Geology 101 Name(s): Lab 5: Sedimentary and metamorphic rocks More sedimentary rocks — Needed: Samples R18 – R28 (Tubs 21 – 31), R33 (Tub 36) and S1 (Tub 94) 1. a. Sedimentary rocks are held together by cement, a non-mineral chemical compound, which forms bonds (though not as strong as chemical bonds) between mineral grains. The three common cements are silica (SiO2), calcite (CaCO3) or iron oxide (rust). How would you identify each cement (think of a test for each)? Iron oxide — Silica — Calcite — b. Look at rock samples R18 and R19. What is the cement that holds each together? R18 _________________ R19 _________________ 2. How does the cement get into these rocks? 3. Now consider the actual mineral grains in rock samples R18, R19, R24 and R25. Because sand-sized dark minerals are very hard to identify, sedimentary petrologists (much to the horror of igneous petrologists) use the term “dark lithic fragments” to categorize lots of little dark minerals. Sample Most common mineral R18 R19 R24 R25 Sedimentary rocks can be classified in a number of ways. For our purposes, the first division to be made is between clastic sedimentary rocks (those that are madeof weathered and eroded grains) and non-clastic or other sedimentary rocks (these include sedimentary rocks of biological and chemical origin). You will use the Sedimentary Rock ID flow chart on the next page. One other thing: fossils (Latin for “dug up”) are the remains of living organisms. If the fossil is literally the body of the organism (or parts such as skeleton or shell), it is called a hard parts fossil; if the fossil merely records the shape of an organism (like a leaf impression in silt) or the passage of an organism (like preserved footprints), then it is called a trace fossil. For any sedimentary rock, if it contains any fossils, use the adjective fossiliferous in front of the rock name. Flow chart for identifying sedimentary rocks — If the rock is made of grains or other materials which have been deposited by wind, water or ice, or else was generated by biological or surface chemical activity, it's a sedimentary rock. First step. If the rock is made of broken up bits of rock (including extremely fine grains) → GO TO Second step alternative A. Else → GO TO Second step alternative B. Second step alternative A. Consider the most common grain size in the rock from the following list. cobble or pebble > 2 mm easily visible to naked eye; "grains" may contain identifiable minerals sand 0.062 — 2 mm visible to naked eye silt 0.005 — 0.062 mm not visible but can be felt between fingers or across teeth clay < 0.005 mm not visible; cannot be felt between fingers or across teeth If the most common grain size is cobble or pebble → conglomerate If the most common grain size is sand → sandstone (arenite) If the most common mineral is quartz → quartz arenite If the rocks is medium gray to red and well-sorted → arkose If the rock is dark-colored and poorly-sorted → greywacke If the most common grain size is silt → siltstone If the most common grain size is clay → shale If the rock splits into irregular or regular layers → mudstone If the rock is massive (no layering) → claystoneSecond step alternative B. Identify the most common mineral in the specimen (use mineral ID chart if necessary). If the most common mineral is quartz → chert If the most common mineral is halite → rock salt If the most common mineral is gypsum → rock gypsum If it is black-colored, not very dense and flaky → coal (also look for plant fibers) If it fizzes, the most common substance is calcium carbonate, usually in the form of the mineral calcite (be careful you are not fizzing the cement) If the rock is not very dense and pure white → chalk If the rock is made of broken-up shells → coquina If the rock is dense and white, gray or black → limestone 4. Fill in the following table for clastic sedimentary rocks. Begin by determining the average grain size of the clasts in the rock (use the grain size terms in the flow chart), then the grain sorting (the choices are: well-sorted, moderately sorted, poorly sorted and unsorted) and the grain roundness (the choices are; well-rounded, sub-rounded, sub-angular and angular). See the diagrams to determine which type of rounding and sorting the grains have. Under fossils, your choices are none, hard parts or trace fossils. Finally, identify the rock, using the flow chart. Clastic sedimentary rocks Sample # Grain size Grain sorting Grain roundness Fossils Rock name R18 R19 R20 R21 R22 R235. Fill in the following table for “other” sedimentary rocks. Begin by determining the rock’s mineral composition. Then add any other details that help identify it. Under fossils, your choices are none, hard parts or trace fossils. Finally, identify the rock, using the flow chart. “Other” (chemical and biological origin) sedimentary rocks Sample # Mineral composition Other defining details Fossils Rock name R24 R25 R26 R27 R28 6. Return to R18 and R19 and circle the correct answers: a. Which rock contains the most stable mineral clasts? R18 R19 (at the Earth's surface) b. Which rock is composed of rounder grains? R18 R19 c. Which rock is more well-sorted? R18 R19 d. Based on a-c, which sample was deposited furthest from its source (and thus is called mature)? R18 R197. The energy of the system (how much force is behind the medium of transport (air or water)) can be characterized by the size of the particles the system can carry. For instance, high-energy systems can carry large grains; low-energy systems can carry small grains. Examine and rank rocks R18, R21 and R20 in order from highest energy to lowest energy depositional system. 8. a. Some limestones (R33) are dense, fine-grained and black. So is basalt (R5). What test can you perform to tell them apart, and how does each behave in the test? b. Which rock has fossils? By the way, in general, why didn’t you worry about fossils in igneous rocks? The place in which the sediment is deposited or the organisms lived is called the depositional environment. Examples of depositional environments include terrestrial environments (like lakes, deserts and rivers), transitional environments (like