IGNEOUS (Fire) Rocks - Identification and Classification
Rocks are naturally occurring inorganic solid objects in/on the earth that are usually made up of two or more natural occurring minerals. There are a few exceptions, whereby some rocks are made up of single minerals. There are over 4000 known minerals, but only a few are considered common rock forming minerals. These will be identified under modes of formation.
By definition, rocks are larger than sand or soil; and include pebbles, boulders, volcanic flows, mountains, tectonic plates, sedimentary basins, etc.
Rocks are identified and classified by their physical, chemical, and mineralogical properties; and by their modes of formation. More advanced studies include geophysical properties, e.g., radioactive, magnetic, seismic, electrical, specific gravity; porosity, permeability; and geochemical properties.
Normal modes of rock formation are: 1. Sedimentary, 2. Igneous and 3. Metamorphic
Igneous Rocks are further differentiated into the following:.
Intrusive: Ultramafic, Gabbro, Diorite, Granite, Syenite
Extrusive: Basalt, Andesite, Dacite, Rhyolite, Trachite
How are Igneous Rocks Named? From the book “Igneous Rocks: A Classification and Glossary of Terms” (Ref. 14), the following statement gives a clue. “Decades of field and microscope studies and more recent quantitative geochemical analyses have resulted in a vast, and sometimes overwhelming, array of nomenclature and terminology associated with igneous rocks. This book presents a complete classification of igneous rocks based on all the recommendations of the International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks. The glossary of igneous terms has been fully updated since the first edition and now includes 1637 entries, of which 316 are recommended by the Subcommission. Incorporating a comprehensive bibliography of source references for all the terms included in the glossary, this book is an indispensable reference guide for all geologists studying igneous rocks, either in the field or the laboratory. It presents a standardized and widely accepted naming scheme that will allow geologists to interpret terminology in the primary literature and provide formal names for rock samples based on petrographic analyses. It is also supported by a website with downloadable code for chemical classifications.” Ref. 14
Fire Rocks: The word igneous comes from the Latin word ignis, which means “fire”, thus igneous rocks are fire rocks. They derive from magma within the earth, and volcanic activity on/ near the surface of the earth.
Earth Formation: Originally, 4.5 billion years ago, the entire earth was a molten mass of magma as gravitational forces gathered up space material orbiting the sun, and melted the material by impact heating, and by gravitational heading due to compaction. This material consisted of 92 elements formed by nuclear fusion and a supernova on another nearby sun, probably a binary sun to our sun. In the molten state of the early earth, most of the heavier elements and minerals migrated towards the center of the earth, and most of the lighter elements and minerals migrated towards the surface of the earth. After billions of years, radioactive elements in the earth’s interior continue to keep the interior of the earth molten; and provide the energy for plate tectonics, volcanoes, and igneous rock formation. The method by which this occurs is explained in the article “Radioactive potassium may be major heat source in Earth's core” (Ref. 10).
Only eight elements make up 99% of the earth’s crust: 1. Oxygen, 47%, 2. Silicon, 27%, 3. Aluminum, 8%, 4. Iron, 5%, 5. Calcium, 3.6%, 6. Sodium, 2.8%, 7. Potassium, 2.6%, and 8. Magnesium, 2%. As the earth cooled, mainly lower density silicate minerals formed an igneous rock crust at the surface of the earth, and hydrogen and oxygen combined to form our water oceans. After billions years of cooling, the earth now has a crust about fifty miles thick under the continents, and about ten miles thick under the oceans; and the interior is still molten. There are over 4000 different minerals in the earth’s crust, but only a few make up the igneous rocks. These minerals are:
- Plageoclase: The white or chalking looking grains in intrusive rocks is plagioclase feldspar. It is the most common mineral in igneous rocks. Plagioclase often has striation, and K-feldspar has none. Plageoclase has a S.G. of 2.64-2.68, whereas K-feldspar is < 2.56.
- Quartz: Quartz is also very common in some igneous rocks, particularly granite. In igneous rocks, it is often medium dark gray and has a rather amorphous shape instead of the clear hexagonal crystals you are used to seeing.
- Potassium (K) Feldspar: Most of the K-feldspars in igneous rocks have a pinkish cast, but white is also a common color. White K-feldspars can be differentiated from white plagioclase by the lack of striations in K-feldspars. Micro K-feldspars in rhyolites may give them a pinkish cast.
- Muscovite: Muscovite is the white mica in igneous rocks, and is not very prevalent. It is usually shiny and silvery, but may oxidize to a yellowish tone. It has perfect cleavage, and can easily be flaked off. Muscovite is hard to spot in hand specimens.
- Biotite: Biotite is the black mica in igneous rocks. Like muscovite, it has perfect cleavage, and can easily be flaked off. It can be differentiated from the dark pyroxene and amphibole mineral in igneous rocks by its hexagonal crystal form. It is easily observable.
- Amphibole: Amphibole (hornblende) is rather common in all igneous rocks. It usually occurs as slender needle-like crystal, and has two cleavage planes 60 and 120 degrees. Its cleavage planes occur in a stair-step fashion, and do not produce perfect flat surfaces. Amphibole is very common in diorite, less so in granite and gabbro.
- Pyroxene: Pyroxene (augite) is common only in mafic igneous rocks like gabbro and peridotite. It occurs as short, stubby, dark green to black crystals, and has 2 cleavage planes nearly at 90 degrees.
- Olivine: Olivine is common only in ultramafic igneous rocks like dunite and peridotite. It occurs as small, light green, glassy crystals; and may have a sugary texture.
Pyroxene and Amphibole are group names of families of minerals in igneous rocks that can be distinguished by their cleavage angles; with pyroxene minerals showing square patterns, and amphibole minerals showing diamond patterns, as displayed below.
 Pyroxene
|
 Amphibole
|
Mafic Minerals: There are 87 recognized minerals in the Amphibole-Family. Hornblende (S.G. = 2.9-3.4) is one of the most common amphibole rock forming minerals in igneous rocks. There are 20 recognized minerals in the Pyroxene-Family. Augite (S.G. = 3.2-3.6) is one of the most common pyroxene rock forming minerals in igneous rocks.
Descriptions of igneous rock forming minerals can be found on the web (Ref. 11), or can be found in most Rock & Mineral books. Intrusive rocks are named by the various sizes and proportions of these rock forming minerals, and by the accessory minerals that are present. The most common accessory minerals in Igneous-Rocks are Tourmaline, Apatite, Zircon, Sphene, Pyrite, Chalcopyrite, and Magnetite (Bell & Wright, 1985).
Intrusive Igneous Rocks: These rocks are formed deep within the earth’s crust from magma migrating up from the earth’s interior; or from previous crustal rocks being melted by radioactive heat, or from tectonic activity. The principle radioactive heat producing elements are the isotopes of uranium (U-238), thorium (Th—232), and potassium (K-40). These elements are presently in granitic crustal rocks at concentrations of about 4 ppm, 18 ppm, and 3.6% respectively. About 1 in every 1000 potassium atoms is radioactive (K-40). The radioactive decay of the above elements is used to age date the formation of igneous rocks. The half live of the above radioactive isotopes are: 1.25 billion years for U-238, 4 billion years for Th-232, and 14 billion year for K-40.
As the magma slowly cools, various rock forming minerals can form at specific temperatures and pressures, and may segregate due to their specific gravity. For further information, look up “Bowen’s Reaction Series”. By definition, Intrusive Igneous Rocks have minerals that are visible to the naked eye. Those visible minerals must be identified in order to classify the rock. Amateurs can identify the minerals in rocks by crystal-form, cleavage and fracture, color, hardness, streak, luster, specific gravity, tenacity, acid test, magnetism, and fluorescence. Professionals may use additional instrumented methods for identifying minerals in rocks, e.g., Polarizing Microscopes, Neutron Activation, Wet Chemistry, Raman Scattering, or X-Ray Powder Diffraction Spectroscopy.
One of the most important items in rock identification is determining the ratio of K-Feldspars to Plageoclase Feldspars. Where the Plageoclase crystals are not striated, this can be achieved by a method of chemically staining the feldspars on a polished surface. The abstract from (Ref. 15) explains the technique as follows: “The identification and chemical analysis of igneous rocks depends measurably on the nature, content and percentage of key feldspar minerals. There is a relatively accurate method of obtaining these percentages that eliminates much of the doubt concerning the feldspar make-up of the rock. It involves preparing a polished slab of the rock and then staining the polished surface with prepared chemicals. The K-feldspar minerals will stain yellow in color and the plagioclase feldspar will stain red making an easy estimate of the composition of these feldspars and a more accurate identification of the rocks chemistry.”
After the proportions of different minerals are identified in the rock, those proportions can be compared to an Igneous Rock Classification Chart to identify the specific rock type. The charts can be found on the net (web), or in most rock and mineral Identification books. The common rock types shown in this program, from a standard Igneous Rocks Identification Kit, are:
1. Biotite Granite, 2. Pegmatite Granite, 3. Syenite, 4. Diorite, Gabbro, 6. Trachyte Porphyry, 7. Felsite, 8. Andesite, 9. Basalt, 10. Obsidian, 11. Pumice, and 12. Scoria.
The mineral makeup & specific gravity of several common igneous intrusive rocks are shown below:
- Granite: 30-43% quartz, 20-50% potash feldspar, 5-35% plagioclase feldspar,
0-10% biotite. (S.G. = 2.5 to 2.8)
- Grano-diorite: 15-30% quartz, 0-20% potash feldspar, 35-55% plagioclase feldspar,
10-15% biotite, 3-15% amphibole, 0-3% pyroxene. (S.G. = 2.7 to 2.8)
- Diorite: 7-15% quartz, 40-55% plagioclase feldspar, 3-13% biotite,10-12% amphibole, 3-30% pyroxene. (S.G. = 2.7 to 3.0)
- Gabbro: 0-7% quartz, 18-40% plagioclase feldspar, 0-3% biotite, 0-10% amphibole, about 30% pyroxene, 3-50% olivine. (S.G. = 2.9 to 3.1)
- Peridotite: 0-18% plagioclase feldspar, 0-20% pyroxene, 55-100% olivine. (S.G. = 3.2 to 3.3)
Extrusive Igneous Rocks: These rocks have the same mineralogy as the intrusive rocks above, but the main mass of minerals is microscopic. This is because of rapid cooling of the magma when it flowed onto the surface of the earth, or intruded the earth at a very shallow depth. Extrusive rocks that are equivalent to intrusive rocks by mineralogy are shown below:
Granite -> Rhyolite (glassy form called obsidian)
Grano-diorite -> Dacite
Diorite -> Andesite
Gabbro -> Basalt (vesicular, amygdaloidal)
Peridotite -> no equivalent extrusive (alters to serpentine)
Since extrusive rocks are hard to differentiate in hand specimens, some classifications simple call the light colored volcanic rocks Felsite and the dark colored volcanic rocks Traprock.
Felsite: Felsite is a very fine grained volcanic rock that may or may not contain larger crystals. Felsite is a field term for a light colored rock that typically requires petrographic examination or chemical analysis for more precise definition. Color is generally white through light grey, reds to tan and may include any color except dark grey, green or black (the colors of traprock)
Traprock: Traprock is a form of plutonic igneous rock that tends to form polygonal vertical fractures, most typically hexagonal, but also four to eight sided. Devils Tower in Wyoming, an extinct volcanic neck, is a good example. The fracture pattern forms when magma of suitable chemical composition (typically basaltic) intrudes as a sill, a thick lava flow, or as a volcanic neck, and then slowly cools. The Deccan traps in India, the Siberian traps in Russia, and the Columbian River Gorge flood basalts in WA and OR, are vast areas of basaltic volcanism.
Geochemical and Geophysical Differentiation: Other methods for differentiating igneous rocks are by potassium content and specific gravity, such as in the table below. The potassium could be determined by wet assay, or measuring the radioactivity of K-40. The technique would also work for extrusive rocks.
Rock Type Ave. Potassium % Ave. S.G. & Range Labeled Sample S.G. .
Peridotite < 0.30 3.2 (3.152-3.276)
Gabbro 0.46 3.0 (2.850-3.120) 2.7 (probably granodiorite)
Diorite 1.10 2.8 (2.721-2.960) 2.7
Granodiorite 2.55 2.7 (2.668-2.785)
Granite 4.26 2.6 (2.516-2.809) 2.4
Pyroxenite (> 90%, augite SG=3.2-3.6 3.2 (3.10-3.318)
Anorthosite (> 90%, plagioclase feldspar) 2.7 (2.640-2.920) 2.7 (labradorite-plagioclase)
Chrometite (> 90%, chromite SG=4.5-4.8) ? 4.2 (from Stillwater Complex)
The best way for rockhounds to learn the different intrusive rocks is by talking to experts, studying books on rocks and minerals, visiting museums, taking field trip to study rocks in place, and by building up a personal collection. After much study, you should be able to identify the common rock types, at a glance. We have abounded Igneous Intrusive Rocks in our area for study. One area is, the Bitterroot Mountain Range, which is part of the Idaho Batholith (Ref. 8).; and another is the Boulder Batholith between Butte and Helena (Ref. 9).
Recommended Study Materials: ( the best are highlighted)
- Home Study – Smithsonian Book “Rock and Gem”, Ronald Louis Bonewitz, 2005
- Home Study – “The Rock Book”, Fenton, Carroll Lane; Fenton, Mildred Adams, 2003
- Field Study – A Field Guide to Rocks and Minerals”, Frederick H. Pough, 1998
- Field Study – Macmillan Field Guides, “Rocks & Minerals”, Pat Bell/David Wright, 1985
- Field Study - “Roadside Geology of Montana”, David Alt & Donald W. Hyndman, 1997
- Web Site - http://en.wikipedia.org/wiki/Igneous_rocks
- Web Site - http://geology.csupomona.edu/alert/igneous/igclass.htm
- Web-Site - http://imnh.isu.edu/digitalatlas/geo/bathlith/bathtxt/bathmn.htm
- Web-Site - http://mathscience.mt.gov/Elkhorn.html
- Web-Site - http://berkeley.edu/news/media/releases/2003/12/10_heat.shtml
- Web-Site - http://geology.csupomona.edu/alert/igneous/igmin.htm
- Chemical & Physical Properties of Rocks – “Handbook of Physical Constants”, GSA 1966
- Classification of Rocks, Quarterly of the Colorado School of Mines, Vol. 50, No. 1, 1955
- Igneous Rocks: A Classification of Terms” R. W. Le Maitre, et al, 2005
Web-Site - http://www.marin.cc.ca.us/~jim/geolprojects/#sample
Metamorphic Rocks – Identification and Classification
Metamorphism derives from Greek words "meat" meaning "changing" & "morpho" meaning "form”.
What are Metamorphic Rocks ?
Metamorphic Rocks are rocks that have recrystallized as a result of changes in the environment from one or more of the following: 1. Temperature, 2. Pressure, 3. Chemistry. These changed rocks may have started out as Igneous Rocks, Sedimentary Rocks, or younger Metamorphic Rocks. The most common temperature and pressure changes are caused by the gravitational gradient of the earth. As rocks are buried by tectonic activities, e.g., subduction, sedimentation, environmental changes are caused by the depth of burial. The Earth’s temperature increases from 20-60C per km of depth. The pressure increases 270 bars/km. Temperature, pressure, and chemical changes can also occur from magmatic activity. Metamorphic rocks make up a large part of the earth’s crust, and are classified by texture and by chemical and mineral assemblage.
The most common metamorphic rocks are: Amphibolites, Marble, Phyllite, Schist, Gneiss, Metabreccia, Quartzite, Serpentine, and Slate. The most common metamorphic minerals are: Amphibole, Chlorite, Muscovite, Sillimonite, Antigorite, Chrysotile, Alkaline Feldspar, Staurlite, Biotite, Garnet, Plagioclase Feldspar, Talc, Calcite, Kyanite, and Quartz.
You will be shown several of the above minerals and an assemblage of rocks from a kit of metamorphic rocks. The rock kit specimens are as follows:
Foliated Rocks
- Biotite Gneiss – may be formed from granites or sedimentary rocks and typically shows layering of mica, quartz, and feldspar.
- Graphite Schist – is formed with a parallel orientation of graphite flakes. Impurities may be present.
- Mica Schist – is composed primarily of small flakes of mica with larger salt and pepper crystals of staurolite.
- Garnet Mica Schist – is composed primarily of flakes of mica with larger crystals of garnet.
- Chlorite Schist – is composed almost completely of the mineral chlorite having a layered texture.
- Slate – is metamorphosed from shale. It has an extremely fine texture and splits cleanly along its rock planes.
- Soapstone – is a rock usually rich in talc. When layering is apparent, it is called a talc schist.
Non-Foliated Rocks
- Quartzite – is usually metamorphosed sandstone which is partially recrystallized.
- Epidosite – is composed primarily of the mineral epidote and quartz.
- Marble – is a recrystallized limestone (calcite) and can be either coarse or fine grained.
- Amphibolite – consists of amphiboles and plagioclase feldspar. It may also show some foliation.
- Hornfel – is usually a fine-grained non-foliated rock. It is usually formed from clays or shale’s.
A simple chart for classifying metamorphic rocks can be displayed as follows:
Foliated |
Degree of Foliation |
Texture |
Characteristics |
Rock |
Excellent |
Fined-grained |
Splits easily into smooth sheets |
Slate |
Medium |
Medium-grained |
Rich in mica, splits easily |
Schist |
Poor |
Coarse banded |
Streaked or banded |
Gneiss |
Non-foliated |
Mineral Content |
Characteristics |
Rock |
Chiefly quartz, some others |
Hard, breaks across mineral grains |
Quartzite |
Chiefly calcite or dolomite |
Granular, reacts with 10 percent HCl |
Marble |
Good rockhound books on metamorphic rocks are the Smithsonian book (Ref. 1), and the Macmillian Field Guide book (Ref. 2). A good web-site is http://www.geo.ua.edu/intro03/Meta.htm. A modern professional classification of metamorphic rocks can be found in a 2007 book by Douglas Fettes and Jacqueline Desmons (Ref. 3). The book description is as follows:
“Many common terms in metamorphic petrology vary in their usage and meaning between countries. The International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Metamorphic Rocks (SCMR) has aimed to resolve this, and to present systematic terminology and rock definitions that can be used worldwide. This book is the result of discussion and consultation lasting 20 years and involving hundreds of geoscientists worldwide. It presents a complete nomenclature of metamorphic rocks, with a comprehensive glossary of definitions, sources and etymology of over 1200 terms, and a list of mineral abbreviations. Twelve multi-authored sections explain how to derive the correct names for metamorphic rocks and processes, and discuss the rationale behind the more important terms. These sections deal with rocks from high- to low- and very-low-grade. This book will form a key reference and international standard for all geoscientists studying metamorphic rocks.”
Metamorphism produces many valuable mineral and rock resources:
A. Quarried Rocks include:
1. Marble which is used for statuary and ornamental building stone.
2. Slate which is used for roofing, flooring, billiard/pool tables, and blackboards.
B. Economic Important metamorphic minerals include:
1. Talc which is ground into powder.
2. Graphite used in pencils and lubricants.
3. Garnet and Corundum used as gemstones and abrasives.
4. Asbestos formerly used as a heat insulator.
5. Kyanite, Andatusite, Sillimanite used a raw material in the ceramics industry.
C. Ore Deposits - result from contact metamorphism where hydrothermal solutions precipitate ore minerals in surrounding rocks:
1. Sulfide deposits (bornite, chalcopyrite, galena, pyrite, sphalerite), Coer de Lane district, ID
2. Iron deposits (hematite).e.g., metamorphic iron formations, Champion Iron Mine, MI
3. Tungsten deposits (scheelite); e.g., Calvert Hill Mine, MT.
4. Precious metal deposits (gold); e.g., Homestake Metamorphic Formation, Lead, SD.
References:
1. Smithsonian Book “Rock and Gem”, Ronald Louis Bonewitz, 2005
2. Macmillan Field Guides, “Rocks & Minerals”, Pat Bell/David Wright, 1985
3. Metamorphic Rocks: A Classification and Glossary of Terms: Recommendations of the
International Union of Geological Sciences Subcommission on the Systematics of
Metamorphic Rocks, Douglas Fettes and Jacqueline Desmons, 2007
Permission granted to reprint material from this bulletin if proper credit is given to the author.
