CEE 437 Rock and Mineral Lab Thomas W. Doe, 2009

Transcription

1 CEE 437 Rock and Mineral Lab Thomas W. Doe, 2009 Part 1. Minerals The minerals section of this laboratory includes both a collection of the rock forming minerals as well as some other minerals that illustrate concepts of crystal form, cleavage, and other mineral properties. There are several minerals here just for general inspection. I will give you some general characteristics and see if you can pick out which is which. Calcite- the minerals that makes up limestones and marbles and is a very important vein and fracture filling as it is very mobile in groundwater. In large crystals calcite shows it rhombic cleavage, however it's crystal growth form is a hexagonal pyramidal with a form commonly called dogtooth spar. Take a calcite rhomb and set it on a piece of paper with a pencil dot. Rotate the crystal. Sketch what you see. What properties of the crystal does this illustrate? How does the demonstrate anisotropy? Fluorite is a cubic mineral that cleaves along an octahedron plane, that is, it cuts off the corners of the cubes. It can have many different colors but is commonly violet or purple. Sulfides -- I have included some sulfides including a massive sample of sphalerite ore (ZnS), galena (PbS), which is cubic and silver-metallic in color, and pyrite, Fe 2 S). Pyrite is a very important and common mineral that can occur in both igneous and sedimentary environments. It particularly occurs whenever there are reducing conditions and the iron bonds with sulfur under anoxic conditions. Evaporite Minerals Halite, Sylvite, Gypsum Halite (NaCl salt) is here as a core sample. It has a cubic crystal structure as does sylvite with very good cleavages on the cubic faces. Note the fluid inclusions (left over water) in the salt. What does their form tell you about the salt crystal structure?

2 Note the clear crystal of gypsum. Note the cleavages. What is the angle between the cleavages? What does this tell you about the crystal symmetry compared with halite? Silicates - Rock-forming Minerals Among the rock forming minerals, I have the following samples: Quartz - I have several prisms of quartz showing their hexagonal form and well formed pyramid terminations. Do you see a cleavage plane? How would the lack of cleavage relate to quartz s crystal structure? Feldspars Start with the single crystal of milky-white potassium feldspar (orthoclase). This is the feldspar typical of granitic rocks. The crystal is a bit rough (it weathered out of a coarse-grained rock and has broken on cleavage planes. How many directions of cleavage do you see? Next there is a polished sample of a Ca-plagioclase. This is more typical of mafic, oceanic rocks like gabbro or basalt. Try to identify the individual crystals present.

3 Note the striations. These are twinning. Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations Left Plagioclase twin, right staurolite twin Ferro-magnesian or Mafic Minerals Olivine - small green crystals, isolated silica tetrahedra structure (no bonds between tetrahedra) important component of mantle and oceanic rocks that are high in Fe and Mg. This mineral has a cubic structure Amphibole and Pyroxene There are no good samples of pyroxene in this lab. The amphibole here is in the form of amphibolite, Note the form of the crystals -- black, elongate prisms. Pyroxene looks similar but is less elongate. Look for amphibole crystals especially in the volcanic rocks. Recall these are the main ferro-magnesian minerals in igneous rocks. Pyroxene has single chains of silica tetrahedral and is higher temperature and higher Fe-Mg content. It will be in more oceanic rocks. Amphibole is a double chain, lower in melting point and FeMg content and appears in intermediate to more silicic (also acidic) igneous rocks of continents and island arcs. Mica Note the samples of muscovite mica. Biotite is the more ferro-magnesian form of the mineral. These samples show clearly the sheet silicate structure that micas and clays have in common. It is closely related (maybe even same family) as clays, but potassium bonds between the sheets make it a bit stronger. Micas appear as important components in igneous rocks, but they are the key minerals in rocks formed by metamorphism of clay-rich rocks shales and claystones. Not surprisingly micas form from recrystallization of clays and degenerate to clays during weathering. Consider what effect having aligned micas, as occurs in schists, slates, and phyllites does to rock property anisotropy!

4 Note silica tetrahedral layer (silica sheet) and octahedral (Al or Mg and O) layers. Look at the figure of silicate structures. What symmetry group would you expect for these minerals? Recognizing Minerals in a Rock Included in this minerals section I have put some samples of pegmatite. Pegmatite is a rock that forms from the very last portions of the granite melt to freeze. At this stage in the freezing process the melt is very rich in water and the ions in the melt are very mobile resulting in unusually large crystals. In these pegmatite samples you can see very clearly the quartz, feldspar, and micas that make up the rock. Note the difference in appearance between the feldspar and the quartz. The quartz is glassy and has no cleavage, while the feldspar is milky white and has a very clear cleavage which presents itself through planar surfaces that reflect light readily. The micas are the flaky, light-colored mineral in the rock.

5 Part 2. Igneous Rocks Acidic of felsic Intermediate Basic or Mafic Ultramafic Continents Intermediate Oceanic Crust Mantle More commonly seen altered to serpentine The classification of igneous rocks is based on crystal size and mineral composition. Aphanitic and Phaneritic are simply fancy terms for visible by naked eye or not. This largely divides slow-cooling rocks, where the crystals have time to grow from magma, from fast cooling rocks which are quenched in lavas. Magma a term for a molten rock below the surface, while lava is what we call it when it on the surface. You can also get phaneritic rocks in the magma-filled fractures that feed (or try to if they don t reach the surface) called dikes.

6 When a magma comes out as a lava, it may have started crystallizing, so one often sees phanereritic crystals in a mass that is aphanitic. The extrusive origin of the rock is determined by the fine-grained, aphanitic mass. In addition to these rocks on this chart, and as described in Waltham, note that there is an important group of volcanic rocks that are deposited from material that blows rather than flows out of volcanoes. These rocks are called pyroclastic or broken up stuff from a fiery source. High silica magma has much lower viscosity than low silica magma. Basalt flows travel long distances as liquid. The Miocene flood basalts (10-16 My ago) of eastern Washington traveled from vents near Walla Walla to the Pacific, and Hawaiian volcanoes have low cone angles. The high viscosity high silica magmas also have high gas and water contents, and tend to explode into ash (volcanic ash, non-welded tuff) and rock fragments or flow as glowing, glassy clouds of gas charges melt (nuées ardentes or glowing clouds welded tuff and pumice). Hence rhyolite is not a very common rock and much of what a volcano like

7 Composite volcanoes (like the Cascade volcanoes) are mixes of usually andesitic lava, which are quite competent, with pyroclastics that have a very big range of properties from competent to very weak. These can pose big engineering challenges. Basalts have their own internal structures some oceanic basalts form blobs or pillows. Surface-based lavas have a layered structure of an upper and lower chill zone that can be very rubbly and scoriaceous (scoria is a frothy material the freezes while it is flowing) and a central zone with various cooling fractures. The rubbly flow tops are prolific aquifers in the eastern part of the state with big porosities and permeabilities. Pick out the following: Scoriaceous flow top Consider how permeable this could be!

8 Basalt with olivine crystals Compare to olivine in mineral section why might this be more brown than green? Basalt Dike Cutting Andesite Note chill zone! Basalt Cobble This is from glacial till, and probably came from Canada. Note zonations of light colored plagioclase crystals. This sometimes indicates changing melt composition while these crystals were growing. Pick out the Andesite Samples These are porphyritic (big crystals in smaller matrix) and from Mt. Rainier. Make a sketch showing plagioclase and amphibole crystals. Pyroclastics Identify pumice and welded tuff samples (one with funny channels on the fracture face). Look carefully and note flow layering and compressed glassy blobs. This is from the proposed Yucca Mountain, Nevada, radioactive waste disposal site. Plutonic Rocks The plutonic rock classification above is rough field approximation. For both plutonics and volcanics there are many further subdivisions which can be scary! Most of these reflect variations in the quartz content and plagioclase type. I am showing them only because field geology reports and maps may use them. For the most part these distinctions don t affect engineering properties.

9 I have a range of rocks here. I will describe them briefly. The ultramafics are a medium to light green peridotite from France and a serpentine. Peridotite is olivine and garnet rich the name peridotite comes from the name for the gem form of olivine peridot. These are rare to see but we will have some like them on the field trip. More often, because they weather and alter relatively easily, you see a rock called serpentine, which is a hydrothermal alteration. Serpentine is the green, slightly greasy feeling rock. Compare likely engineering properties of peridotite and serpentine. Serpentine is found a few places in the Cascades (it causes stability problems on Blewett Pass along US-97). It is also the state rock of California. The diorites are actually rather dark with reddish feldspars. They are from an underground lab in Sweden. Don t spend much time on these, but note if they have quartz. There is also a fine grained gabbro. Pick it out. I have several granites. Compare the minerals you can see to the pegmatite in the mineral section. One of these is a thin sheet. This does not reflect the rock structure but is a spall of the intersection of two underground galleries in highly stressed rock. Most CE problems won t involve failure of intact hard

10 rock, but this is in an exception. Why it spall there?

11 For reference interest, I have added two interesting samples. I have two bags of decomposed granite. These were intact when I first collected them and have fallen apart since carrying them around. What is the likely mineral change going on here? There is also a breccia (rock made of angular broken pieces) made from granite. This is from a fault zone underground at a hydroelectric development site the pressure tunnel. This is a piece of what was producing mode of the water flow in the tunnel. Note the high porosity and consider how permeable it could be. Also, note the crystals growing in the voids. What is the crystal form? Is it calcite (usually a good guess)? If not, why not. What else could it be?

12 Part 3: Sedimentary Rocks Sedimentary rock are those deposited by/from water (and wind). The big groups are siliciclastics, carbonates, and evaporites. Siliciclastic simply means broken up stuff made of mainly silica minerals. Siliciclastics Classification is rather simple: Take some time to identify the samples of shale/claystone, conglomerate, sandstone and siltstones. Note the two samples that have two rock types sandstone and silty claystone. Look for cross bedding and make a sketch when you find it. Cross bedding is the formed from the angle-of-repose foreslopes of ripples and dunes. How does this tell you which side the rock is up? See picture below for help.

13 Cross beds from ancient sand dunes There are also some flames and small slumps show sediment instability. Make a little sketch. We see more of these on the field trip in larger scale. Carbonates Carbonate rocks are sedimentary rocks made of dominantly calcite and sometimes dolomite (Mg replaces one of the calciums in the lattice this happens after deposition). They are formed mainly in the tropics from shells, coral debris, and other organic material. They are also clastic and one can have lime sands, lime muds, lime breccias, and lime conglomerates. From the samples of carbonates pick out and admire: 1. A limestone that is a layer of lime mud and well-cemented lime sand sketch

14 2. A dark oil saturated limestone that is high porosity and composed of sand size grains 3. An oolite (highly rounded grains that start as sand and take on coatings as they are agitated in by wave activity 4. A limestone made mainly of oyster shells. Evaporites I have a single core of anhydrite here. Not much to talk about but you saw the salt core in the mineral section. Salt is a relatively low density and very low viscosity rock (all rocks have viscosity but they need time, temperature, and pressure to flow). Salt is well known for flowing under gravity instability to form domes and other flow features. Salt domes are common on the Gulf coast and active flow structures are known in the Rockies. They are important as oil traps, for hosting gas storage caverns, and radioactive waste disposal in New Mexico. What kind of permeability would salt have? What does salt being there tell you about access to fresh water?

15 Part 4: Metamorphic Rocks Foliated Rocks Slate, Phyllite, Schist, Gniess Metamorphic rocks are those that recrystallize under conditions of temperature and pressure, usually due to burial. They are classified by the types of mineral/chemistry and the inferred degree of recrystallization. Original Material sandstone, limestone, shale, basalt) Metamorphic Grade (Temperature, Pressure) Source of Metamorphism (Regional, Contact)

16 The sources for metamorphic rocks are mostly sediments/sedimentary rocks. As we discussed in lecture, these tend to be made of quartz-rich sand and silts and clays. Clay metamorphism mainly proceeds by recrystallization of clay minerals into their cousins, micas. These rocks have strong foliation due to preferred orientation of the mica. Foliation comes from folia which is Latin for leaves. What does foliation do to property anisotropy? The rock types are based on mica crystal size: Slate dull color, strong foliation Phyllite (thin sheets -- the root is the Greek for Leaves -- like phyllo dough) some mica sheen but not visible crystals Schist visible micas, strong schistosity.

17 Identify these in the metamorphic group. The slate is from Home Depot. The light colored schist is Manhattan Schist from Central Park in New York, and the phyllite is from near Cle Elum. Normal clays will produce a mainly light-colored muscovite (light mica) schist. Metamorphism of more mafic (Fe-Mg rich) materials will produce biotite schists with various accessory minerals. The dark schist here is from Icicle Creek near Leavenworth. It represents the metamorphism of a variety of basalts and serpentines by the granite body that forms Mount Stuart (and the Enchantments). Note the garnet crystals. Eventually at the highest grades (and approaching melting) the mineralogy goes to something granitelike quartz, feldspar, micas, and some ferromagnesians. These are called gneiss. They can look like granite but the foliation strongly suggests a metamorphic rather than igneous origin. Quartzite and Marble The metamorphic progression for quartz sandstones and limestone is straightforward with no major changes in mineral content as the quartz recrystallizes and remains quartz, and the calcite recrystallizes and remains calcite. Dolomites, due to the addition of Mg to the calcite structure, can also recrystallize out an Mg-olivine if there is appropriate silica present. Pick out and note the samples of quartzite and marble. On the core sample, look at both ends. Are the materials the same? Note

18 There is a core that is whitish colored, but closer inspection shows two rock types present, marble and quartzite. Look carefully at both ends, and using what you learned about cleavage in the mineral sections, determine which is which. The last two rocks are from the contact zone of the Snoqualmie Granite batholith/pluton from Denny Creek, Snoqualmie Pass. One, which is looks a lot like basalt is actually a hornfels (see below) and the other is a very fine grained granite (white) with broken bits of the host rock (turned to hornfels?) embedded in it.