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An overview of the engineering properties of igneous rocks, focusing on their classification based on composition, texture, and structure. It discusses the origin and characteristics of igneous, sedimentary, and metamorphic rocks, and the importance of petrographic analysis in determining their properties for engineering applications. The document also touches upon the use of igneous rocks in concrete production and the significance of petrography in predicting their engineering properties.
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BUREAU OF RECLAMATION
PETROGRAPIIY AND ENGINEERING·
PROPER11ES OF IGNEOUS ROCKS
hy Rit~bard C. 1\lielenz
95 cents
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
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
Excavation and concreting of altered zones in rhyolite dike in the spillway foundation. Davis Damsite. Arizona-Nevada.
Fl'ontispiece
Rocks are important to engineers. Most modern hydraulic structures rest upon rock ·foundations. some rest laterally against rock abutments, and essentially all are· composed, at least in part, of rock material. Conse- quently, the design of engineer~ng works and method~ of construction depend to a great extent on the properties of rocks-- their strength, elasticity, permeability, durability' density. volume change, and solubility. It follows. then, that adequate engineering investigations of rock should be made to reveal these properties and their effect on construction.
The properties of rocks depend upon their mineral composition~ texture, and struc- ture. Mineral composition controls such properties as hardness, density, and solu.., bill~. Texture and structure comprise the fabric in which the individual components of the rock are arranged; they control the properties of the rock as a whole, such as strength. permeability, and durability.
When we think specifically in terms of the properties of the rocks at a site or of the rock materials to be used in the construc- tion, rules for rock classification and no- menclature might seem to be irrelevant. On the contrary, petrographic classification of rocks is based upon composition, tex- ture, and structure--the sa_me character- istics upon which the rock _properties depend.
Therefore, petrographic examination and identification will indicate the properties to be expected in the rock ..
Petrography is being applied increasingly to engineering problems. All concrete ag- gregates and riprap materials to be used in construction of Bureau projects are exam- ined and evaluated petrographically. Sam- ples of rock as well as earth materials are analyzed and tested petrographically in con- nection with planning, design, construction, and maintenance problems. For example, during the past 10 years 282 samples of riprap and 1, 078 samples of concrete ag- gregate were examined and evaluated for engineering use.
Because of the increasing application of petrography for engineers a greater rmm.ber and variety of rock nanies are appearing in construction and feasibility reports on engi- neering problems. Moreover, rock names are strange to most engineers, and they are far from self-explanatory. As a result, many questions have been directed to the Petrographic Laboratory regarding the basis and method of petrographic classification. This mc::nograph has been prepared to answer these questions, and to bring to ~e engineer a summary view of· the range of rock com- position, texture, and structure, and the relationship of these to rock properties.
Rocks, as a whole, are divided into three classes: igneous, metamorphic, and sedi- mentary. Igneous rocks originated through solidification of molten material either at or below the surface .of the earth. Igneous rocks comprise such types as the common granites which formed from tremendously large masses of molten material intruded from depths of the earth into the crust, then
cooled and crystallized far below the sur- face. Igneous also are the lavas poured from volcanos, and those which did not quite reach the earth's surface but froze at small depths as small intrusive masses, such as dikes and sills.
Metamorphic rocks were formed through recrystallization, in an"essentially solid
structure and are cha.ract~rized by a tightly lmit interlocking equigranular texture.
. Igneous rocks~ more so than sedimentary or metamorphic rocks, lend themselves to classification on a fundamental basis, since they are formed in a relatively narrow range of physical-chemical environments, and because they represent the solidified phase of siliceous melts typified by chemical com- positions which vary within comparatively narrow limits. Igneous rocks differ from one another because of variations in en- vironmental conditions of formation and variations in chemical composition. En- vironmental conditions include (1) the tem- perature of the melt (magma), (2) the rate of cooling, (3) the pressure. and (4) the viscosity of the melt. The chemical com- position of a magma may control many of its physical-chemical properties, such as melting point and viscosity. The most im- portant aspects of chemical composition are silica content (which ranges from 45 per- cent to 77 percent iJ) common igneous rocks) and the content of gases and vapors occuring within the melt. Viscosity increases with silica content; whereas viscosity is de-
Igneous rocks are classified through the simultaneous consideration of two factors: (1) the texture and structure of the rock, and (2) the chemical and mineralogic composi- tion. Textur~s and structures of igneous rocks are controlled by the physic8.1-chemi- cal conditions existing within the magma during solidification. In turn, these condi- tions are most strongly influenced by the depth at which solidification took place (which controlled the rate of cooling and the hydrostatic pressure) and the physical
characteristics of the melt itself (mainly temperature and viscosity). The miner- alogic composition of igneous rocks is largely controlled by chemical composition. of the magma; but in part the minerals which develop arE' determined by the envi- ronmental conditions of solidification.
For example, volcanic rocks which are characterized by rapid cooling commonly are crystallized only partially, since some of the melt was congealed as glass before crystallization was completed. The min- erals which exist in equilibrium with half- crystallized melt may be different from those which would exist in equilibrium with the last residues of the melt. Thus, a ba- saltic magma. the mineral olivine, forms early in the progressive crystallization. But, if the melt is comparatively rich in silica, the olivine may subsequently be con- verted through reactions with the melt to pyroxene. The final completely crystallized rock could be designated as gabbro and would be completely free from olivine; yet the partially glassy basalt which could be formed by quick chilling of the partially crystallized magma would contain crystals of olivine. Comparable progressive changes occur in all igneous melts, especially the plagioclase or soda-lime feldSpar which becomes increas- ingly sodic and less calcic during crystalli- zation.
Thus, an engineering- classification of igneous rock must use the mineralogic com- position as a basis. ·A knowledge of the chemical composition is valuable for cor- relation of, for instance, separate but ad- jacent masses, or where the minerals arP. not determinable microscopically. However, the rocks can be adequately classified ac- cording to this scheme only after chemieal analyses have been performed. This is an expensive and time-consuming process.
The properties of rocks depend uJ)on their texture, structure, and composition; conse- quently, when these factors are precisely· evaluated, the physical and chemical qual- ities of the rocks can be predicted. How-
ever, this prediction cannot be made from determination of the petrographic identity alone, even though petrographic identity is established from texture. struc~re, and composition. Texture, structure. and com-
position can· be changed by secondary proc- esses, and the innate properties of the rock thus caused to vary significantly. An ori- ginally weak rock may be strengthened by slight metamorphism. An originally strong rock may be weakened by weathering or hy- drothermal alteration; or an originally mas- sive rock ~ be rendered pervious by frac- tures resulting from geologic processes. To the extent that rocks are altered, de- composed, or fractured, they partake more and r;1ore of proper1Ies tjpicaliy. relateci to rocks of other origins. Thus, with increase in content of clayey decomposition products, and breakdown of the original internal texture and structure, igneous and metamorphic rocks progressively will approach the sedi- mentary c1ays and shales in their properties. Therefore, determination of petrographic identity of a rock is merely the first step in the application of petrography to engi- neering.
Texture, structure, and composition of rocks are determined by petrographic meth- ods, comprising microscopical examination, microchemical analysis, X-ray diffraction analysis, and certain physical tests. With skill, experience, and good judgement, the engineering petrographer can anticipate the degree to which the observed charac- teristics will be reflected in adverse prop- erties of the material. ·With continued re- search, the correlation between the petro- graphic characteristics and the engineering properties will be improved further. By his examination, analyses, and semiquan- titative tests, the petrographer establishes the need 'for detailed testing of materials. He must determine the probability that the stratification, schistosity, or fractures will reduce critically the strength, dura-
bility, or impermeability of th·e material, the degree to which the expansive poten- tialities of clay constituents will deleteri- ously affect the engine_er~·properties, and the engineering significance of other physical ·and chemical attributes of material.
·Many examples of the interrelation of petrographic character and engineering properties of materials can be cited. Petro- graphic examination Qf schists from the
demcnstrated that schistosity, stratification, and microscopi~ fractures in quartz grains were present. It was anticipated that these internal structures would control the failure of specimens in compression. Subsequent tests corroborated this prediction. Portions of the shore o! Lake Roosevelt, Washington, are composed of the Nespelem formation known to contain a series of stratified silts and ciays having petrographically unstable $racteristics. As a result of wetting by the reservoir waters, the strata of clay softened and became ·so unstable that exten- sive sliding occurred, involving areas ex- tending as much as 1, 000 feet back from the shoreline. Apparently insignificant variations in composition of rocks actually can cause gre~t changes in properties. At a proposed dam si~e on the Colorado River in Utah, the great ·decline in strength of sandstones as a result of wetting was found ·to correlate with variations in the amount of determinable interstitial clay, which in no case constitutes more than 5 percent of the rock. Petrographic examination of the clay- stones and shale forming the foundation of the Malheur River Siphon, Owyhee Project, Oregon, would have revealed their poten- tialities for expansion with wetting, a proc- ess which within 8 years raised the siphon supports more than 1 foot in some places.
ENGINEERING PROPERTIES OF IGNEOUS ROCKS
Plutonic rocks
Igneous rocks which form in the so-called plutonic, or deep, zone of the earth's crust are characteristically medium-to-coarse- _grained in texture and massive in structure.
The series of rocks from granite through monzonites, quartz monzonites, granodi- orites and quartz diorites, to gabbros include by far the most common types. When en- countered in place, plutonic igneous rock podie~ are usually large and may be regarded
·limestone gravel is it possible to obtain good concrete with the apparently sound, highly granitic sand-gravel aggregate of the Kan- sas-Nebraska Area. Natural granitic ag- gregates in the vicinity of Denver, Colorado, produce satisfactory but not superior con- crete only with adequate control of mixing, placing, and cur~.
With weathering or hydroth.ermal altera- tion, plutonic igneous rocks may produce compounds which themselves contribute to reduced qualitY of concrete. For example, an altered anorthosite from Soledad Canyon, California, containing the zeolite laumontite (or leonhardite) is known to have caused rapid deterioration of cast stone and stucco in the vicinity of Los Angeles. An appar- ently sound, crushed granodiorite containing a zeolite is associated with surface scalirig and efflorescence of concrete on the down- stream face of a dam in Southern California.
At Green Mountain Dam, Colorado, it was noted that a small number of boulders in stockpiled riprap had weathered to a rub- ble. The rock had been identified petro- graphically as monzonite porphyry. Exam- ination of the excavation from which the rock had been obtained revealed shear zones along which the rock had lost its physical coherence by deep-seated alteration. The rock was in sound physical condition except along these zones. Exposure of the rock to natural weathering resulted in rapid disinte- gration of the altered material. Examination during 1959 of the riprap that had been placed on the dam about 18 years previously showed that the small number of contaminating boulders had completely disintegrated by exposure to the weather while the sound rock remained unaffected.
These comments in regard to the quality of plntonic igneous rocks should not be taken to imply that granitic rocks in general in- ·variably contribute to inferior quality of concrete. On the contrary, plutonic igneous rocks, or sands and gravels containing par- ticles of plutonic igneous rocks, have been widely and successfully used as concrete aggregate. It is to be anticipated that ad- verse qualities will be reduced by (1) de- creased grain size of the crystals composing the rock, (2) increased interlocking of the grains, such as will accompany metamor- phism under· high pressure and temperature,
and (3) decreased content of orthoclase and microcline, the feldspars characterized by very low thermal coefficient of expansion.
Hypabyssal rocks
Igneous rock solidified at moderate depths in the earth's crust (the so-called "hypa- byssal" zone) are charac:terized by textures, structures, and composition intermediate between those of the plutonic and volcanic rocks. Consequently, their physical and chemical properties are likewise inter- mediate. Because of the high viscosity of highly siliceous melts and the common as- sociation of abundant gases and vapors with the more siliceous magmas, rocks of the granite-rhyolite family exhibit. a very close relation between environment of solidifi- cation and internal texture and structure. Rocks of the gabbro-basalt family do ~ot demonstrate a comparably close relation of this kind; thus the characteristics of the rocks may not always reveal the mode of occurence of the igneous bodies.
Hypabyssal igneous rocks occur as com- paratively small bodies, which are called · "dikes" if they are thin and tabular in form and cut across the structure of the country rock, or "sills" if they are thin and tabular in form and intrude along structures (such as stratification) of the country rock. Hy- pabyssal intrusions of irregular or lenticular shape, such as laccoliths, are.less common than are dikes and sills.
Because of their small· size, these intru- sions typically constitute merely a portion of large construction sites.· If they are ex- ploited as sources of construction materials, only small quantities may be available. For example, an andesite intrusion occurs at the left abutment of Palisades Dam site, Idaho, but the remainder of the site is under- lain by sedimentary rocks. Because the competence of the igneous rock and the in- creased strength of the sedimentary rocks indurated under the influence of the heat of the intrusion, the left abutment was selected by the designers as the site of the spillway structure. Unfortunately, close jointing in the ancfesite restricted use of the rock as riprap because of the limited quantity of larger sizes available. At Davis Damsite, Arizona_.Nevada, granite and granite gneiss
constituting the greater part of the founda- tion and the abutments are cut by rhyolite dikes deeply altered (even· to clay) along contacts. Because of this incompetence, these zones, in some places more than 10 feet in width, required excavation to good rock prior to construction. Basalt dikes cutting the foundation were strong and com- petent, and presented no design problems. Rock of suifable size, quality, and gradation for slope protection could not be obtained from the excavated rock; therefore, a quarry was opened at some distance from the dam.
Along the Feeder Canal, Columbia Basin Project, Washington, sills and flows of basalt were involved in ancient landslides and possibly in renewal of sliding of the shales and siltstones of the Latah formation. These slides greatly impeded progress of construction.
Volcanic rocks
Igneous rocks which form at or near the surface of the earth are characteristically very fine-grained or partially to completely glassy. They may be massive, or they may contain few or many vesicles owing to release of gas from the melt, or they may be banded as a result of flow of the plastic lava. Thus, the structure of the ·volcanic rock may be massive, vesicular. -pumiceous, or flow-banded. Comparable ·to the granite-gabbro series is the rhyolite- · "basalt series of volcanic rocks. ·
Lava flows are sheet-like in form; that is, the area which they cover is large com- pared to their thickness. Because.of their greater fluidity, basic lavas, such as basalt, commonly extend over large areas even though the flows are thin. Acidic lavas, such as rhyolite, usually are limited in area and typically are thicker for a given distance of flow: Because of their small thickness or limited areal extent, lava flows are likely to underlie merely a portion of a large con- struction site.
Lava flows are commonly interbedded with tuffaceous or other fragmental volcanic ~aterial blown from the volcano. Also. particularly in the basic (basaltic) lavas, tunnels or tubes may occur wherever the still fluid lava flowed o~t from beneath a
hardened surface crust. These zones of fragmental material- -the lava tunnels, the fractured zones, and to some extent, the vesicular zones--contribute to the charac- teristically high permeability of thin bedded lavas. Because of the higher viscosity and commonly higher contents of gases and va- pors of acidic and intermediate lavas, rhyo- lite. dacite, and andesite tuffs and agglom- erates are considerably more common and widespread than tuffs and agglomerates of basaltic composition.
The crystals composing the greater portion of volcanic rock types are minute, usually being invisible to the unaided eye. Typi- _cally. volcanic glass, representing the un- crystallized residue of the melt, is present constituting in some instances, merely in- _terstitial segregations, in others, virtually the entire rock. Most volcanic rocks are porphyritic; i.e. , embedded within the fine- grained or 'glassy groundmass of the rock are crystals which are considerably larger than those in the groundmass. Rhyolites commonly are highly glassy because of the high. viscosity of the melt, whereas basic volcanics. such as basalt, usually contain merely small amounts of glass held inter-
. · stitially in the groundmass. Because of the content of glass, volcanic rocks are usually hard and brittle. rhyolites typically being the ·most brittle because of their greater glass content.
Volcanic glasses are unstable chemically, and are therefore decomposed easily by weathering or hydrothermal activity. Con- sequently. during examination of volcanic formations to establish their structural stability. diligent investigations should be made to assure that all zones of alteration are discovered. For example, along some ·sections of the Main Canal. Deschutes Proj- ect. Oregon, locally intense alteration of andesite and rhyolite tuffs to bentonitic clays .is responsible for drastic instability of cut slopes and displacements in the canal sec- tion. Because of their chemical properties, glassy rhyolites. andesites, and dacites are deleteriously reactive with the alkalies re- leased during hydration of portland cement. .As a consequence of their widespread occur-
portant rocks participating in cement-ag- gregate reaction. In addition. because of -their chemical activity. acidic. intermediate,
sence of zones of shearing and faulting can be established by correlation of strata a- cross the site. For example, at Canyon Ferry Damsite the sedimentary formation constituting the foundation and abutments contained thin sills of altered andesite, dis- ti.Dguisbable onzy- .after petrographic analysis, whose continuity proved the absence of sig- nificant faulting beneath tbe river alluvium.
·Petrography is a valuable tool for deter- mination of the properties of rocks, either when applied independently or as a means to select those quantitative tests necessary to measure specific properties. The latter function is the more important. Most tests to measure properties of rock materials are expensive and time- consuming, and they uSually require carefully selected samples so protected that at the time of test they truly represent the character of the rocks and mater~s in place, especially with regard to moisture content and fractures. eon.:. sequently, it is wise to determine the ne- cessity for certain tests before they are requested. For example, testing the .quality of an aggregate by performing tests of con- crete ~ntaining the aggregate may be a- voided by application o~ physical and chem- ical tests and petrographic examination of ·the aggregate. Tests of concrete need be performed only when the results of-~e phys- ical and chemical tests of the aggregates are anomalous, or if the petrographic examina- tion indicates adverse properties not eval- uat~ by the aggregate tests. Determination of volume increase of rock materials with wetting is significant only if clay minerals of -the montmorillonite type (bentonitic) ~e _present. The presence and abundance of these clays can be determined quickly by petrographic and X-ray diffraction analysis. If the number of samples available for test "is so great as ·to preclude testing.of all,
petrographic examination to select speci- mens representative of the group frequently will frimplify·the test program without sacri- ficing significance .of the results.
Engineering petrography and engineering .geology in coordination serve to relate the ·properties of individual specimens subjected to laboratory tests to the properties of the rock formations in place. For example, _the petrographer and geologist may be called upon to decide: To what extent should the fractures. joints, and planes of shear in the rock in place be cause for reducing, below the measured strength of rock specimens, the strength of the _rock mass; or to what ·extent might the swelling clays remain stable by virtue of their impermeability; or to what degree would discontinuities, such as joints, bedding planes, and faults, augment the known permeability of the rock itself. Not infrequently. the results of tests of rock specimens. hOwever selected, may be more delu~ tha.n: instructive. Only experience, geologic and petrographic skill, and good judgement will permit adequate translation of test results into the data required for engi- neering design.
The petrographer can assist the engineer in design, construction, and maintenance problems. By working with the engineering geologist, the petrographer facilitates selec- tion. exploration, and subsurface investi- gation of construction sites. Tilrough appli-- cation of petrography to materials testing, specific ~ests to be applied and samples to be tested can be seiected with minimum ·hazard of iriefficiency. Petrography is ef- fective in predicting the engineering prop- erties of rocks because those properties are determined by the rocks' texture, structure, and composition, chal-acteristics which can be. discerned by petrographic methods.
~ .MINERALOGIC AND TEXTURAL (^) CLASSIFICATION
OF IGNEOUS ROCKS
On the basis of mineralogic composition and internal texture and structure a chart has been prepared which illustrates the
9
characte_ristics of all widespread igneous rocks.* 'Ibis chart. which appears following page 10, Dl8IY be used far systematic classifi-
cation of described rocks if all pertinent mineralogic and textural data are known; or definitions of rock types can be obtained from the chart. It is to be noted that many
tified only after microscopic examination; thus ad~quate facilities for preparation and examination of specimens are necessary for complete petrographic analysis. The organ- ization of the chart is described below.
Textures
Rocks shown in the same vertical column are similar in chemical and mineralogic composition. For example, except for some natural glass which may occur in the rhy- olite, granites and rhyolites are essentially the same in composition. However, as is noted in the column headed "Typical Tex- tures, " the characteristic texture and struc- tures of the rocks in any given vertical col- umn are different. Rocks which solidified at comparatively great depths in the earth are characterized by completely crystalline, fine- to coarse-grained textures. These rocks are shown in the bottom lines of the chart. They possess textures similar to granite and represent igneous bodies which solidified in the so-called plutonic zone of the earth's crust.
In the four rows of boxes above the bottom two rows, the rocks are generally com- pletely crystalline, but they are fine-grained or porphyritic, since they commonly rep- resent small, rapidly cooled igneous bodies, · such as dikes. They formed in the so- called hypabyssal zone at moderate depths in the earth.
The three rows of boxes at" the top of the chart represent r~cks which solidified at or
*Expanded from charts prepared by J. F. Kemp, A Handbook of Rocks for Use without the Petrographic Microscope, D. Van No- strand Company, P. 46; and G. D. Louder- back, Index Table of Igneous Rocks, Min-. eralogic and Textural Classification.
near the surface of the earth and which are therefore exceedingly fine grained or are partially or completely glassy. These rocks are generally related to volcanism. The topmost row represents fragmental mate- rials thrown from volcanic vents. At the extreme left side of the chart a note has been added to describe the interrelation of occurrence, chemical composition, and texture of igneous rocks.
Mineralogic Composition
Along any horizontal row the rocks are different in chemical and mineralogic com- position, and therefore are classified dif- ferently even though they may have originated In precisely the same physical situation. Feldspars are the most abundant minerals in the crust of the earth, and they occur in ~p.ost igneous rocks. Therefore, igneous rocks are classified on the basis of the kind and relative abundance of the feldspars. Feldspars may be classified as alkali feid- spars which are of the potassium or sodic ahlminum silicates, and plagioclase or soda- lime feldspars, which represent an iso- morphous series between NaAlSi30s and CaA1 2 Si 2 o8• AJkali feldspars predominate in igneous rocks which are rich in alkalies (e. g. , phonolites) or high in silica (e. g., rhy- olites), whereas plagioclase feldspars occur in small or large proportions in virtually all igneous rocks. A note concerning the relation of chemical composition to min- eralogy of igneous rocks may be found at the bottom of the rock chart.
A horizontal row in the upper part of the chart indicates the range of silica content characteristic of the various groups of rocks. It is to be noted that these proportions rep- resent Si02 determinable only by chemical analysis, as distinguished from silica crys- tallized as quartz.
Boxes within the chart which are empty represent exceedingly rare types of classifi- cations not exemplified by described rocks.
'•
KS Compiled^ by^ R.C.^ Mielenz
SODA -LIME (^) FELDSPARS PREDOMINATE FELDSPARS^ ABSENT (Or nearly so) 13187 to 01100 Potash n~t uncommon^ feldspars in^ alteredore^ rare; bosolts,eg.spilite^ but^ albite^ is^ Smallof potash^ proportions feldspar^ Samepar mayolko!i occur^ felds-
GOCLASE AND ANDESINE LABR'1l.DORITE,^ BYTOWNITE^ ANDESINE^ FELDSPARSOME^ SODA-LIME MAY BE SOMECONSTITUTE^ SODA-LIME UP TOFELDSPAR I0%0FROCK.^ MAY AND ANORTHITE (^) TO BYTOWNITE (^) PRESENT LABRADCIRITE TO ANORTHITE -NEPHELINE QUARTZ -OUARTZ (^) -OliVINE +OLIVINE +LEUCITEor +NEPHELINEor -LEUCITE-OLIVINE -NEPHELINE-LEUCITE ("'5%) (or•S%}^ +NEPHELINE +LEUCITE +PYROXENE +OLIVINE or (^) +PYROXENE +HORNBLENDE BIOTITE HORNBLENDE AUGITE, ALKALI AUGITE (^) - (^) HORNBI.ENDE DIOPSIOE AUGITE PYROXENE^ PYROXENES and ALKALI PYROXENE MICA HYPERSTHENE Hornblende^ AMPHIBOLES MICA 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 (^) ANDESITE ASH, BASALT ASH, (^) OLIVINE BASALT, TEPHRITE OR ,BRECCIA, (^) BRECCIA, TUFF, BRECCIA, TUFF, (^) ASH, BRECCIA, TUFF. BASANITE ASH, UFF, OR (^) OR AGGLOMERATE OR AGGLOMERATE OR AGGLOMERAT~ BRECCIA, TUFF, OR 'LOMERATE AGGLOMERATE_
(^1) ATE GLASSES BASIC GLASSES ULTRA BASIC GLASSES E, SCORIA (^) SCORIA, VARIOLITE, TACHYLITE
BASALT. .I OLIVINE BASALT
-Olivine.^ LIMBURGITE -Olivine NEPHELINITE +Basic^ soda-lime liCIT£ ANDESITE (^) If diabosic texture: TEPHRITE^ LEUCITITE^ feldspar +Olivine +Olivine^ AUGITITE PIC RITE DIABASE OLIVINE DIABASE (^) BASANITE NEPHELINE BASALT (^) +MelilitePICRITE^ BASALT LEUCITE BASALT (^) MELILITE BASALT OLIVINE DIABASE (^) DIABASE THERALITE CITE ANDESITE (^) (Rarely (^) prphyriticJ ·ESSEX!TE 'ORPHYRY PORPHYRY DOLERITE
'ORITE (^) DIQRITE DIABASE^ DIABASE^ THERA^ LITE^ IJQLITE HORNBLENDITE +BGsic^ soda-lime PORPHYRY (^) .PORPHYRY (Rarely^ porphyritic)^ ESSEXITE^ Rarely porphyritic feldspar IlLITE I^ +Olivine^ PICRIT.E DRPHYRY) DOLERIT~^ MISSOURITE^ DUNITE GABBRO APLITE OLIVINE 4LCHITE DIORITE^ GABBRO APLITE NORITEAPLITE^ APLITE BEERBACHITE
.RTZ KERSANTITE^ KERSANTITE^ OLIVINE KERSANTITE (^) • Melilite :RSANTITE SPESSARTITE^ SPESSARTITE 'SSARTITE (^) CAMPTONITE ODINITE OLIVINE^ ALNOITE SPESSARTITE
'TZ DIORITE GABBRO GMATITE DIORITE^ PEGMATITE^ OLIVINE GABBRO A LITE PEGMATITE^ NORITE (^) PEGMATITE 'GiriATITE} (^) PEGMATITE
a 0 v ~Augite^ or^ Dialloge^ +Augite^ or^ DiaiJ::fe
GABBRO OLIVINE GABS (^) -Olivine ~^ +Hy{)!!rsthene^ or^ 1-Hy{)!!rsthene^ or^ THERA^ LiTE^ IJOLITE^ PYROXENITE^ PERIOOTiT£ DIORITE Enstatite Enstatite.
. /^ -PyroxeneNORITE OLIVINE^ NOR^ IT^ E^ ESSEXITE^ +Olivine^ HORNBLENDITE^ DUNITE
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 1 ied^ 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
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.
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.
