HNRT 227
Laboratory Experiment #8 with Ms. Crowell
FALL 2015
The Rocks of Earth and the Surface of Mars
Purpose: To understand the rocks of terrestrial planets and how they are formed, and to interpret and analyze the surface of planets and what that teaches us about the conditions that exist today and may have existed eons ago.
Earth Rocks Introduction
Minerals are defined as naturally occurring, inorganic, solids with a definite chemical composition and a regular, internal crystalline structure. The keys to this definition are the chemical composition and the crystalline structure. Different chemical compositions result in different minerals. A good example is the mineral plagioclase. Plagioclase is a member of the feldspar group, but there is more than one type of plagioclase. Albite and anorthite are two examples. Albite has a chemical composition of NaAlSi3O8, while anorthite's chemical compositon is CaAl2Si2O8. Very similar, but still different, therefore two different minerals.
Different crystalline structures, or how the atoms and molecules are arranged, result in different minerals. A good example is diamond and graphite. Both minerals are composed of carbon (C). The same chemical composition, but two different crystalline structures - therefore, two different minerals.
Determination of the actual chemical composition and crytalline structure of a mineral is difficult without the proper equipment. In an introductory level lab it is impossible for us to determine these two aspects of a mineral. Fortunately, these two aspects determine a mineral's physical properties. How the atoms and molecules are arranged and the strength of the bonding between the atoms result in different physical properties for different minerals. While many minerals share common physical properties, when all of a mineral's physical properties are examined, it often results in a unique set of physical properties which can be used to identify the mineral.
Below you will find a chart which defines the physical properties and provides the means for determining the physical property of a mineral sample. These definitions and methods are simplified. Consult your lab manual for detailed discussion.
Mineral Physical Properties ChartPHYSICAL
PROPERTY / Definition* / Testing Method
Cleavage / Breakage of a mineral along planes of weakness in the crystral structure. / Examine the mineral for areas where the mineral is broken. Look for areas where the light reflects from planar surfaces. This can be easily confused with a crystal face and is the most difficult properties for students to master.
Color / Visible light spectrum radiation reflected from a mineral. / Look at the sample and determine its color - white, black, green, clear, etc.
Crystal Form / Geometric shape of a crystal or mineral. / Examine and describe the geometric shape of the mineral - cubic, hexagonal, etc. Not commonly seen in most introductory lab samples.
Fracture / Breakage of a mineral, not along planes of weakness in the crystral structure. / Examine the mineral for areas where the mineral is broken. Describe the breakage as either irregular or conchoidal (has the appearance of broken glass)
Hardness / Resistance to scratching or abrasion. / Use minerals of known hardness from the Mohs Hardness Kits. Scratch the unknown mineral with a known hardness to determine which mineral is harder. Continue doing this with harder or softer minerals from the kit until the hardness is determined.
Luster / Character of the light reflected by a mineral. / Look at the sample to determine if the mineral is metallic in appearance (looks like a chunk of metal) or non-metallic (doesn't look like a chunk of metal).
Magnetism / Electromagnetic force generated by an object or electrical field. / Use a magnet to determine if the magnet is attracted to the sample.
Reaction to HCl / Chemical interaction of hydrochloric acid and calcium carbonate (CaCO3). / Place one small drop of HCl on a sample a watch for a reaction - effervesces (bubbles).
Specific Gravity / Ratio of the mass of a mineral to the mass of an equal volume of water. / Generally not determined in an introductory lab. Look this information up in your lab manual once the mineral has been identified.
Streak / Color of the mineral when it is powdered. / Grind a small amount of a mineral into a powder on a porcelain streak plate and determine the color of the powder.
Taste / Nerve ending reaction in the tongue to different chemicals. / Lick the mineral. (not recommended in an introductory lab - you don't know who has handled or licked the sample before you).
Other Properties / Fluorescence, Radioactivity / Requires special equipment such as a UV lamp and geiger counter. These are not commonly tested for in an introductory lab.
* Definitions simplified or modified from Bates, R.L. and J.A. Jackson (eds.), 1987, Glossary of Geology. American Geological Institute, Alexandria, VA, 788 p.
Igneous rocks are rocks that solidify from molten material (magma). Cooling of the magma can occur beneath the surface (plutonic) or on the surface (volcanic). Igneous rocks can be identified by the determination of the composition and texture of the rock. Once these two characteristics have been identified, the Igneous Rock Identification chart can be used to help identify the rock name.
COMPOSITION
\\\\\\\\\\\\\\\\
TEXTURE\\\\\\\ / Felsic / Intermediate / Mafic / Ultramafic
Pegmatitic / Granite Pegmatite / Diorite Pegmatite / Gabbro Pegmatite
Phaneritic / Granite / Diorite / Gabbro / Dunite
Aphanitic / Rhyolite / Andesite / Basalt
Porphyritic / Rhyolite / Andesite / Basalt
Glassy / Obsidian / Basaltic Glass
Vesicular / Pumice / Scoria
Pyroclastic / Volcanic Tuff
Composition of igneous rocks is properly identified by determination of the rock’s chemical composition. This, however, requires chemical equipment and apparatus that is unavailable in this lab. Fortunately determination of the exact chemical composition is not necessary. Color is often an indicator of the composition of a rock or mineral and can be effectively used to identify the composition of most igneous rocks. Light colors, including white, light gray, tan and pink, indicate a felsic composition. Felsic compositions are rich in silica (SiO2). Dark colors, such as black and dark brown, indicate a mafic or ultramafic composition. Mafic compositions are poor in silica, but rich in iron (Fe) and magnesium (Mg). Intermediate compositions have an intermediate color, often gray or consisting of equal parts of dark and light mineral. Beware that even though an igneous rock may have a felsic composition (light color), the rock can contain dark colored minerals. Mafic rocks may contain light colored minerals as well. As mentioned above, the composition of most igneous rocks can be identified using this system, formally known as the Color Index. However, there are exceptions. The two most notable are obsidian and dunite. Obsidian is volcanic glass which erupts as a lava flow. Most obsidian is felsic in composition, yet typically it will have a very dark color (dark brown to black). Dunite has an ultramafic composition yet is apple green to yellowish green in color. Dunite is composed almost entirely of the mineral olivine which usually contains both iron and magnesium.
The texture of an igneous rock does not refer to the roughness or smoothness of the surface. Textures are based primarily on crystal size. Pegmatitic texture is composed of very large crystals (larger than 2-3 cm). Phaneritic texture is composed of crystals that are large enough to see but smaller than pegmatitic texture, and the entire rock is composed of crystals. Aphanitic texture is a fine grained texture but the crystals are too small to see. Porphyritic texture is composed of crystals of two different sizes. Typically the large crystals (phenocrysts) are visible while the smaller crystal are not (referred to as groundmass). Glassy texture is the most readily recognized. The rock is composed entirely of glass. Few, if any, crystals will be visible. Vesicular texture is formed when lava solidifies before gases are able to escape. The result is a "bubbly" appearance. Lastly, pyroclastic texture is composed of volcanic fragments. These fragments or clasts can be very fine (ash) or coarse (lapilli) or very coarse (bombs and blocks).
Sedimentary rocks are rocks composed of sediment. Sediment is deposited in a number of environments of deposition, by both moving air and moving water. Sedimentary rock identification is primarily based on composition. Texture will still be used but in a different sense than for igneous rocks.
Texture of sedimentary rocks in this lab will be taken to indicate origin or type of sediment found in the rock. Three types of sedimentary rock "texture" are clastic, chemical, and biologic.
Clastic sedimentary rocks contain clasts. These are fragments or pieces of rock or minerals. The composition of clastic sedimentary rocks is divided into three types - clay/silt, sand and gravel. Clay and silt are less than 1/256 mm. These are not visible to the unaided eye. Sand is clasts between 1/16 and 2 mm in size, and gravel is greater than 2 mm.
Chemical sedimentary rocks are identified by identifying the mineral from which they are composed. Four minerals that need to be identified are quartz, halite, gypsum and calcite. Quartz has a hardness of 7 and is very difficult to scratch, even with a good quality knife blade. Gypsum is relatively soft (Hardness =2) and can be scratched easily with a fingernail. Halite is common table salt and is most easily identified by taste. However, this is not a sensible practice in a large lab with many different people handling the samples. Halite has a hardness of 2.5 and cannot be scratched by a fingernail (unpolished fingernail). Calcite readily reacts with a small drop of HCl.
Biologic sedimentary rocks are which form as the result of the accumulation of organic material or biologic activity. Coal is usually obvious to most students even though few people seem to have ever actually examined it up close. The dark brown to black color is the most obvious characteristic. Coquina and limestone are both composed of calcite. Coquina is composed almost entirely of shell or fossil fragments. Limestone may or may not contain fossils. Both will react to HCl. Limestone containing fossils is referred to as fossiliferous limestone.
Sedimentary Rock Identification ChartTEXTURE / GRAIN SIZE / COMPOSITION / ROCK NAME
Clastic / 2 mm / rock fragments, quartz, feldspar / Conglomerate
1/16 - 2 mm / quartz, feldspar / Sandstone
<1/16 mm / quartz, clay minerals / Mudstone
1/16 mm / feldspar, quartz / Arkose
Chemical / calcite / Limestone
silica (quartz) / Chert
gypsum / Rock Gypsum
halite / Rock Salt
Biologic / organic material, plant fragments / Bituminous Coal
calcite, shell and skeletal fragments / Coquina
calcite with some fossils / Fossiliferous Limestone
Metamorphic rocks are rocks that have undergone a change from their original form due to changes in temperature, pressure or chemical alteration. The classification of metamorphic rocks is based on the minerals that are present and the temperature and pressure at which these minerals form. Determination of this information is not easily accomplished in this lab. Therefore, a simplified system is used based on texture and composition.
Texture is divided into two groups. Foliated textures show a distinct planar character. This means that the minerals in the rock are all aligned with each other. This planar character can be flat like a piece of slate or folded. Non-foliated textures have minerals that are not aligned. Essentially, the minerals are randomly oriented.
Foliated textures show four types of foliation. Slaty cleavage is composed of platy minerals that are too small to see. Typically, these rocks split along parallel, planar surfaces. Phyllitic foliation is composed of platy minerals that are slightly larger than those found in slaty cleavage, but generally are still too small to see with the unaided eye. The larger size gives the foliation a slighly shiny appearance. Schistose foliation is composed of larger minerals that are visible to the unaided eye. Platy minerals tend to dominate. Gneissic banding is the easiest of the foliations to recognize. It is composed of alternating bands of dark and light minerals.
Non-foliated textures are identified by their lack of planar character. Further identification of non-foliated rocks is dependent on the composition of the minerals or components in the rock. Anthracite coal is similar to bituminous coal. Both are black in color, and is composed of carbon. Anthracite coal is generally shiny in appearance and breaks with a conchoidal fracture (broken glass also shows this type of fracture). Metaconglomerate is composed of pebbles and gravel that have been flattened due to directed pressure. Quartzite is composed of quartz sand grains. Quartz has a hardness of 7, which makes it difficult to scratch. Marble is composed of calcite and will readily react to a small drop of HCl.
Metamorphic Rock Identification ChartTEXTURE / FOLIATION / COMPOSITION / TYPE / PARENT ROCK / ROCK NAME
Foliated / slaty / mica / Regional / Mudstone / Slate
phyllitic / quartz, mica, chlorite / Regional / Mudstone / Phyllite
schistose / mica, quartz / Regional / Slate / Schist
schistose / amphibole, plagioclase / Regional / Basalt or Gabbro / Amphibolite
gneissic banding / feldspar, mica, quartz / Regional / Schist / Gneiss
Non-Foliated / carbon / Contact or Regional / Bituminous Coal / Anthracite Coal
quartz, rock fragments / Contact or Regional / Conglomerate / Metaconglomerate
calcite / Contact or Regional / Limestone / Marble
quartz / Contact or Regional / Sandstone / Quartzite
Mars Surface Introduction