alkali feldspar and plagioclase feldspar. It may contain small quantities of mafic minerals, but
is typically light in color and coarse in texture, where the crystals of the quartz and the
feldspars can be seen with the naked eye.
Gabbro – is a plutonic or intrusive igneous rock that is defined by the low percentage of
quartz (less than 20 percent) and the high percentage of plagioclase feldspar (greater than 90
percent). This is a mafic rock that is black in color and is of the same composition as basalt.
Volcanic igneous rocks
Rhyolite – is a volcanic or extrusive igneous rock with the same composition as granite. It is
light in color and usually varies from a white to a pink color. It has a fine texture with few
crystals visible with the naked eye (phenocrysts) and vesicles (holes formed from the rapid
release of gases).
Pumice – is a form of volcanic glass (high silica or quartz) content. It is typically light in color
and hardened from a frothy gas-filled molten lava. It is usually a light gray color with
extensive vesicles.
Scoria - is a volcanic material that consist of irregular, vesicular fragments that range in size
from 64 to 2 mm. It is often black or red in color with a predominant mafic composition.