Soil Mechanics Laboratory
Fall 1999
Rock Identification Lab
Objective: To learn the basic principles of rock identification.
Method: 1) Read through the descriptions of minerals and of igneous, sedimentary and metamorphic rocks in the attached handout. 2) Identify each of the samples in the lab on the basis of these descriptions. 3) Develop a flow chart that can be followed to identify each of the rocks and minerals displayed. (The first division in the chart should give methods for identifying the main type of rock, i.e., mineral, igneous rock, sedimentary rock, or metamorphic rock. A illustration of the start of the chart is given on the next page.)
Deliverable: At the end of lab today, turn in the following,
· a list giving the number of each sample and its rock or mineral type
· your completed flow chart
Mineral/Rock Descriptions (Taken from Hamblin and Howard, 1980)
Quartz is one of the most common minerals in the earth’s crust, as it is a major constituent in many igneous, sedimentary and metamorphic rocks. If allowed to grow in an unrestricted environment, it will form well-developed hexagonal crystals that terminate in a pyramid. In most igneous rocks and in veins, quartz is one of the last minerals to form, so it fills interstices between other minerals and thus has an irregular shape. The color, crystal size, and general appearance of quartz will vary greatly, so that a large number of varieties have been named. Regardless of color, crystal size, shape, mode of origin, etc., all quartz is characterized by a hardness of 7 (it will scratch glass and steel), conchoidal fracture, and glassy luster.
The feldspars are the most abundant mineral in the earth’s crust, and, like quartz, they are a basic constituent for many igneous, sedimentary, and metamorphic rocks. All are aluminum silicates of potassium, sodium, or calcium and are closely related in form and physical properties. Two main subgroups are recognized: the plagioclase feldspars and the potassium feldspars. Distinction between varieties of feldspars can be made best by a chemical analysis, or optical or x-ray measurements of the crystal structure. The color is due to small amounts of iron or magnesium and certain aspects of the crystal structure and is not diagnostic of a particular variety. The most important physical properties of the feldspars are two-directional cleavage at approximately right angles, hardness of 6, and pearly luster.
The micas are a distinctive group of minerals with cleavage so perfect in one direction that each crystal can be separated into sheets thinner than a piece of paper. The two leading varieties are muscovite (or white mica) and biotite (or dark mica). In addition to its perfect cleavage, mica is characterized by a pearly to vitreous luster. The cleavage sheets are elastic and flexible, and the mineral is notably soft. Micas are common in light-colored granitic rocks, and to a lesser extent in light-colored volcanics. They are also widespread in a variety of low- to intermediate-grade metamorphic rocks and impure sandstones.
Pyrite is a yellow metallic mineral composed of FeS2 which commonly crystallizes into cubes. It has a hardness between 6 and 6.5 and breaks with a conchoidal fracture.
Granite is probably the most familiar of all igneous rocks. Its texture is phaneritic (individual crystals are large enough to be plainly visible to the naked eye) but the average crystal size ranges from less than one-half inch to more than one inch in diameter. Biotite, amphibole, and plagioclase generally have well-developed crystal faces whereas quartz and potassium feldspars have poorly developed crystal faces.
The texture of diorite is essentially the same as that of granite. The two rocks differ only in composition. Whereas granite contains potassium feldspar, quartz, and calcium plagioclase, diorite is composed predominantly of plagioclase and ferromagnesian minerals.
Basalt is one of the easiest rocks to recognize, because it is characteristically black, dense, and massive. Individual crystals cannot be seen with the naked eye, but under the microscope tiny needles of plagioclase crystals commonly form a feltlike network surrounding crystals of pyroxene and olivine. Many basalts are clinkerlike in appearance, with about half of the total volume consisting of small holes termed vesicles. Vesicular textures develop as gases rise toward the top of the flow and are trapped in the cooling lava.
Obsidian is a massive volcanic glass. It breaks with a conchoidal fracture and has a bright, glassy luster. In spite of its composition, it is characteristically jet black due to the presence of countless dustlike particles of magnetite or ferromagnesian minerals. Although obsidian is not crystalline, it does contain skeletal crystal embryos called crystallites. Many aphanitic rocks contain appreciable quantities of glass filling interstices between crystals.
Pumice is a very porous volcanic glass. Its texture consists of subparallel silky glass fibers tangled together. It originates when relief of pressure in a volcano permits rapid expansion of gases through the upper part of the ascending column of obsidian lava. The lava swells into a froth or foam with innumerable minute bubbles and solidifies.
Conglomerates consist of coarse rock fragments (greater than 2 mm in diameter), held together by a matrix of sand, clay, and cement. The individual pebbles are usually well rounded and moderately well sorted. Pebbles in conglomerates may consist of any mineral or rock, but resistant materials such as quartz, quartzite, and chert are especially common.
Sandstones are clastic (consisting predominantly of fragments and debris of other rock material) sedimentary rocks consisting mostly of grains ranging from 1/16 to 2 mm in diameter. The grains are generally well rounded and show other effects of abrasion. Quartz is usually the dominant mineral, although feldspar, garnet, mica, and other minerals may be present in varying amounts.
Shales are fine-grained clastic rocks consisting of particles less than 1/256 mm in diameter. They are characteristically laminated or thin bedded. Quartz, mica, and the clay minerals are the dominant constituents, but the particles are too small to be seen without a high magnification.
Limestones are sedimentary rocks that contain more than 50 percent calcium carbonate. Impurities, which may range in amount to 50 percent, include clay, quartz, iron oxide, rock fragments, and other material. The calcite may be precipitated chemically or organically or may be of detrital origin. The many varieties of limestones are classified on the basis of texture or some other significant property. Typical varieties include crystalline limestone, microcrystalline limestone, oolitic limestone, coquina, and chalk.
Coal is composed of highly altered plant remains and various amounts of clay. It is opaque and noncrystalline, with colors ranging from light brown to black. Coalification results from the burial of peat and is classified according to the degree of change. Prolonged heat and pressure produce lignite (brown coal), bituminous (soft) coal, and anthracite (hard) coal.
Slates are fine-grained metamorphic rocks possessing a type of foliation known as slaty cleavage. In many slates, the traces of original bedding are expressed by changes in color or grain size and are commonly at an angle to the foliation. Common minerals are quartz, muscovite, and chlorite, but crystals are generally so small that they can be seen only under high magnification. Slates are characteristically dense and brittle and are colored gray, black, red, or green. They are low-rank metamorphic rocks derived principally from shales.
Schists are metamorphic rocks in which the foliation is due to the parallel arrangement of relatively large crystals of platy minerals. Muscovite, chlorite, and talc are the important platy constituents. Feldspars are rare, but quartz, garnet, and hornblende are common accessory minerals.
Gneisses are metamorphic rocks in which the foliation results from layers of different mineral groups. Feldspars and quartz are the chief minerals, with minor amounts of mica, amphibole, and other ferromagnesian minerals. Gneisses thus resemble granite in composition, but are distinguished from them by the foliation. The foliation in a gneiss may range from semi-continuous layers of light and dark minerals to highly contorted, well-defined layers. In many deposits, layers expressing the foliation are several inches thick.
Marble is a nonfoliated metamorphic rock composed principally of calcite or dolomite. The crystals are commonly large and interlock to form a dense crystalline rock. Bands or streaks or organic impurities resulting from flowage or extreme deformation are common in some deposits. Colors may be white, pink, blue gray, or brown. Like limestone, its softness and its effervescence with hydrocholoric acid characterize a marble.
The beginning of your flow chart should look something like this: