G201 Igneous Rock Classification: Texture + Composition = Name

Texture: Texture is related to how an igneous rock cooled. There are several textures you should be able to identify in igneous rocks.

Fragmental Glassy Interlocking Aphanitic Phaneritic Porphyritic Vesicular Composition: refers to the chemical composition of the magma. Color is a useful guide to identifying the chemical composition and hence the magma type that cooled to produce an igneous rock.  Mafic (Mg Fe rich) magmas form dark colored minerals (e.g. olivine, pyroxenes) when they cool.  Felsic (silicic) (Al Si rich) magmas form light colored minerals (e.g. quartz, feldspar) when they cool.  So, the overall darkness or lightness of an igneous rock can be used to infer what type of magma cooled to produce the igneous rock. magma type % dark minerals color value Mafic 100-75 black to dark gray (100-75%) Intermediate Mafic 75-50 dark gray to medium gray (75-50%) Intermediate Felsic 50-25 medium gray to light gray ( 50 to 25%) Felsic 0-25 light gray to white (25-0%) Composition can also be determined by measuring the SiO2 content which requires chemical analysis.

Name Most igneous rocks have an interlocking texture, for these use the table below. Mafic Intermediate Mafic Intermediate Felsic Felsic aphanitic Basalt Andesite Dacite Rhyolite phaneritic Gabbro Diorite Granodiorite Granite If the rock is porphyritic then modify the above names e.g. porphyritic granite, porphyritic andesite.

For rocks with fragmental or glassy textures use the table below. Texture Name Glassy no holes (flesic) -> obsidian no holes (mafic) ->tachylite (rare) Vesicular glassy, mostly holes -> pumice more rock than holes -> scoria Fragmental fragments < 2mm -> tuff fragments > 2mm -> volcanic breccia glassy fragment (water quenched) -> hydroclastic Common Igneous Minerals Mineral Color Luster Cleavage Shape Quartz

Feldspars

Amphiboles

Pyroxenes

Olivine G201 Lab-Igneous Rocks Name______ Make observations based on the specimens in class, not based on pictures in the text.  Not all specimens are identical, look at more than one and generalize!

How large (in millimeters) is the field of view of your hand lens? ______

Granite #3639 This granite has a phaneritic texture.

Grainsize? fine medium coarse mixed Texture? glassy interlocking fragmental

Make a representative sketch of the granite as you see it through the handlens:

What percentage is dark minerals? ______% What percentage is light minerals? ______%

How many light colored minerals are in the granite? ______How can you tell them apart? Describe each and identify!

How many dark colored minerals are in the granite? ______How can you tell them apart? Describe each and identify!

Did the granite solidify above ground (fast cooling) or below ground (slow cooling) or some combination of the two? Explain your choice briefly.

Dacite This dacite has a porphyritic texture. Grainsize? fine medium coarse mixed Texture? glassy interlocking fragmental

Make a representative sketch of the granite as you see it through the handlens:

What color is the matrix? ______What color are the phenocrysts? ______

Do the phenocrysts have a cleavage? yes no Which best describes the phenocrysts? angular rounded

Which best describes the three dimensional shape of the phenocrysts? pencil flake blocky

This rock formed when a magma cooled and solidified. Which solidified first the matrix or the phenocrysts? Explain your reasoning!

Did this rock solidify above ground (fast cooling) or below ground (slow cooling) or some combination of the two? Explain your choice briefly.

Basalt (#5) in cardboard box Note the holes! These holes formed as gas was escaping from the cooling solidifying magma. Describe the size and shape of the holes! What percentage of the rock is holes?

Basalt #4 (no dot) Note this basalt contains white blebs. Do you think these blebs formed when the white mineral crystallized from the cooling basalt or by the white mineral filling holes in already cooled basalt? Explain your reasoning!

Basalt #10 (yellow dot) Note this basalt contains white blebs. Do you think these blebs formed when the white mineral crystallized from the cooling basalt or by the white mineral filling holes in already cooled basalt? Explain your reasoning! Sample Grainsize Igneous Color Visible Minerals Rock Name How did # Texture Index It form? 2 6 7 9 10 11 12 15 16 17 27 29 30 Sedimentary Rock Handout

Clastic Sedimentary Rocks Clastic sedimentary rocks consist of consolidated sediment and have a fragmental texture. Clastic sedimentary rocks are named after the size of the particles that make up the rock. Look up the size range in millimeters for each particle type on the sand gauge and page 167 in Marshak.

Grain Size Particle Name Diameter (mm) ROCK NAME Boulder Conglomerate Coarse Gravel Cobble (if grains are rounded) Pebble Breccia (if grains are angular) Granule V. Coarse Sand Sandstone Medium Sand Coarse Sand  quartz sandstone (just quartz) Medium Sand  arkosic sandstone (quartz & feldspar) Fine Sand  lithic sandstone (rock fragments) Very Fine Sand  greywacke (rock fragments & clay) Fine Mud Silt Siltstone Clay Mudstone (if not layered) Shale (if layered)

Biochemical Sedimentary Rocks Biochemical sedimentary rocks have either an interlocking texture or a fragmental texture in which all the clasts have formed by biological processes (e.g. shells stuck together.) Biochemical sedimentary rocks are classified based on composition.

Composition ROCK NAME Gypsum Evaporite Halite (table salt) Chalcedony (SiO2) Chert Calcite Limestone  fossiliferous limestone (fossils in lime mud)  coquina (entirely broken fossils)  micrite (lime mud)  chalk (microscopic plankton shells, usually powdery)  travertine (interlocking texture, precipitated directly from water) Dense organic matter Coal Identifying Sedimentary Minerals and Materials

Calcite CaCO3 can be most any color (usually light colored) harder than a fingernail but softer that steel nail has cleavage reacts strongly with acid Dolomite CaMg(CO3)2 can be most any color (usually brown) harder than a fingernail but softer that steel nail has cleavage reacts weakly with acid Halite NaCl colorless to white hardness close to that of fingernail has cleavage water soluble, salty taste. Chalcedony SiO2 can be most any color harder than steel nail no cleavage, extremely fine grained to amorphous, has conchoidal fracture Gypsum CaSO42H2O colorless to white softer than fingernail has cleavage Organic (C, H, N, S, O) black in color, shiny to dull Matter softer than a steel nail no cleavage, sometimes conchoidal fracture low heft LAB Sedimentary Rocks Name______ BE KIND TO THE SPECIMENS!

DETRITAL SEDIMENTARY ROCKS 1. Gray Box 2 (red dot) vs. Gray Box 9 (gold dot) Examine the specimens in both boxes. Using the handout and the sand gauge determine the particle size of the specimens in each box.

Box 2: ______Box 9: ______

Which is the sandstone and which is the siltstone? How can you tell?

Notice that the grains in 2 are all of the same type. Describe the color, luster and cleavage of these grains. Can you identify the mineral?

Which was deposited by more rapidly flowing water? Explain your reasoning!

Suggest a depositional environment for each!

2 ______9 ______

2. Gray Box 6 (black dot) vs. Gray Box 7 (white dot) Note that specimens in each box consist of a mixture of small and large particles. Determine the particle size of the large particles in the specimens in each box.

Box 6: ______Box 7: ______

Which is the conglomerate and which is the breccia? How can you tell?

In which box were the large grains transported further before being deposited? Consider both the grain size and the grain shape when answering this question!

Suggest a depositional environment for each!

6 ______7 ______CHEMICAL SEDIMENTARY ROCKS 3. Limestone (Gray Box 5, orange dot) Examine all the specimens in box 5. Base your answers on all the specimens in the box.

What color is limestone?

What is the grain size of limestone?

What do all the specimens in the box have in common?

4. Chert (Gray Box 8, brown dot) Chert is biogenic, composed of millions of microscopic shells. These shells are made of one mineral. What is that mineral? How do you know?

5. Chalk and Diatomite (Gray Boxes B & N) Both chalk and diatomite are made of microscopic plankton shells. The difference is that shells in chalk are made of calcite while the shells in diatomite are made of silica.

Describe how chalk and diatomite are similar in appearance.

Which box contains diatomite? How could you tell? UNKNOWNS Use the sedimentary rock hand out to classify the following rocks.

Unknown # I What is the particle size of the grains in this rock? ______

What is the composition of the grains in this rock?

What type of environment did it form in? Was it marine or terrestrial? Be as specific as possible.

What is the name of this sedimentary rock? ______

Unknown #J What is the particle size of the grains in this rock? ______

Unknown #P What is the particle size of the grains in this rock? ______

What is the name of this sedimentary rock? ______Unknown #O What is the particle size of the grains in this rock? ______

Unknown #R What is the particle size of the grains in this rock? ______

Unknown #V What is the particle size of the grains in this rock? ______

What is the name of this sedimentary rock? ______Unknown #F What is the particle size of the grains in this rock? ______What is the composition of the grains in this rock?

Unknown #L What is the particle size of the grains in this rock? ______

Unknown #U What is the particle size of the grains in this rock? ______

What is the name of this sedimentary rock? ______Metamorphic Rock Handout Foliated Metamorphic Rocks

Layered metamorphic rocks are named according to the type of layering they have.

LAYERING GRAIN SIZE ROCK NAME Slatey Cleavage: rock breaks along finely Fine grained Slate spaced fractures that are controlled by the orientation of microscopic clay grains. Schistossity: rock consists of oriented Fine grained Phyllite mineral grains which control how the (satin sheen) rocks breaks. Medium to coarse grained Schist (glittery) Gneissic Banding: rock consists of Medium to coarse grained Gneiss alternating layers of light and dark minerals. Migmatitic: rock consists of coarse grained Medium to coarse grained Migmatite non-foliated layers surrounded by gneissic layers.

Unlayered Metamorphic Rocks

Unlayered metamorphic rocks are named according to their composition.

COMPOSITION ROCK NAME Calcite Marble Quartz Quartzite

Metamorphic Minerals

A large variety of minerals form during metamorphism, the two most common are described below.

Chlorite-A variety of mica. Typically greenish in color though sometimes whitish to yellowish to colorless. Vitreous to pearly luster. Softer than steel nail, hardness close to fingernail. Good cleavage. Occurs as thin sheets.

Garnet- Usually red, sometimes brown to white. Vitreous to greasy luster. Harder than steel nail. No cleavage. Occurs as equant crystals, often as twelve sided dodecahedrons.

In addition to these minerals metamorphic rocks also contain the following common silicate minerals/mineral groups: quartz, feldspar, biotite, muscovite, amphibole, pyroxene. Olivine is uncommon in metamorphic rocks. Calcite is common in metamorphic rocks. Lab-Metamorphic Rocks Name______

Comparing Protoliths and Metamorphic Rocks During metamorphism rocks are subjected to increased temperature and/or pressure and/or are infiltrated by fluids. In response to these changes the minerals in the rock recrystallize while remaining solid (i.e. they don't melt!), this changes the appearance of the rock and transforms the starter rock (the protolith) into a metamorphic rock. Metamorphic recrystallization can produce several types of changes including; changes in grain size, changes in grain shape, changes in the minerals present and changes in layering. Amazingly these changes usually occur with very little change in the chemical composition of the rock (i.e. no atoms are removed or added to the rock), what happens is that the atoms in the rock are rearranged destroying existing crystal structures and creating new crystal structures (i.e. recrystallization). Compare the following protoliths with their metamorphosed counterparts and describe the changes produced by metamorphism. Consult your previous labs to help describe the protoliths.

1. Sandstone (protolith) (gray box 2) vs. Quartzite (metamorphosed sandstone) (blue box 19) a) Touch the sandstone and quartzite, they should feel different. Describe their different 'feels'.

b) Examine the grains in the sandstone and quartzite using a hand lens. Make a sketch of the grains in each rock as you see them through the hand lens; draw several grains (ten or so) showing how they meet along their edges, please draw the grain boundaries clearly!

Sandstone Quartzite

Texture? interlocking fragmental interlocking fragmental c) Describe in words the change in the shape of the grains.

d) The sandstone is made of mostly one mineral; what is that mineral? ______e) The quartzite is also made mostly of one mineral. Describe this mineral and identify. 2. Limestone (gray box 5) to Marble (blue box 6) a) The limestone is made of mostly one mineral; what is that mineral? ______b) The marble is also made mostly of one mineral. Describe this mineral and identify.

c) What type of fossils are in the limestone? ______d) Are there any fossils in the marble? yes no (circle correct response) e) What happened to the fossils during metamorphism?

3. Basalt (red box 9) to Serpentinite (blue box 21 ) a) What is the grain size of the basalt? fine medium coarse (circle correct response) b) What is the grain size of the serpentinite? fine medium coarse (circle correct response) c) The serpentinite (see Fig. 7-18) is made mostly of the mineral serpentine. Describe the color, luster, cleavage and hardness of serpentine. d) Look up the chemical composition of serpentine: ______e) Is serpentine a hydrous mineral (does it contain water)? yes no f) How did the serpentine grains in the rock form?

g) The basalt is unlayered while its metamorphosed equivalent (the serpentinite) is layered. How did the layers in the greenstone form? 3. Progressive Changes During Metamorphism As temperature and/or pressure increase many metamorphic rocks will go through a series of changes. For example when a mudstone is metamorphosed it is first converted to a slate, then a phyllite, then a schist, and finally to a gneiss. Describe the changes in grain size, layering and mineralogy which accompany these changes in rock type. Describe Changes in Describe Changes in Describe Changes in Grain Size Layering Minerals Present Shale (gray box 4) to Slate (blue box 4)

Slate (blue box 4) to Phyllite (blue box 3)

Phyllite (blue box 3) to Schist (blue box 2)

Schist (blue box 2) to Gneiss (blue box 9)

Generalizations based on table above (circle correct statement)

During progressive metamorphism grain size tends to: decrease or increase or stay the same?

During progressive metamorphism layering tends to become: thinner or thicker or stay the same? 4. Unknowns Describe each unknown and then name the unknown using the handout provided in class.

Unknown #14 blue box grain size: fine medium coarse layering: none slatey cleavage schistossity gneissic banding

How many different minerals can you identify? ______

Describe each and try to identify.

rock name: ______

Unknown #12 cardboard box grain size: fine medium coarse layering: none slatey cleavage schistossity gneissic banding

rock name: ______Unknown #5 cardboard box grain size: fine medium coarse layering: none slatey cleavage schistossity gneissic banding

rock name: ______

Unknown #25 cardboard box grain size: fine medium coarse layering: none slatey cleavage schistossity gneissic banding

rock name: ______