Practical-Rock-ID-Guide

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1
Fossils are present.
100%
5
Sedimentary
Clastic, Biochemical, or Organic
Layers are present.
99%
Examine the sample to determine whether the layers are beds
(sedimentary), colour banding from foliation (metamorphic) or
rock cleavage (metamorphic).
Examine the texture, grain size, mineral composition, and
colour to determine which specific kind of these three
sedimentary rock types it is.
2
Garnet is present.
100%
6
Metamorphic
Foliated
OR
Examine the texture, grain
size, and rock cleavage to
determine which foliated
metamorphic rock it is.
3
Foliation is present.
100%
!
4
99%
95%
Sedimentary or Metamorphic
Exception: Igneous rocks can have small amounts of calcite in
veins and filling vesicles.
70%
Glitters
Examine the mineral
composition to determine
which non-foliated
metamorphic rock it is.
Some coarse-grained clastic sedimentary rocks have clasts
aligned by flowing water and could resemble metaconglomerate. Look for foliation throughout, not just in the
arrangement of clasts..
Igneous
Mafic or Ultramafic
Exception: Some metamorphic rocks where chemical
reactions with olivine in an igneous protolith are incomplete.
Examine the texture and composition to determine which mafic
or ultramafic rock it is.
OR
Look for evidence of flat
crystal faces,
semi-transparent crystals, &
uniform composition to
confirm that it is marble.
7
Examine the texture, grain size, and rock cleavage to determine
which foliated metamorphic rock it is.
Olivine is present.
Calcite is present.
Non-Foliated
Metamorphic
Sedimentary or Metamorphic
8
A crystalline rock has
some well-formed
crystals that are much
larger than other
crystals surrounding
them.
Vesicles or amygdules
are present.
90%
Doesn’t Glitter
Look for a dull lustre, light
grey or tan colour, and
absence of visible crystals
as evidence of limestone.
Igneous or Metamorphic
In igneous rocks these are phenocrysts. Similar crystals in
metamorphic rocks are porphyroclasts or porphyroblasts.
Examine the sample for other igneous or metamorphic textures,
or minerals that are exclusive to igneous or metamorphic rocks.
100%
!
99%
Igneous, Volcanic
Some sedimentary rocks have irregularly-shaped openings.
Vesicles are openings that resemble bubbles.
Lower than
average density
OR
Examine the colour to
determine whether the rock
is pumice or scoria.
Average density
Examine the colour, lustre,
and look for phenocrysts
to determine whether it is
rhyolite, andesite, basalt,
or obsidian.
Karla Panchuk (2017) CC BY 4.0
9
10
Contains rounded or
broken fragments that
are not fossils.
Breaks or crumbles
relatively easily.
80%
Sedimentary, Clastic, Medium-Coarse Grained
Clastic, Biochemical, or Organic
13
Examine the grain size and shape to determine whether the
rock is sandstone, conglomerate, or breccia.
80%
Weathering and fractures can cause any rock type to break easily.
14
It glitters when turned
slightly in the light.
Examine the texture and grain size to determine which clastic
sedimentary rock it is.
11
3 or more colours
70%
65%
Feels Smooth
OR
Look at the lustre. Obsidian
has glassy lustre. Shale can
have a dull lustre and makes
a knock when you hit it on a
surface. Slate can have a soft
shine and clink when hit.
!
Sedimentary, Clastic
Exception: highly vesicular igneous rocks can break or crumble
easily.
!
Dark with no visible
grains.
Feels Rough
Check if the rock is light for
its size. If yes, and vesicles
are abundant, it is scoria. If
not, basalt.
Weathering can make a rough rock feel smooth.
70%
Metamorphic, Medium-High Grade or Igneous, Intrusive
80%
Tiny sparkles
OR
Lots of sparkles without
colour banding is schist.
Fewer sparkes with colour
banding is gneiss.
Igneous, Felsic or Metamorphic, High Grade
Larger flashes
Look at the mineral
composition to determine
what kind of intrusive
igneous rock it is.
Exception: Some coarse-grained sedimentary rocks could appear
multicoloured.
!
100%
Do not count colours from secondary features such as veins,
staining, or weathering reactions.
No pattern
OR
Examine the grain size to
determine whether the rock
is granite or porphyritic
rhyolite.
Colour bands
Examine the rock for
foliation to confirm that it
is gneiss.
15
When the sample is wet
then wiped dry, a darker
patch remains.
75%
!
Sedimentary, Clastic
This effect is due to porosity, and can occur in muddy
limestones and on some weathered surfaces.
Look at the grain size to determine the type of clastic
sedimentary rock.
Karla Panchuk (2017) CC BY 4.0
12
Feels gritty.
65%
!
Sedimentary, Sandstone
Whether something feels gritty (like sandpaper) or not is
subjective. A rough texture is not necessarily a gritty texture.
Examine the grain size and check for the presence of quartz to
help confirm that the rock is sandstone.
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