This laboratory session is a brief introduction to a variety of rock forming, ore, and industrial minerals. The specimens we will examine provide examples of the major features of minerals that can be used for identification and, more importantly for us, have influence on their engineering properties.
The collection of ore minerals gives a good introduction to some the key distinguishing features of minerals: density, luster, color, and cleavage.
The first samples worth a look are the pyrite (FeS) and galena (PbS). Both have distinct cubic crystalline forms and metallic lusters. They differ in color as well as the fact that pyrite lacks a cleavage, while galena as a pronounced cubic cleavage (the samples are rather small so please take my word for this). Galena is a main ore of lead, while pyrite is generally of little value except occasionally in the production of sulfuric acid. A close relative of pyrite, chalcopyrite, (Fe,Cu) S is a main ore mineral of copper particularly from deposits that are not oxidized near the surface. Another common copper ore mineral, malachite (CuCO
) occurs in enriched, near surface zones. The bronze colored mineral in the rock containing the veins is either chalcopyrite or bornite, the sulfide of cobalt. Rarely, copper appears in its native form shown in another sample here in an unusual fracture filling from the upper peninsula of Michigan. The underlying rock is greenstone, a low grade metamorphic product of volcanic, Fe-Mg rich basalts.
Pyrite is a common mineral in many geologic environments over a wide range of pressure and temperature conditions. It is a common component of sulfide-rich ore deposits. In near-surface environments, the presence of pyrite indicates reducing conditions; however in the presence of oxygen it readily rusts into iron oxides pose problems in ore wastes where acid drainage can be problematic. Pyrite is can substitute for other minerals as shown by the fossil scallop that has undergone pyrite replacement.
Also shown here is a sample of sphalerite, ZnS, the main ore mineral of zinc from the
Balmat Mine in upstate New York. Sphalerite is known for its submetallic luster.
The most common iron ore minerals are magnetite and hematite. Using the compass make your best guess which one this specimen represents. Many iron ore deposits are very old (Precambrian, >800 million years) and had sedimentary origins probably from iron-fixing bacteria that no longer live in such abundances in the current surface chemical environment (though iron-fixing bacteria can be seen in many underground mines and tunnels as bright red slimes on water seeps. They are but one type of bacteria found at depth. A significant revolution in geologic thinking is currently underway as recent discoveries of biota are great depths and extreme conditions are changing our view of
how deep life can go! The engineering implications of bacteria on rock properties is still to be explored.
For comparison with the other ore minerals note the dodecahedral garnet crystal. This is non-gem quality specimen. Note the lack of cleavage. Compare this with the purple fluorite (CaF
). Fluorite also produces crystals with cubic symmetry, however, the cleavage is very pronounced, and unlike the galena, is along the octahedral rather than the cubic planes.
The crystalline structures of minerals give rise to many anisotropic properties (except for minerals of the cubic system which are isotropic). Elastic and thermoelastic properties show these behaviors. Optical properties do as well. We are exploring hand specimen identification, but most actual mineral work on rocks is done using microscopic methods that use the optical properties of crystals, particularly the refractive index and the birefringent (light splitting) properties. The gemstones here show those behaviors. Note first the black pendant. This is a diopside (a common rock forming mineral seen here in an unusual gem-quality state). Under light note the formation of the “star” which one can see when observing light along the symmetry axis of the crystal. The star is a cross, reflecting the near orthogonality of the two secondary symmetry axes. The second specimen is a small sapphire. Sapphires are the mineral corundum (second in hardness only to diamond). The mineral carries different names for its gem forms depending on the colors induced by impurities (e.g. ruby, sapphire, etc.). We are looking down the “c” axis, or the principal symmetry axis, of this hexagonal crystal. Note the high refractive index of the gemstone. Also, using the flashlight, try to detect the characteristic sixarmed star of the star-cut sapphire.
The mineral in this group represent the major rock-forming minerals of igneous and metamorphic rocks. Reflecting the dominance of silicon and oxygen in the earth’s crust, they are silicate minerals.
Rock-forming silicates are all based on the fundamental structure of the silica tetrahedron, which consists of a central silicon atom with oxygen atoms at the tetrahedral vertices. Like its similarly valance cousin carbon, silica tetrahedral can combine into a variety of structures from isolated tetrahedral to chains, double chains, sheets, and three dimensional frameworks. The form of the silica structure affects greatly the mechanical properties of the mineral and the rocks these minerals dominate.
The form of the structure varies with chemical composition. Silica combines with Fe and
Mg to form the ferromagnesian minerals. The structure varies with the ratio of Fe and
Mg to silica. Olivine, (Fe, Mg)
, contains isolated tetrahedral surrounded by the Fe and Mg atoms. The green rock with the mm-sized crystals is a garnet peridotite, a rather rare rock representative of the earth’s mantle (rock’s below the crust). These minerals
are rather far from their conditions of origin and weather more readily than other more silica rich rocks. They also are commonly altered by high temperature fluids
(hydrothermal fluids) to a greasy-feeling mineral called serpentine (see specimen from one of our field trip sites). Rocks that are rich in Fe and Mg, and that dominate oceanic crust, are know as “mafic” and rocks like peridotite and serpentine are known as
“ultramafic”. How would you compare the engineering properties of peridotite (or other unweathered crystalline rocks) with serpentine?
Most igneous rocks contain some ferromagnesian component. As the proportion of silica increases, the Fe and Mg are tied up in minerals with progressively higher silica content and ordering of the structure of the silica tetrahedra. These mineral families are, in decreasing order of Fe and Mg, the pyroxenes and amphiboles. Pyroxenes contain chains of linked silica tetrahedra (Si
) 4, while the amphiboles have double chains linked on one of the tetrahedra vertices (Si
The gem sample in the optical properties group is a pyroxene called diopside. There is also a sample of amphibolite, a rock dominantly made of amphibole in the collection here. The sample is somewhat weathered so please treat it carefully. Through the hand lens note the typical lathe form of the amphibole crystals. Both pyroxenes and amphiboles have very well developed cleavages. Due to the chain nature of the geometry, these crystals (particularly the amphiboles) tend to be long needles or prisms.
By making an additional tetrahedral connection, the double chains turn into the sheets silicates of the mica group (Si
. Depending on the other elemental constituents, micas take various forms of biotite (black) or muscovite (clear). The micas have a very distinctive weak cleavage and form crystals with hexagonal symmetries. Micas are common constituents in plutonic igneous rocks (those that cool at depth to have visible crystals throughout), however, they dominate some metamorphic rocks, such as the schists, and the preferred alignments of the mica crystals create a very strong strength anisotropy. Note the sample of biotite garnet schist. Chlorite is complex green mica that is produced in the low-grade metamorphism of Fe and Mg rich rocks like basalt. If you have a chance, go back and look at the native copper sample in the ores group. This is metamorphosed basalt that likely has a significant chlorite content. Chlorite is also a common fracture coating or filling.
The clay minerals are also sheet silicates, closely related to the micas. Clays are the weathering products of feldspars (to be discussed below) and micas, and differ from the micas by the addition of hydroxyl layers between the silica sheets. The structure of the hydroxyl layers distinguishes the different clay mineral groups. Clay minerals are highly reactive with water and some groups, such as the montmorillonites, can readily absorb or lose water depending on the conditions accompanied by significant volume changes.
These properties of clays make them responsible for many of the most significant geoengineering problems. Clay minerals are very small and their crystalline characters are only observable by electron microscope or x-ray methods. Upon metmorphosis under the high temperature and pressure conditions that accompany burial, clays recrystallize to micas and eventually to feldspars under high enough conditions. Note the sample of
Opalinus Clay from Switzerland. This claystone is being considered as a host for radioactive waste disposal because of its low water permeability and chemical reactivity with radioactive cations. This rock is about 40%-50% clay minerals. Even though it is a mixed illite-smectite clay (smectite is the really bad actor) experiments in the underground lab where this rock is being studied have to take great care in water because of the clays tendency to swell and disrupts the laboratory tunnels walls and floors. The small white scales are gypsum that has come out of pore water during drying of the surface.
The third most significant element in the crust’s composition is Al. In igneous and metamorphic rock the element is tied up mainly in the feldspar mineral group. Feldspars are made up of silica tetrahedra with open frameworks. Positively charge atoms such as
Al, Ca, Na, and K occupy the open portions of the framework. The two main feldspar groups are the plagioclases and the orthoclases (or K-feldspars). Plagioclases (Na and Ca feldspars) are found mainly with the ferromagnesian rich mafic rocks. The gray, crystalline sample here is anorthosite, a relatively uncommon rock made up almost entirely of plagioclase. The deep purple, iridescent sample is also a plagioclase, labradorite. The surfaces of this sample are not natural crystal surfaces, but have been cut an polished. You can make out the large crystals and their stripes which are formed by plagioclases typical “twinning”, where crystals with different orientations intergrow in directions along which their lattices are compatible. Labradorite is a currently a very popular decorative rock for working surfaces in offices and home kitchens. If you look carefully at the anorthosite you can see some smaller crystals of what I think are pyroxenes.
K-feldspars, or orthoclase, are more commonly associated with continental rocks. It is often milky white or pink. It has a pronounced two pronounced cleavages 90 degrees from one another parallel the crystal’s main symmetry axis. Find the K-feldspar crystals and observe these properties. You will also see a dark pebble containing pinkish latheshaped crystals that are also K-feldspar (I think). These are from a volcanic rock where the crystals began to form prior to the eruption that rapid chilled the remaining, finegrained groundmass around the larger crystals.
The end of the silica story lies with quartz, SiO
. Quartz occurs in those rocks where silica is left over after all the other elemental constituents have been accommodated in other minerals. Quarts is a framework silicate where all tetrahedra corners are occupied leaving no openings for other elements. The structure is tough and gives quartz a considerable hardness and durability. Quartz does not weather at the earth’s surface and this tends to accumulate in the continental and shallow crust. Quartz only fracture concoidally and has no cleavage. Its crystals are hexagonal often with distinctive pyramidal interpretation. The healing powers of these crystals have yet to be verified.
There are samples of a granitic relative, pegmatite, for observation. Pegmatite is a waterrich magma that is often the last to crystallize from magmas underground. Pegmatites are close to granites in composition and have similar mineral constituents, but they often have very large crystals, and the residues of rare elements often concentrate in the
pegmatites creating rare minerals and gemstones. Observe the pegmatite sample and identify its main mineral constituents.
Carbon is not one of the most abundant elements in the crust, however, its role in life and in the earth’s waters and atmosphere give it an importance far disproportionate to its quantitative ranking. Although there are some very rare carbonate-rich igneous rocks, far and away the bulk of carbon in the earth’s crust is tied up in limestones (CaCO
) and their close cousins the dolomites (Ca,Mg)CO
, which form by alteration of limestone in groundwaters. The bulk of limestone is formed in the tropics due to biogenic, mainly reef-building, activity with some contribution from the microscopic shells of plankton.
The dominant mineral of carbonate rocks is calcite (CaCO
) which is distinguishable by its hexagonal “dogtooth-spar” crystals, with their distinctive rhombic cleavages. Note the samples of both naturally-terminated crystals and the cleaved fragments. Test your cleaving powers on some of the smaller pieces. Calcite is generally milky to clear and it is rather soft (softer than a knife blade, unlike most silicates). I did not bring HCl to class, but a dilute acid (like weak HCl or acetic acid) will produce a fizzing reaction on calcite.
Calcite is soluble in water, and it is readily mobilized by groundwater which causes pressure solution, transport, and re-deposition. Limestones are closely associated with cavern formation under favorable conditions.
For comparison with the crystals note the fragment of a limestone core. The biologic origins of this sample are unmistakable. Note the shell fragments and the pellets of coated limestone grains that likely bounced about in tropical waves building their coatings of calcite by precipitation from the sea water. You can also see pore spaces between shells that are filled in with clear sparry calcite that was likely deposited in porous voids after burial. Limestones like this are major aquifers and oil reservoirs depending on how much porosity remains. You can also see some cavities that have not filled completely leaving small, well-formed crystals of calcite.
In addition to the limestone, there are two samples of evaporite minerals, One is a core of rock salt (feel free to taste, remembering that you are probably not the first to try) and the other is selenite, CaSO
O. In its un-hydrated form this composition produces the mineral anhydrite. Commonly deposits at depth go through an anhydrite to gypsum transition with a large volume change that can have engineering implications.
Halite or rock salt (NaCl) is a cubic crystal. You can see this form in the fluid inclusions in the interior of this sample. Contrast the crystal forms of halite and selenite. Could you tell these apart without (yuck) licking? Note also that selenite is one of the minerals softer than fingernail (try this in a discrete place on the sample). Halite and gypsum are evaporite minerals mainly formed by precipitation from seawater in areas of restricted flow and high temperatures. In evaporating basins, anhydrite precipitates first, followed
by halite, and under extreme conditions, sylvite (KCl), and important commercial source of potash.
Evaporites are highly soluble, and dissolve in groundwater Salt is famous for its rheology, and the ability to plastically deform under low deviatoric stress.
Using the materials provided, determine the density of the rock sample shown. Which are more likely from oceanic crust? Which from continental crust?