what should I notice

If a mineral breaks along smooth flat planes, what should I notice?

Cleavage. Count the number of different cleavage directions, not every flat surface.

If a mineral breaks irregularly instead of along flat planes, what should I notice?

Fracture. Curved shell-like fracture is called conchoidal fracture.

If a mineral is scratched, what property am I testing?

Hardness; compare it with fingernail, penny, glass, etc.

If a mineral leaves a colored powder on a streak plate, what property am I testing?

Streak; the powder color can be more useful than the mineral's surface color.

What should I notice first when identifying a mineral?

Luster, hardness, streak, cleavage/fracture, crystal shape, and special properties like HCl reaction.

If a mineral has 3 cleavage directions at 90°, what should I think?

Halite; cubic cleavage.

If a mineral has 3 cleavage directions NOT at 90°, what should I think?

Calcite; it also reacts strongly with HCl.

If a mineral has 2 cleavage directions near 90°, what should I think?

Pyroxene.

If a mineral has 2 cleavage directions around 60° and 120°, what should I think?

Amphibole.

If a mineral is very soft and scratches with a fingernail, what should I think?

Gypsum.

If a mineral is glassy, hard, and has no cleavage, what should I think?

Quartz; commonly shows conchoidal fracture.

If a mineral has a reddish-brown streak, what should I think?

Hematite, even if the outside looks dark or metallic.

If a mineral peels into thin flexible sheets, what should I think?

Mica; light = muscovite, dark = biotite.

What should I notice first in an igneous rock?

Texture/crystal size: coarse, fine, glassy, vesicular, or porphyritic.

Large visible igneous crystals tell me what?

Slow cooling underground → intrusive.

Tiny or invisible igneous crystals tell me what?

Fast cooling at/near the surface → extrusive.

Two different crystal sizes in an igneous rock tell me what?

Porphyritic texture → two stages of cooling.

Lots of holes in an igneous rock tell me what?

Vesicular texture from trapped gas bubbles.

A black glassy rock with no visible crystals should make me think what?

Obsidian; extrusive igneous rock that cooled extremely quickly.

A very light-colored, extremely vesicular rock should make me think what?

Pumice.

A dark/red vesicular rock that is heavier than pumice should make me think what?

Scoria.

Coarse light-colored igneous rock with visible crystals?

Granite.

Fine light-colored igneous rock?

Rhyolite.

Coarse black-and-white salt-and-pepper igneous rock?

Diorite.

Fine gray intermediate igneous rock?

Andesite.

Coarse dark igneous rock?

Gabbro.

Fine dark igneous rock?

Basalt.

What should I notice first in a metamorphic rock?

Foliation/banding, mineral alignment, recrystallization, grain size, and whether it reacts with HCl.

Very fine, dull metamorphic rock that splits into flat sheets?

Slate.

Fine metamorphic rock with a silky/shiny wavy surface?

Phyllite.

Metamorphic rock with obvious shiny mica flakes?

Schist.

Metamorphic rock with alternating light and dark bands?

Gneiss.

Crystalline metamorphic rock that fizzes with HCl?

Marble.

Very hard crystalline metamorphic rock that does not fizz?

Quartzite.

Dense, fine-grained metamorphic rock with no foliation?

Hornfels.

What does foliation tell me?

Minerals were aligned/rearranged by directed pressure during metamorphism.

Slate → phyllite → schist → gneiss tells me what?

Increasing metamorphic grade.

How do I distinguish slate from shale?

Slate = metamorphic and has slaty cleavage; shale = sedimentary and has bedding.

How do I distinguish marble from limestone?

Marble = metamorphic and recrystallized; limestone = sedimentary. Both may fizz.

How do I distinguish quartzite from sandstone?

Quartzite is recrystallized and breaks through grains; sandstone has visible cemented sand grains.

What should I notice first in a sedimentary rock?

Grain size, grain shape, layers/bedding, fossils, composition, and HCl reaction.

Rounded pebble-sized clasts should make me think what?

Conglomerate.

Angular pebble-sized clasts should make me think what?

Breccia.

Visible sand-sized grains should make me think what?

Sandstone.

Very fine-grained rock that splits into thin layers should make me think what?

Shale.

Sedimentary rock that fizzes strongly with HCl should make me think what?

Limestone because it contains calcite.

Sedimentary rock with obvious shells or fossils should make me think what?

Fossiliferous limestone.

Black sedimentary rock made from plant material?

Coal.

Crystalline sedimentary rock made mostly of halite?

Rock salt; an evaporite.

Soft crystalline sedimentary evaporite?

Rock gypsum.

Rounded clasts tell me what about transport?

They were transported farther or abraded more.

Angular clasts tell me what about transport?

They were transported a shorter distance from the source.

Large/coarse sediment grains usually indicate what kind of environment?

Higher-energy environment.

Fine mud/clay sediments usually indicate what kind of environment?

Low-energy, quiet water.

What should I notice when looking for an unconformity?

A missing interval of geologic time caused by erosion or nondeposition.

Tilted older layers beneath younger horizontal layers?

Angular unconformity.

Parallel sedimentary layers separated by an erosional/time gap?

Disconformity.

Sedimentary rock lying on igneous or metamorphic rock?

Nonconformity.

If one feature cuts through another, which is younger?

The cutting feature is younger — cross-cutting relationship.

If rock fragments are inside another rock, which is older?

The included fragments are older than the rock containing them.

In undisturbed sedimentary layers, where is the oldest layer?

Bottom; principle of superposition.

Sedimentary layers normally form in what orientation?

Approximately horizontal; original horizontality.

What should I notice when identifying a fossil?

Overall shape, symmetry, chambers, coils, segments, and whether it is an impression or a 3-D form.

A coiled spiral shell should make me think what?

Gastropod.

Small circular or star-shaped disk pieces should make me think what?

Crinoid.

A shell with symmetry through each individual valve should make me think what?

Brachiopod.

Many small repeating chambers or radial patterns should make me think what?

Coral.

An empty impression of an organism is called what?

Mold.

A 3-D replica formed when a mold fills with material is called what?

Cast.

A thin dark carbon outline of an organism is what type of preservation?

Carbonization.

When I see crystals, can I immediately call the rock igneous?

No. Both igneous and metamorphic rocks can contain crystals.

When I see layers, can I immediately call the rock sedimentary?

No. Gneiss has metamorphic banding; sedimentary layers are bedding.

When I see no foliation, can I rule out metamorphic rock?

No. Marble, quartzite, and hornfels are nonfoliated metamorphic rocks.