Download Rock or Mineral?

Rock or Mineral?
• It is not easy to tell the difference
between rocks & minerals
because there are so many kinds
of them. It takes years of study to
be able to accurately identify a
mystery rock & where the
specimen came from.
• All rocks are made of 2 or more
minerals, but minerals are not made
of rocks.
MINERALS
A mineral is the same all
the way through. It is a
sample or a specimen
rather than a rock.
There are about 3000
known minerals on
earth.
Minerals: Building Blocks
Minerals are born of fluid, heat,
and pressure. They are forged
underground, where forces that
have been at work for billions of
years continue to make more
minerals. A mineral is a naturally
occurring, inorganic (nonliving)
solid having a specific chemical
composition.
Chemical Formula
Every mineral has a unique arrangement
of atoms that determines the mineral type.
All minerals have a chemical formula,
which is an analysis of the types and
amounts of elements present in a mineral.
The chemical formula of the mineral
hematite is Fe2O3. The letters describe the
element type (Fe = iron, O = oxygen), and
the subscripted numbers describe the
amount of those atoms in each molecule. A
Hematite molecule has 2 iron (Fe) atoms
and 3 oxygen (O) atoms.
Classes of
Minerals
The Carbonates
The Silicates
The Sulfides
The Oxides
Quartz Family
The Halides
The Sulfates
Elements
http://www.rocksandminerals4u.com/minerals.html
Physical Properties
most useful for mineral identification:
•
•
•
•
Cleavage
Fracture
Hardness
Specific
Gravity
• Streak
• Associated
Minerals
• Notable
Localities
•Color
•Luster
•Transparency
•Twinning
•Crystal
Systems
•Technical
Crystal
Habits
•Descriptive
Crystal
Habits
PROPERTIES OF MINERALS
These are the most common characteristics
used when describing minerals.
• Color – this varies depending on the chemicals present and is the
least informative in identifying a mineral variety
• Luster – what the surface looks like in the light
• Specific Gravity – how heavy it feels, heft
• Crystal Form – shape of crystal, shape the mineral would take if it
had room to grow in a cavity,
not massive – some minerals have a number of different crystal
shapes
• Cleavage – pattern when mineral is broken –planes or conchoidal
fracture
• Tenacity - toughness, how cohesive the mineral is, if it falls apart
• Hardness – what it can scratch & what scratches it
• Transparency - The ability to transmit light. Many rocks that are
opaque when in a chunk, are translucent when cut into very thin
slices. Gems stones are often valued on how transparent they are.
• Special Properties– magnetism, chatoyancy (cat’s eye),
fluorescence, odor, streak, burn test, conductivity, double
refraction, & radioactivity.
COLOR
• Color is one of the physical properties most commonly used
to describe minerals, but it is not a very good property to use t
identify minerals.
• Some minerals are nearly always the same color like azurite
(blue) and sulfur (yellow)
• Many minerals come in a variety of colors – the changes are
caused by chemical impurities or through exposure to heat
• Color can change when the surface is exposed (tarnishes or
oxidizes)
• Some minerals have common names (varietal names) that
describe a specimen with a certain color eg. Quartz – rock
crystal (colorless), smoky quartz (brown), citrine (yellow),
amethyst (violet), rose quartz (pink)
• Color can be described as metallic or non-metallic and is
often described along with luster though different.
• Rocks are often distinctive or named because of a certain
color which occurs because of their mineral content.
Color Descriptions Metallic or
Nonmetallic
Intensity of colors:
Dark
very dark
light, pale
deep
dull
shiny
bright
Color is distributed:
streaked
splotchy
mottled
speckled
layered
banded
Other words that
describe luster:
shiny, sparkly
shimmering,
opalescent
frosted, milky
Words
LUSTER - describes the way light
reflects off of the surface of a
mineral:
Description
Example
dull / earthy very dull, mainly in minerals that are porous
kaolinite, orthoclase
waxy
opal, chalcedony
like the surface of a candle
greasy / oily
nepheline
pearly
like a pearl, play of colors, light
silky
has a shiny surface like a piece of silk cloth
glassy /
vitreous
resinous
adamantine
talc, muscovite mica
gypsum, kernite, ulexit
& in fibrous minerals
looks like glass
quartz, rock-forming
minerals, obsidian
looks like freshly-broken shellac, usually yellow- sphalerite
brown
high luster, almost brilliant, "diamond-like"
sphalerite
sub-metallic silvery or metallic luster but mineral is
transparent or translucent
metallic
very shiny, like processed metals, highly
reflective, opaque minerals
hematite
pyrite, gold, silver
SPECIFIC GRAVITY
•Specific Gravity (SG) indicates how many times more the mineral weighs
compared to an equal amount of water (SG 1).
• So if you have a bucket of silver, it would weigh 10 times as much as a
bucket of water.
•If you have a bucket of calcite, it would only weigh about 2 1/2 times as
much as a bucket of water.
•That is why we think of metals as being "heavy“ or "heft" of an object.
•They are heavy compared to other things that we are used to picking up.
Description
very light
light
SG
Mineral Examples
<2
borax
2 - 2.5 gypsum, halite, selenite, ulexite
average
2 – 3 calcite, dolomite, feldspar, muscovite mica, quartz, talc,
turquoise,
above average / 3 - 4 biotite mica
slightly heavy
heavy
4 – 5 almandine garnet, apatite, barite, celestite, fluorite
very heavy
5 – 10 galena, hematite, magnetite, nickel-iron, pyrite
extremely heavy> 10 gold, silver
super heavy
20+
platinum
TENACITY
Tenacity is how tough or how easily a
mineral will break, split, crumble or change
shape.
•elastic ~ can be bent & resume previous
shape ~ mica
•ductile ~ can be pulled to make thin
threads ~ gold
•flexible
•fragile ~ break into pieces easily
•friable ~ crumbles easily
•malleable ~ flatten into thin sheets w/o
break ~ gold
•sectile ~ can be cut for shavings ~ gypsum
•tough
CLEAVAGE
Cleavage is when a mineral breaks
with smooth flat surfaces.
Cleavage can be described as
perfect, good, imperfect, poor.
It can also be described as:
•Perfect 1 way ~ breaks on one
perfect cleavage plane, crystals
break into slices, sheets peel off
•Perfect 2 ways ~ breaks into
elongated boxy shapes, 90 degree
angles
•Perfect 3 ways ~ breaks into
perfect rhombs, pieces look like
squished boxes
•No cleavage ~ does not break
regularly
FRACTURE
Fracture is when a mineral breaks,
but the surface is not regular, does
not show cleavage.
conchoidal ~ curved break like what
happens with thick glass or bottle
bottom, shell shaped, can be rough or
smooth
•jagged ~ metals, sharp point that
scratches or snags fingertips, hackly
•splintery ~ fibrous
•uneven ~ rough surface, not smooth
TRANSPARENCY ~ Transmitting Light Through Minerals
transparent •clear, see right through it when it is
celestite, quartz (rock
sliced thin
crystal),
•called "gemmy", desirable for gemstones selenite
translucent •see shapes & shadows through it when it calcite, quartz, sphalerite
is sliced thin
•chemical impurities can cause the
mineral to be cloudy
opaque
•can’t see light through it at all when it is metals, gypsum
sliced thin
•rarely used for gemstones
STREAK ~ powdered residue left on a black or white tile.
Not necessarily the same color as the mineral.
HARDNESS
Friedrich MOH’S SCALE OF
HARDNESS
Scratching tools:
fingernail (2.2)
copper penny (3.5)
pocket knife or common nail
(5.2)
piece of glass (5.5)
steel file or concrete nail (7.5)
piece of corundum (9)
Moh’s
Scale
Rating
1 Very Soft
2 Soft
Description
Easily crumbles. Can be scratched with a
fingernail (2.2)
Can be scratched with a fingernail (2.2)
3 Soft
Can be scratched with a copper penny (3.5)
Gypsum,
Soapstone
Calcite
4 Semi-Hard
Can be scratched with a common nail (5.2)
Fluorite
5 Hard
Can be scratched with a common nail (5.2).
Apatite
6 Hard
N. B. Mineral of hardness 6 or more will
scratch glass.
Can be scratched with a concrete nail (7.5).
Feldspar
7 Very Hard
8 Very Hard
Mineral Example
Talc
Quartz
Topaz
9 Extremely
Used in industrial tools for cutting, grinding & Corundum
Hard
sanding.
10 The Hardest Diamond is used to cut all minerals including Diamond
diamonds.
Notes for testing:
Each mineral can scratch the
minerals with lower hardness
ratings.
Each mineral can scratch itself.
Don’t press hard, normal scratching
should do.
Weathered surfaces are softer.
Corners or edges of crystals are
softer.
Small pieces seem softer than large
pieces.
When you scratch, take a close look
at the scratch line - which often looks
white. Is it really a scratch or is it a
powder line made from the tool you
used because it was softer than the
item you were trying to scratch?
Crystalllographic
Systems
•Minerals usually form
distinct crystals and their
shape plays an important
part in their identification.
• The study of crystals is
called crystallography and
includes the study of
natural crystal but crystals
formed by metal alloys,
chemicals, and other
synthetic materials.
•Specific tools, such as an
x-ray spectrometer, are
used to find and
distinguish new minerals
and verify the
identification of specimens
by the arrangement
of atoms and/or ions.
Crystals: The Form
Most minerals occur naturally as crystals.
Every crystal has an orderly, internal pattern
of atoms, repeating that pattern.
Symmetry is a regular, repeated pattern of
component parts.
The shape of the resulting crystal mirrors the
internal arrangement of the atoms.
As crystals grow, differences in temperature
and chemical composition cause variations.
Crystals need ideal growing conditions and
room to grow or they mesh together to form a
conglomerated mass like most rocks.
The museum-quality specimens shown in the
images here, grew in roomy environments that
allowed the geometric shapes to form.
Crystals Grouped by Properties
There are four main categories of crystals, as grouped by
their chemical and physical properties:
Covalent Crystals
Many covalent crystals have extremely high melting points.
Examples :diamond and zinc sulfide crystals.
Metallic Crystals
Individual metal atoms of metallic crystals sit on lattice sites
Metallic crystals are very dense and have high melting points
Ionic Crystals
Ionic crystals are hard and have relatively high melting
points. Table salt (NaCl) is an example of this type of crystal.
Molecular Crystals
Molecular crystals tend to be soft with relatively low melting
points. Rock candy is an example of a molecular crystal.
The two most common methods of grouping crystals are according
to their crystalline structure and their chemical/physical properties:
Crystal Grouped by Lattices (Shape)
There are seven crystal lattice systems.
Cubic or Isometric - not always cube shaped! You'll also find
octahedrons (eight faces) and dodecahedrons (10 faces).
Tetragonal - similar to cubic crystals, but longer along one axis than
the other, forming double pyramids and prisms.
Orthorhombic - like tetragonal crystals except not square in cross
section (when viewing the crystal on end), forming rhombic prisms
or dipyramids (two pyramids stuck together).
Hexagonal - six-sided prisms. When you look at the crystal on-end,
the cross section is a hexagon.
Trigonal - possess a single 3-fold axis of rotation instead of the 6fold axis of the hexagonal division.
Triclinic - usually not symmetrical from one side to the other, which
can lead to some fairly strange shapes.
Monoclinic - like skewed tetragonal crystals, often forming prisms
and double pyramids.
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