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Petrography and Engineering Properties of Igneous Rocks

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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
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