ENGINEERil~G MONOGRAPHS No. I
United States Department of the Interior BUREAU OF RECLAMATION
PETROGRAPIIY AND ENGINEERING· PROPER11ES OF IGNEOUS ROCKS
hy Rit~bard C. 1\lielenz
Denver, Colorado October 1948 95 cents (R.evised September 1961) United States Department of the Interior
STEWART L. UDALL, Secretacy
Bureau of Reclamation
FLOYD E. DOMINY, Commissioner
G~T BLOODGOOD, Assistant Commissioner and Chief Engineer
Engineering Monograph
No. 1
PETROGRAPHY AND ENGINEERING PROPERTIRES ·OF IGNEOUS RO<;:KS
by Richard C. Mielenz
Revised 1959. by William Y. Holland Head. Petrographic Laboratory Section Chemical Engineering Laboratory Branch Commissioner's Office. Denver
Technical Infortnation Branch Denver Federal Center Denver, Colorado ENGINEERING MONOGRAPHS are published in limited editions for the technical staff of the Bureau of Reclamation and interested technical circles in Government and private agencies. Their purpose is to record devel opments, innovations, .and progress in the engineering and scientific techniques and practices that are employed in the planning, design, construction, and operation of Rec lamation structures and equipment. Copies 'may be obtained from the Bureau of Recla- · mation, Denver Federal Center, Denver, Colon.do, and Washington, D. C. Excavation and concreting of altered zones in rhyolite dike in the spillway foundation. Davis Damsite. Arizona-Nevada.
Fl'ontispiece
CONTENTS
Page Introduction ...... 1 General Basis of Classification of Rocks ...... 1 Relation of the Petrographic Character to the Engineering Properties of Rocks ...... 3 Engineering J?roperties of Igneous Rocks ...... :. 4 Plutonic Rocks ...... 4 Hypabyssal Rocks ...... 6 Volcanic Rocks...... 7 Application of Petrography to Engineering Problems of the Bureau of Reclamation ...... 8 A Mineralogic and Textural Classification of Igneous Rocks . . . . . • ...... 9 Textures ...... , ...... 10 Mineralogic Composition...... 10 Chart ...... Follows page 10 Photographs ...... Begin on page 11
INTRODUCTION
Rocks are important to engineers. Most Therefore, petrographic examination and modern hydraulic structures rest upon rock identification will indicate the properties ·foundations. some rest laterally against rock to be expected in the rock .. abutments, and essentially all are· composed, at least in part, of rock material. Conse Petrography is being applied increasingly quently, the design of engineer~ng works to engineering problems. All concrete ag and method~ of construction depend to a gregates and riprap materials to be used in great extent on the properties of rocks- construction of Bureau projects are exam their strength, elasticity, permeability, ined and evaluated petrographically. Sam durability' density. volume change, and ples of rock as well as earth materials are solubility. It follows. then, that adequate analyzed and tested petrographically in con engineering investigations of rock should be nection with planning, design, construction, made to reveal these properties and their and maintenance problems. For example, effect on construction. during the past 10 years 282 samples of riprap and 1, 078 samples of concrete ag The properties of rocks depend upon their gregate were examined and evaluated for mineral composition~ texture, and struc engineering use. ture. Mineral composition controls such properties as hardness, density, and solu.., bill~. Texture and structure comprise the Because of the increasing application of fabric in which the individual components petrography for engineers a greater rmm.ber of the rock are arranged; they control the and variety of rock nanies are appearing in properties of the rock as a whole, such as construction and feasibility reports on engi strength. permeability, and durability. neering problems. Moreover, rock names are strange to most engineers, and they are When we think specifically in terms of the far from self-explanatory. As a result, properties of the rocks at a site or of the many questions have been directed to the rock materials to be used in the construc Petrographic Laboratory regarding the basis tion, rules for rock classification and no and method of petrographic classification. menclature might seem to be irrelevant. This mc::nograph has been prepared to answer On the contrary, petrographic classification these questions, and to bring to ~e engineer of rocks is based upon composition, tex a summary view of· the range of rock com ture, and structure--the sa_me character position, texture, and structure, and the istics upon which the rock _properties depend. relationship of these to rock properties.
GENERAL BASIS OF CLASSIFICATION OF ROCKS
Rocks, as a whole, are divided into three cooled and crystallized far below the sur classes: igneous, metamorphic, and sedi face. Igneous also are the lavas poured mentary. Igneous rocks originated through from volcanos, and those which did not quite solidification of molten material either at or reach the earth's surface but froze at small below the surface .of the earth. Igneous depths as small intrusive masses, such as rocks comprise such types as the common dikes and sills. granites which formed from tremendously large masses of molten material intruded Metamorphic rocks were formed through from depths of the earth into the crust, then recrystallization, in an"essentially solid
1 state, of igneous or sedimentary rocks sub _fectly is the mode of origin related to the jected to high temperature, high pres·sure, composition, texture, and !:!trUcture that a · or high shearing stresses within the crust of petrographer ccin almost always determine the earth. They include slates, schists, the origin of the rock and hence classify the phyllite, gneisses, marbles, and many other rock from the characterist~cs determinable less ~mman rock types. in th~ hand specimen. Consequently, rocks could be fitted into a threefold classification SedimentarY rocks originated through dep merely on the b3.sis of composition, texture, osition and consolidation of materials weath and structure, without primary consideration ered and eroded from the earth's surface. of origin. In fact, the petrographer ordi 'lbe decomposed and broken material after narily must determine whether a rock is be in~ transported over the earth 1 s surface sedimentary, metamorphic, or igneous by by water, wind, gravity, or ice was depos examination of individual fragments, for the ited in valleys. Jakes, or along the margins circwnstances under which the rock formed . of the oceans. fllbsequently, they may have usually no longer exist, but the large-scale been tightly or .loosely cemented by mineral geologic relations of the rock formation · substances pre·cipitated from groundwater,· commonly are now known. or they may have .been more or less indu Certain textures, structures, and com rated by compaction·due to the weight of positions are typical.of each petrographic overlying materials or as a resUlt of earth classification because they are controlled stresses. The process of consolidation by the physical and chemical conditions of may have proceeded rapidly or ·slowly de formation. Thus, sedimentary rocks are pending upon many factors, such as the formed by grain-by-grain accumulation. of composition of the interstitial groundwater, small or large, angular or rounded parti the clayey, sandy, or gravelly character cles. subsequently compacted or cemented of the sediments, and the weight of over together. creating a fabric in which the burden. grains are not interlocked or keyed together at their margins. but which are knit ·by Because of the great variety of textures, secondary cementing substances or by ad structures, and compositions occuring in hesive or cohesive forces. Stratification, rocks of each of the three classes, sub the most typical structure of sedimentary classes or families of rocks can be dis rocks, represents discontinuities in the tinguished. Thus, sedimentary rocks com · conditions of formation, such as change in. posed of grains 2 mm to 0. 05 mm in diam composition of grain size of the sediments eter cemented by substances in the intergran deposited, or in the type of mineral sub ular spaces are called sandstones, whereas stances deposited interstitially by ground sedimentary rocks composed of mineral and water. Metamorphic rocks develop from rock grains less than 0. 05 mm in diameter _originally igneous or sedimentary rocks are ~ed siltstones. Sedimentary roc:;Ks subjected to temperatures. pressures, and composed of clay are called claystones, if shearing stresses so high that the compo they are massive: shale, if they are charac nents are crushed and recrjstallized, com terized by a flaky or platy structure. Rocks monly with development of new minerals. composed by calcium carbonate are called The recrystallization proceeds without fu limestones, if fine-grained:· marble·S., if sion, by progressive dissolution of the old medium or coarse-grained. With incipient and precipitation of the new minerals, the metamorphism. a sandstone is designated a mass of the rock being essentially solid metasandstone, but with recrystallization throughout the process. Metamorphic rocks and development of new minerals, the rock such as schists and slates which formed may be changed to a mica schist. which is under conditions of high shearing stress a metamorphic rock possessing a stl"'ong are characterized by planar internal struc planar structure. A .metamorphic rock tures, being composed in part of platy min composed of alternating schistose zones and erals like mica, or prismatic minerals like massive granular zones is called a gneiss. th~ amphiboles which develop in response to the stress conditions. 'Metamorphic rocks such as gramilites and ·some marbles formed 'Although the threefold classification of' at elevated temperature and pressure but rocks is based upon their origin, so per- with_ little shearing stress are massive in
2 structure and are cha.ract~rized by a tightly characteristics of the melt itself (mainly lmit interlocking equigranular texture. temperature and viscosity). The miner alogic composition of igneous rocks is . Igneous rocks~ more so than sedimentary largely controlled by chemical composition . or metamorphic rocks, lend themselves to of the magma; but in part the minerals classification on a fundamental basis, since which develop arE' determined by the envi they are formed in a relatively narrow range ronmental conditions of solidification. of physical-chemical environments, and because they represent the solidified phase For example, volcanic rocks which are of siliceous melts typified by chemical com characterized by rapid cooling commonly positions which vary within comparatively are crystallized only partially, since some narrow limits. Igneous rocks differ from of the melt was congealed as glass before one another because of variations in en crystallization was completed. The min vironmental conditions of formation and erals which exist in equilibrium with half variations in chemical composition. En crystallized melt may be different from vironmental conditions include (1) the tem those which would exist in equilibrium with perature of the melt (magma), (2) the rate the last residues of the melt. Thus, a ba of cooling, (3) the pressure. and (4) the saltic magma. the mineral olivine, forms viscosity of the melt. The chemical com early in the progressive crystallization. position of a magma may control many of But, if the melt is comparatively rich in its physical-chemical properties, such as silica, the olivine may subsequently be con melting point and viscosity. The most im verted through reactions with the melt to portant aspects of chemical composition are pyroxene. The final completely crystallized silica content (which ranges from 45 per rock could be designated as gabbro and would cent to 77 percent iJ) common igneous rocks) be completely free from olivine; yet the and the content of gases and vapors occuring partially glassy basalt which could be formed within the melt. Viscosity increases with by quick chilling of the partially crystallized silica content; whereas viscosity is de- magma would contain crystals of olivine. • creased, and ease of crystallization, at any Comparable progressive changes occur in all given temperature below the melting point, igneous melts, especially the plagioclase or is increased by increasing content of volatile soda-lime feldSpar which becomes increas constituents. ingly sodic and less calcic during crystalli zation. Igneous rocks are classified through the simultaneous consideration of two factors: Thus, an engineering- classification of (1) the texture and structure of the rock, and igneous rock must use the mineralogic com (2) the chemical and mineralogic composi position as a basis. ·A knowledge of the tion. Textur~s and structures of igneous chemical composition is valuable for cor rocks are controlled by the physic8.1-chemi relation of, for instance, separate but ad cal conditions existing within the magma jacent masses, or where the minerals arP. during solidification. In turn, these condi not determinable microscopically. However, tions are most strongly influenced by the the rocks can be adequately classified ac depth at which solidification took place cording to this scheme only after chemieal (which controlled the rate of cooling and the analyses have been performed. This is an hydrostatic pressure) and the physical expensive and time-consuming process.
RELATION OF THE PETROGRAPHIC CHARACTER TO THE ENGINEERING PROPERTIES OF ROCKS
The properties of rocks depend uJ)on their ever, this prediction cannot be made from texture, structure, and composition; conse determination of the petrographic identity quently, when these factors are precisely· alone, even though petrographic identity is evaluated, the physical and chemical qual established from texture. struc~re, and ities of the rocks can be predicted. How- composition. Texture, structure. and com-
3 position can· be changed by secondary proc bility, or impermeability of th·e material, esses, and the innate properties of the rock the degree to which the expansive poten thus caused to vary significantly. An ori tialities of clay constituents will deleteri ginally weak rock may be strengthened by ously affect the engine_er~·properties, and slight metamorphism. An originally strong the engineering significance of other physical rock may be weakened by weathering or hy ·and chemical attributes of material. drothermal alteration; or an originally mas sive rock ~ be rendered pervious by frac ·Many examples of the interrelation of tures resulting from geologic processes. petrographic character and engineering To the extent that rocks are altered, de properties of materials can be cited. Petro composed, or fractured, they partake more graphic examination Qf schists from the and r;1ore of proper1Ies tjpicaliy. relateci to foundation of Hiwassee Dam, North Carolina, rocks of other origins. Thus, with increase demcnstrated that schistosity, stratification, in content of clayey decomposition products, and microscopi~ fractures in quartz grains and breakdown of the original internal texture were present. It was anticipated that these and structure, igneous and metamorphic internal structures would control the failure rocks progressively will approach the sedi of specimens in compression. Subsequent mentary c1ays and shales in their properties. tests corroborated this prediction. Portions Therefore, determination of petrographic of the shore o! Lake Roosevelt, Washington, identity of a rock is merely the first step are composed of the Nespelem formation in the application of petrography to engi known to contain a series of stratified silts neering. and ciays having petrographically unstable $racteristics. As a result of wetting by the reservoir waters, the strata of clay Texture, structure, and composition of softened and became ·so unstable that exten rocks are determined by petrographic meth sive sliding occurred, involving areas ex ods, comprising microscopical examination, tending as much as 1, 000 feet back from microchemical analysis, X-ray diffraction the shoreline. Apparently insignificant analysis, and certain physical tests. With variations in composition of rocks actually skill, experience, and good judgement, the can cause gre~t changes in properties. At engineering petrographer can anticipate a proposed dam si~e on the Colorado River the degree to which the observed charac in Utah, the great ·decline in strength of teristics will be reflected in adverse prop sandstones as a result of wetting was found erties of the material. ·With continued re ·to correlate with variations in the amount of search, the correlation between the petro determinable interstitial clay, which in no graphic characteristics and the engineering case constitutes more than 5 percent of the properties will be improved further. By rock. Petrographic examination of the clay his examination, analyses, and semiquan stones and shale forming the foundation of titative tests, the petrographer establishes the Malheur River Siphon, Owyhee Project, the need 'for detailed testing of materials. Oregon, would have revealed their poten He must determine the probability that the tialities for expansion with wetting, a proc stratification, schistosity, or fractures ess which within 8 years raised the siphon will reduce critically the strength, dura- supports more than 1 foot in some places.
ENGINEERING PROPERTIES OF IGNEOUS ROCKS
Plutonic rocks The series of rocks from granite through monzonites, quartz monzonites, granodi- Igneous rocks which form in the so-called orites and quartz diorites, to gabbros include plutonic, or deep, zone of the earth's crust by far the most common types. When en are characteristically medium-to-coarse countered in place, plutonic igneous rock _grained in texture and massive in structure. podie~ are usually large and may be regarded
4 as extending indefinitely in depth. For ex ·soda-lime (plagioclase) feidspar·~ and Df ample, the sites of Grand Coulee, Arrow dark-colored minerals increases as the rock, Kortes, and Anderson Ranch Dams silica content decreases. The basic plagi lie entirely within granite. oclase feldspars and the dark-colored· min erals are more susceptible to weathering The individual crystals or grains com or hydrothermal alteration. Consequently, posing plutonic igneous rocks usually range diorites and gabbros are more likely to from about 1 millimeter to 1 centimeter in ·exhibit local or general decomposition than size; although considerably larger. crystals are granites and monzonites. Howev~r, dO occur in some less common types (e. g .• all primary constituents of plutonic igneous the pegmatites). Because some of the min rocks are sufficiently stable to withstand erals precipitated from the melt crystallize chemical attack by the alkalies released early and others late, granites are charac during hydration of portland cement. and· terized by a poorly interlocked texture with even if calcined and pulverized, they produce the last crystallized minerals being molded inferior or worthless pozzolans. against the essentially smooth surfaces of the early formed crystals. However, in Despite their chemical stability, even gabbros, the main constituents, plagioclase some unaltered granitic rocks ·may con feldspars and pyroxenes,. develop simul tribute to poor durability of concrete. For taneously. Consequently, gabbros typically example, the bond of cement paste onto the. possess a well-interlocked internal texture. smooth, impermeable surfaces of the large As a result of the poorly interlocked texture, exposed crystals of quartz and·feldspars may granites tend to disintegrate with weathering. be inadequate, or, because of the poorly impact, or abrasion to their granular com interlocking texture of tYPical granites, the ponents. The excellent service of granitic grains may be loosened hi even moderate rocks as road aggregate relates in part to weathering, thus decreasing strength, elas the. ease with which disintegration occurs, as ticity, and durability of the rock. Large a consequence of which a·uiuformly graded crystals possessing adverse properties road metal is produced by traffic and natural may affect the quality of the aggregate. For weathering. Conversely, gabbros charac example, orthoclase and microcline crystals teristically are tough and disintegr-ate less whose thermal coefficient of expansion dif readily, their average Young's modulus of fers markedly from that of the cement paste elasticity being about 1. 7 times that of gran may contribute to rupture of bond and dis ite. tress of the enclosing-mortar. These feld- . spars are characterized~ a thermal co Because the plutonic igneous rocks develop efficient of 0 to 0. 6 x 10- per degree F. by slow and complete crystallization of the With changes in temperature of concrete, molten magma, porosity and permeability severe stresses arise in the cement paste, of the igneous rock masses are typically low. bond between aggregate and cement fails, However, jointing and fracturing of plutonic and consequent overall deterioration of the . masses are commonly sufficient to permit concrete may occur. No deterioration of localized passage of groundwater. The this kind is known to be associated with fracturing and jointing are unusually abundant diorites or gabbros, probably because the ·because the originally massive structure plagioclase feldspars possess a thermal. precludes local and progressive release of . coefficient of expansion considerably greater earth stresses. At Kortes Damsite, Wyo than that of the alkali feldspar and-because ming, intense hydrothermal alteration con-· the individual grains of these rocks are more trolled by jointing has produced zones of . firm.Jy interlocked. decomposed granite as wide as 7 inches in which the rock contains secondary mont Other granitic aggregates are connected morillonite-type (bentonitic) clay and is so with inferior quality of concrete. For ex weakened as to yield to granulation in the anlple, granitic aggregates from near Ander hand. son Ranch Dam, Idaho, and Vallecito Dam, Colorado, produce concrete of satisfactory In the granite-gabbro. seri~s the content of durability ·only if special care is used in its quartz and alkali feldspar (such as_ ortho manufacture. Extensive investigations have clase) de_creases and the con~ent of basic shown th~t only by the addition of crushed_
5 ·limestone gravel is it possible to obtain good and (3) decreased content of orthoclase and concrete with the apparently sound, highly microcline, the feldspars characterized by granitic sand-gravel aggregate of the Kan very low thermal coefficient of expansion. sas-Nebraska Area. Natural granitic ag gregates in the vicinity of Denver, Colorado, produce satisfactory but not superior con Hypabyssal rocks crete only with adequate control of mixing, Igneous rock solidified at moderate depths placing, and cur~. in the earth's crust (the so-called "hypa byssal" zone) are charac:terized by textures, With weathering or hydroth.ermal altera structures, and composition intermediate tion, plutonic igneous rocks may produce between those of the plutonic and volcanic compounds which themselves contribute to rocks. Consequently, their physical and reduced qualitY of concrete. For example, chemical properties are likewise inter an altered anorthosite from Soledad Canyon, mediate. Because of the high viscosity of California, containing the zeolite laumontite highly siliceous melts and the common as (or leonhardite) is known to have caused sociation of abundant gases and vapors with rapid deterioration of cast stone and stucco the more siliceous magmas, rocks of the in the vicinity of Los Angeles. An appar ently sound, crushed granodiorite containing granite-rhyolite family exhibit. a very close relation between environment of solidifi a zeolite is associated with surface scalirig cation and internal texture and structure. and efflorescence of concrete on the down Rocks of the gabbro-basalt family do ~ot stream face of a dam in Southern California. demonstrate a comparably close relation At Green Mountain Dam, Colorado, it of this kind; thus the characteristics of the rocks may not always reveal the mode of was noted that a small number of boulders occurence of the igneous bodies. in stockpiled riprap had weathered to a rub ble. The rock had been identified petro graphically as monzonite porphyry. Exam Hypabyssal igneous rocks occur as com ination of the excavation from which the paratively small bodies, which are called rock had been obtained revealed shear zones · "dikes" if they are thin and tabular in form along which the rock had lost its physical and cut across the structure of the country coherence by deep-seated alteration. The rock, or "sills" if they are thin and tabular rock was in sound physical condition except in form and intrude along structures (such along these zones. Exposure of the rock to as stratification) of the country rock. Hy natural weathering resulted in rapid disinte pabyssal intrusions of irregular or lenticular gration of the altered material. Examination shape, such as laccoliths, are.less common during 1959 of the riprap that had been placed than are dikes and sills. on the dam about 18 years previously showed that the small number of contaminating Because of their small· size, these intru boulders had completely disintegrated by sions typically constitute merely a portion exposure to the weather while the sound rock of large construction sites.· If they are ex remained unaffected. ploited as sources of construction materials, only small quantities may be available. For These comments in regard to the quality example, an andesite intrusion occurs at of plntonic igneous rocks should not be taken the left abutment of Palisades Dam site, to imply that granitic rocks in general in Idaho, but the remainder of the site is under ·variably contribute to inferior quality of lain by sedimentary rocks. Because the concrete. On the contrary, plutonic igneous competence of the igneous rock and the in rocks, or sands and gravels containing par creased strength of the sedimentary rocks ticles of plutonic igneous rocks, have been indurated under the influence of the heat of widely and successfully used as concrete the intrusion, the left abutment was selected aggregate. It is to be anticipated that ad by the designers as the site of the spillway verse qualities will be reduced by (1) de structure. Unfortunately, close jointing in creased grain size of the crystals composing the ancfesite restricted use of the rock as the rock, (2) increased interlocking of the riprap because of the limited quantity of grains, such as will accompany metamor larger sizes available. At Davis Damsite, phism under· high pressure and temperature, Arizona_.Nevada, granite and granite gneiss
6 constituting the greater part of the founda hardened surface crust. These zones of tion and the abutments are cut by rhyolite fragmental material--the lava tunnels, the dikes deeply altered (even· to clay) along fractured zones, and to some extent, the contacts. Because of this incompetence, vesicular zones--contribute to the charac these zones, in some places more than 10 teristically high permeability of thin bedded feet in width, required excavation to good lavas. Because of the higher viscosity and rock prior to construction. Basalt dikes commonly higher contents of gases and va cutting the foundation were strong and com pors of acidic and intermediate lavas, rhyo petent, and presented no design problems. lite. dacite, and andesite tuffs and agglom Rock of suifable size, quality, and gradation erates are considerably more common and for slope protection could not be obtained widespread than tuffs and agglomerates of from the excavated rock; therefore, a quarry basaltic composition. was opened at some distance from the dam. The crystals composing the greater portion Along the Feeder Canal, Columbia Basin of volcanic rock types are minute, usually Project, Washington, sills and flows of being invisible to the unaided eye. Typi basalt were involved in ancient landslides _cally. volcanic glass, representing the un and possibly in renewal of sliding of the crystallized residue of the melt, is present shales and siltstones of the Latah formation. constituting in some instances, merely in These slides greatly impeded progress of _terstitial segregations, in others, virtually construction. the entire rock. Most volcanic rocks are porphyritic; i.e. , embedded within the fine grained or 'glassy groundmass of the rock Volcanic rocks are crystals which are considerably larger than those in the groundmass. Rhyolites Igneous rocks which form at or near the commonly are highly glassy because of the surface of the earth are characteristically high. viscosity of the melt, whereas basic very fine-grained or partially to completely volcanics. such as basalt, usually contain glassy. They may be massive, or they merely small amounts of glass held inter- may contain few or many vesicles owing . · stitially in the groundmass. Because of the to release of gas from the melt, or they content of glass, volcanic rocks are usually may be banded as a result of flow of the hard and brittle. rhyolites typically being the plastic lava. Thus, the structure of the ·most brittle because of their greater glass ·volcanic rock may be massive, vesicular. content. -pumiceous, or flow-banded. Comparable ·to the granite-gabbro series is the rhyolite- · Volcanic glasses are unstable chemically, "basalt series of volcanic rocks. · and are therefore decomposed easily by weathering or hydrothermal activity. Con Lava flows are sheet-like in form; that sequently. during examination of volcanic is, the area which they cover is large com formations to establish their structural pared to their thickness. Because.of their stability. diligent investigations should be greater fluidity, basic lavas, such as basalt, made to assure that all zones of alteration commonly extend over large areas even are discovered. For example, along some though the flows are thin. Acidic lavas, ·sections of the Main Canal. Deschutes Proj such as rhyolite, usually are limited in area ect. Oregon, locally intense alteration of and typically are thicker for a given distance andesite and rhyolite tuffs to bentonitic clays of flow: Because of their small thickness .is responsible for drastic instability of cut or limited areal extent, lava flows are likely slopes and displacements in the canal sec- to underlie merely a portion of a large con tion. Because of their chemical properties, struction site. glassy rhyolites. andesites, and dacites are deleteriously reactive with the alkalies re Lava flows are commonly interbedded leased during hydration of portland cement. with tuffaceous or other fragmental volcanic .As a consequence of their widespread occur ~aterial blown from the volcano. Also. rence. these rocks are by fa.r the most im particularly in the basic (basaltic) lavas, portant rocks participating in cement-ag tunnels or tubes may occur wherever the gregate reaction. In addition. because of still fluid lava flowed o~t from beneath a -their chemical activity. acidic. intermediate,
7 ·and alkaline volcanic rocks and tuffs are concrete to such an extent that another ag excellent sources of pozzolans for use in gregate had to be substituted. Deposits of concrete. highly basaltic Sands and gravels near Pasco, Washington, which are satisfactory sources Basalts also are susceptible to decom of concrete aggregate, are underlain by position by weathering and hydrothermal deeply weathered sands and gravel but of action. But, because basalts typically con-. similar composition. Because of the sus- tain less glass than rhyolites and andesites, -ceptibility of basalt to weathering. the older equivalent alteration ~rdinarily impairs the portions of deposits have been rendered structural quality of basaits less than it im unsuitable for use. Exploration must be pairs the quality of rhyolites and andesites. conducted to determine the extent and depth On the other hand, the decomposition of the of. the satisfactory aggregate to prevent con glass in the basalt can decrease critically tamination by the unsound material. Also, its durability and soundness. For example, because of the ease with which basalts are an altered dolerite proposed for use as con decomposed by groundwater, basaltic sands crete aggregate on a project in Oregon was and gravels commonly become coated with_ found to contain about 30 percent of the clay opal. which represents the silica leached mineral nontronite, formed by alteration of from the basalt pebbles. These opal coat interstitial glass and minerals. Tests of ings are deleteriously reactive with cement. concrete proved that this rock caused rapid alkalies. freezing and thawing breakdown of concrete when used as aggregate, even though the Although basaltic glasses are not resistant crushed rock passed tests of soundness. to weathering they are not delete~iously The deterioration of the concrete was deter- reactive with cement alkalies. Their in . mined to be caused in part by large volume. nocuous character apparently relates to the changes .of the dolerite. low content of silica and the high content of magnesium and calcium. Because of their toughness, ,:;trength, and a surface texture The occurrence of montmorillonite clay· conducive to good bond with cement, ba~ts in basaltic fine aggregate at Prineville Dam, as group constitute one of the best sources of Oregon, caused premature stiffening of the concrete aggregate.
APPLICATION OF PETROGRAPHY TO ENGINEERING PROBLEMS OF THE BUREAU OF RECLAMATION
Petrography serves the engineer in var .alteration observed at the surface was caused ious ways: First. detailed petrographic by weathering and hence is limited in depth, study of rocks assists the engineering geol or by hydrothermal processes and is likely ogist in establishing the geologic structure to continue downwardly. The decision would and interrelation of formations at construc ,greatly influence preliminary estimates of tion sites. Second, petrography assists exploration and construction costs. for if in determination of the engineering prop hydrothermal, deep-:- seated alterations were erties of the rock materials in place at :the indicated, the advisability of extensive pre site and of materials to be used in the con construction exploration would be estab struction. lished. At Anderson Ranch Dam site, in tense hydrothermal alteration rendered the The engineering geologist requires ap granite incoherent to depths in excess of plication of petrography to obtain the maxi 300_ feet below the original surface, and mum geologic information from limited ex serious ~dslidmg occurred during excava.:. posure or relatively few drill cores so that tion of the site. preliminary est~mates will be as valid as possible and so that future· explorations Precise description and identification of might be the most appropriate. For ex rock formations aid the geologist in problems ample. the geologist may wish to know if the of correlation. Thus. the presence or ab-
8 sence of zones of shearing and faulting can petrographic examination to select speci be established by correlation of strata a mens representative of the group frequently cross the site. For example, at Canyon will frimplify·the test program without sacri Ferry Damsite the sedimentary formation ficing significance .of the results. constituting the foundation and abutments contained thin sills of altered andesite, dis Engineering petrography and engineering ti.Dguisbable onzy- .after petrographic analysis, .geology in coordination serve to relate the whose continuity proved the absence of sig ·properties of individual specimens subjected nificant faulting beneath tbe river alluvium. to laboratory tests to the properties of the rock formations in place. For example, ·Petrography is a valuable tool for deter _the petrographer and geologist may be called mination of the properties of rocks, either upon to decide: To what extent should the when applied independently or as a means to fractures. joints, and planes of shear in the select those quantitative tests necessary to rock in place be cause for reducing, below _measure specific properties. The latter the measured strength of rock specimens, function is the more important. Most tests the strength of the _rock mass; or to what to measure properties of rock materials are ·extent might the swelling clays remain stable expensive and time-consuming, and they by virtue of their impermeability; or to what uSually require carefully selected samples so degree would discontinuities, such as joints, protected that at the time of test they truly bedding planes, and faults, augment the represent the character of the rocks and known permeability of the rock itself. Not mater~s in place, especially with regard infrequently. the results of tests of rock to moisture content and fractures. eon.:. specimens. hOwever selected, may be more sequently, it is wise to determine the ne delu~ tha.n: instructive. Only experience, cessity for certain tests before they are geologic and petrographic skill, and good requested. For example, testing the .quality judgement will permit adequate translation of of an aggregate by performing tests of con test results into the data required for engi crete ~ntaining the aggregate may be a neering design. voided by application o~ physical and chem ical tests and petrographic examination of The petrographer can assist the engineer ·the aggregate. Tests of concrete need be in design, construction, and maintenance performed only when the results of-~e phys problems. By working with the engineering ical and chemical tests of the aggregates are geologist, the petrographer facilitates selec anomalous, or if the petrographic examina tion. exploration, and subsurface investi tion indicates adverse properties not eval gation of construction sites. Tilrough appli- uat~ by the aggregate tests. Determination cation of petrography to materials testing, of volume increase of rock materials with specific ~ests to be applied and samples to wetting is significant only if clay minerals of be tested can be seiected with minimum -the montmorillonite type (bentonitic_) ~e ·hazard of iriefficiency. Petrography is ef _present. The presence and abundance of fective in predicting the engineering prop these clays can be determined quickly by erties of rocks because those properties are petrographic and X-ray diffraction analysis. determined by the rocks' texture, structure, If the number of samples available for test and composition, chal-acteristics which can "is so great as ·to preclude testing.of all, be. discerned by petrographic methods.
~ .MINERALOGIC AND TEXTURAL CLASSIFICATION OF IGNEOUS ROCKS
On the basis of mineralogic composition characte_ristics of all widespread igneous and internal texture and structure a chart rocks.* 'Ibis chart. which appears following has been prepared which illustrates the page 10, Dl8IY be used far systematic classifi-
9 cation of described rocks if all pertinent near the surface of the earth and which are mineralogic and textural data are known; therefore exceedingly fine grained or are or definitions of rock types can be obtained partially or completely glassy. These rocks from the chart. It is to be noted that many are generally related to volcanism. The of the rocks shown in the chart can be-iden topmost row represents fragmental mate tified only after microscopic examination; rials thrown from volcanic vents. At the thus ad~quate facilities for preparation and extreme left side of the chart a note has examination of specimens are necessary for been added to describe the interrelation of complete petrographic analysis. The organ occurrence, chemical composition, and ization of the chart is described below. texture of igneous rocks.
Textures Mineralogic Composition Rocks shown in the same vertical column are similar in chemical and mineralogic Along any horizontal row the rocks are composition. For example, except for some different in chemical and mineralogic com natural glass which may occur in the rhy position, and therefore are classified dif olite, granites and rhyolites are essentially ferently even though they may have originated the same in composition. However, as is In precisely the same physical situation. noted in the column headed "Typical Tex Feldspars are the most abundant minerals tures, " the characteristic texture and struc in the crust of the earth, and they occur in tures of the rocks in any given vertical col ~p.ost igneous rocks. Therefore, igneous umn are different. Rocks which solidified rocks are classified on the basis of the kind at comparatively great depths in the earth and relative abundance of the feldspars. are characterized by completely crystalline, Feldspars may be classified as alkali feid fine- to coarse-grained textures. These spars which are of the potassium or sodic rocks are shown in the bottom lines of the ahlminum silicates, and plagioclase or soda chart. They possess textures similar to lime feldspars, which represent an iso granite and represent igneous bodies which morphous series between NaAlSi30s and solidified in the so-called plutonic zone of CaA12Si2o8• AJkali feldspars predominate in the earth's crust. igneous rocks which are rich in alkalies (e. g. , phonolites) or high in silica (e. g., rhy In the four rows of boxes above the bottom olites), whereas plagioclase feldspars occur two rows, the rocks are generally com in small or large proportions in virtually pletely crystalline, but they are fine-grained all igneous rocks. A note concerning the or porphyritic, since they commonly rep relation of chemical composition to min resent small, rapidly cooled igneous bodies, eralogy of igneous rocks may be found at the · such as dikes. They formed in the so-called bottom of the rock chart. hypabyssal zone at moderate depths in the earth. A horizontal row in the upper part of the chart indicates the range of silica content The three rows of boxes at" the top of the characteristic of the various groups of rocks. chart represent r~cks which solidified at or It is to be noted that these proportions rep resent Si02 determinable only by chemical *Expanded from charts prepared by J. F. analysis, as distinguished from silica crys Kemp, A Handbook of Rocks for Use without tallized as quartz. the Petrographic Microscope, D. Van No strand Company, P. 46; and G. D. Louder Boxes within the chart which are empty back, Index Table of Igneous Rocks, Min-. represent exceedingly rare types of classifi eralogic and Textural Classification. cations not exemplified by described rocks.
10 A MINERALOGIC AN. Modified ofterJ.F.Kemp and G.D.Louderback ·0 F I (:. I I .6 FeLDSPARS I Q)- 1 ...... # Alkali feldspars include: CHIEF FELDSPARS ALKALl FELDSPARS ALKAL .... ~ IN RO.CK ~~iH~~g~ ~ ·- orthoclase, microcline, PREDOMINATE FELDS ! oooooo Q: ::l albite,onorthoclose,per- .... _-~ ...... ~&.~.5! '2: thite,sonidine (in vofmics. Ratio of olkoli to sodo-lime t. / Ifsoda-lime feldspars~- •0 02~5l~g 10010 67 33 Not critical t a: ~-- Soda-lime feldspars ·feldspars. J prisec5Sprefix'blkali"used i: -~ ...... _.., I _gO... (pkjgioclose)ore on isanor- Soda-lime feldspars in OLIGOCLASE TO ANDESINE {WHERE ALB •o"' ...... co phous series of minerals ALBITE OLIG #atat .... aeae ~ Cl> normal rock ITE IS PRESENT, PREFIX'~LKALI•IS USED ... NoAISilOs to CoAI,SizOe :8:~~ ~~~~~0 -~ :: NEPHELINE .!.!~.!.!.! ;e.!! Other minerals whose presence is necessary or ·~ l&lf whose virtual obsehce is characteristic. +QUARTZ -QUARTZ or +Qu. "lCI> ( ... 5%} lor .. S%1 (>· I ~ LEIJCITE ~~~~~~ 0~ +Signifies presence in significant amounts•. l
SODA -LIME FELDSPARS PREDOMINATE FELDSPARS ABSENT (Or nearly so) Potash feldspars ore rare; but albite is Small proportions 13187 to 01100 Same olko!i felds- n~t uncommon in altered bosolts,eg.spilite of potash feldspar par may occur SOME SODA-LIME FELDSPAR MAY LABR'1l.DORITE, BYTOWNITE ANDESINE SOME SODA-LIME GOCLASE AND ANDESINE CONSTITUTE UP TO I0%0FROCK. AND ANORTHITE TO BYTOWNITE FELDSPAR MAY BE PRESENT LABRADCIRITE TO ANORTHITE -NEPHELINE +LEUCITE +NEPHELINE -LEUCITE -NEPHELINE QUARTZ -OUARTZ -OLIVINE -LEUCITE (or•S%} -OliVINE +OLIVINE or or ("'5%) +NEPHELINE +LEUCITE +PYROXENE +OLIVINE or +PYROXENE +HORNBLENDE BIOTITE HORNBLENDE AUGITE, ALKALI AUGITE PYROXENE - HORNBI.ENDE DIOPSIOE AUGITE PYROXENES and ALKALI PYROXENE MICA MICA HYPERSTHENE Hornblende AMPHIBOLES MICA % to62% 65%to50% 60%to50% 55%to45% :so%to40% 50%to40% 55%to43% 45%to30%
common Very comrrion Very common Very common Very rare Very rare Rare Uncommon ... ACITE TEPHRITE OR ANDESITE ASH, BASALT ASH, OLIVINE BASALT, ,BRECCIA, BASANITE ASH, BRECCIA, TUFF, BRECCIA, TUFF, ASH, BRECCIA, TUFF. UFF, OR BRECCIA, TUFF, OR OR AGGLOMERATE OR AGGLOMERATE OR AGGLOMERAT~ 'LOMERATE AGGLOMERATE_
1ATE GLASSES BASIC GLASSES ULTRA BASIC GLASSES
E, SCORIA SCORIA, VARIOLITE, TACHYLITE -Olivine . LIMBURGITE -Olivine NEPHELINITE +Basic soda-lime BASALT . OLIVINE BASALT feldspar .I LEUCITITE liCIT£ ANDESITE If diabosic texture: TEPHRITE AUGITITE PIC RITE +Olivine +Olivine DIABASE OLIVINE DIABASE PICRITE BASALT BASANITE NEPHELINE BASALT +Melilite LEUCITE BASALT MELILITE BASALT OLIVINE DIABASE DIABASE THERALITE CITE ANDESITE (Rarely prphyriticJ ·ESSEX!TE 'ORPHYRY PORPHYRY DOLERITE RTZ I OLIVINE -Olivine PYROXENITE PERIDOTITE DIABASE DIABASE THERA LITE +BGsic soda-lime 'ORITE DIQRITE IJQLITE HORNBLENDITE (Rarely porphyritic) ESSEXITE feldspar PORPHYRY .PORPHYRY Rarely porphyritic IlLITE I +Olivine PICRIT.E DRPHYRY) DOLERIT~ MISSOURITE DUNITE GABBRO APLITE OLIVINE DIORITE GABBRO 4LCHITE NORITE APLITE APLITE APLITE BEERBACHITE
KERSANTITE KERSANTITE OLIVINE .RTZ KERSANTITE :RSANTITE SPESSARTITE SPESSARTITE • Melilite 'SSARTITE OLIVINE ALNOITE CAMPTONITE ODINITE SPESSARTITE
GABBRO 'TZ DIORITE OLIVINE DIORITE PEGMATITE GMATITE GABBRO PEGMATITE A LITE NORITE PEGMATITE 'GiriATITE} PEGMATITE
0 ~Augite or Dialloge +Augite or DiaiJ::fe a v GABBRO OLIVINE GABS -Olivine +Hy{)!!rsthene or 1-Hy{)!!rsthene or THERA LiTE IJOLITE PYROXENITE PERIOOTiT£ ~ DIORITE Enstatite Enstatite. ESSEXITE +Olivine / NORITE OLIVINE NOR IT E HORNBLENDITE DUNITE . -Pyroxene MISSOURITE ANORTHOSITE • Prj~XJ8f.ouTE ~1 If SiOe GENERAL INCREASINGLY DARK COLOR- - moy be present. The series granite-syenite -monzonite-granodiorite-diorite- gabbro is characterized by :imultaneously become less sodic and more calcic in their isomorphous series. The feldspars ore essentially 1ied by increases in the proportions of dark colored minerals. The mineralogic changes ore caused by differing crease and the proportions of MgO ond,ta some exteM;·of CoO increose.Rocks related in composition to granites termedia te ~ "Basic • igneous rocks include go bbros and fom i ly. Feldspar-and feldspothoid- free rocks .ore called ' resignoted os"alkoline•types. i Deeply weathered pebbles of basalt selected from physically unsound gravel in a deposit near Pasco. Washington. The gravel and sand greatly reduce the freezing and thawing durability of concrete when used as ag gregate. Natural size.
Photomicrograph of pyroxene andesite. Crystals of pyroxene (gray) and laths. of plagioclase feldspar (white) are held in a groundmass composed of minute crystals and glass. The sample was obtained from the proposed Chifio Damsite. New Mexico. Magnification X 28.
11 Photomicrograph of rhyolite showing marked flow-banding. The rock is composed largely of glass (cloudy) containing scattered minute crystals and vesicles (white). The rock was investigated as a source of riprap for Ochoco Dam, Oregon. Magnification X 28.
Photomicrograph of vesicular basalt com posed of pyroxene crystais (gray) and laths of plagioclase feldspar (white, elongated) held in a groundm.ass composed of minute crystals and glass. Vesicles (voids) are irregular in shape and appear white. The ·rock was obtained from the proposed Chiflo .Damsite, New Mexico. Ma~ication X 28.
12 Monzonite porphyry constituting the right abutment and powerhouse foundation. Green Mountain Dam. Colorado. The large crys tals (phenocrysts) are anorthoclase feld spal" and the smaller crystals are plagio clase feldspar. The groundmass is com posed of exceedingly minute crystals. About natural size.
Photomicrograph of gabbro showing typical well interlocked texture and massive struc ture which are conducive to high strength and elasticity. The rock is composed of augite (gray) and p1agioclase feldspar (white). Mag nifieation ;x 12
13 Photomicrograph of obsidian showing well formed crystals of quartz (Q). and the alkali feldspar sanidine (S) embedded in a glassy groundmass. The rock is deleteriously reactive with cement alkalies when used as a concrete aggregate. The sample was obtained from near Georgetown. Colorado. Magnification X 28.
Photomicrograph of typical granophyre show ing microcrystalline internal texture and massive structure composed of intercrys tallized quartz and feldspar. The rock is a deleterious constituent of the aggregate used in construction of Stewart Mountain Dam, Arizona. Magnification X 13.
14 Photomicrograph of altered rhyolite tuff showing irregular fragments of glass em bedded in a Jine-grained matrix composed largely of the clay mineral beidellite. When pulverized and calcined at 14000 F., the material produces a satisfactory pozzolan. The sample was obtained near Wagon Wheel Gap, Colorado. Magnification X 90.
Photomic:rograph of biotite diorite, show- . iDg typical moderate~ well.interlocked tex ture aad ~ssive structure. The rock·is composed of plagioclase feldspar (P), shoW ing well-deve~oped internal twinning, and biotite (B). Nicol prisms in crossed posi tion. Magnification X 13.
15
Photomicrograph of biotite granite gneiss~ a rock of metamorphic origin similar to a granite in composition. The rock is com posed of microcline feldsPar (M), quartz (Q), and biotite (B). The well interlocked texture contributes to high strength and elasticity. The rock was encountered in the excavation of Ra.mshorn Tunnel, Colorado Individual crystals of microcline feldspar Big Thompson Project, Colorado. Mag constituting whole pebbles in the 1-1/2- nification X 28. to 3/4-inch gravel used as aggregate in deteriorating pavements at Kimball, Ne braska. The feldspars reduce the durability of concrete because of their abnormally low coefficient of thermal expansion, and their poor bonding characteristics. Natural size.
17 Photomicrograph of a granite. showing typ ical poorly interlocked texture and massive structure. The rock is composed essentially of quartz (Q}. plagioclase feldspar (P). and microcline feldspar {M). The rock was investigated as a source of riprap for Heart Butte Dam. Missouri Basin Project. North Dakota. Magnification X 16.
18