Download Properties2

Under different conditions, a given
mineral may grow as a massive aggregate
where its many crystals show no distinct
habit (too small or too tightly packed to
develop symmetrical faces).
massive magnetite
octahedral magnetite
amethyst SiO2
topaz
“Prismatic” and “columnar” are used interchangeably. They describe crystals that
are elongated but with an equant cross
section (may be 3-, 4-, 6-sided or even
more complex).
Habits of elongated crystals are
described as:
acicular
(needle-like)
prismatic
(angular cross section)
columnar
(rounded cross section)
The elongation of crystals can be extreme...
fibrous habit
(fiber-like)
natrolite
If the fibers are
flexible, the habit is
called asbestiform (as
with chrysotile).
Crystals may grow as tightly packed
aggregates. Bundles of fibrous crystals
often develop smooth, rounded
surfaces.
hematite
jasper
malachite
• mammillary
• botryoidal
• reniform habits
cerussite PbCO3
Crystals that are
elongated but with a
distinctly flattened
cross section, are said
to be “bladed”.
kyanite AlSi2O5
Aggregates with a
branching look have
a “dendritic”
habit.
psilomelane
silver, Ag
snow,
H2O
copper, Cu
Clusters of crystals can adopt many
distinctive shapes.
These bladed crystals of
gypsum show no
preferred orientation.
But some clusters look
strikingly like“rosettes”.
Cleavage: crystal breaking along planes
Three feldspars: different colours and
composition (Na-, Ca- or K-silicates) but
same hardness, vitreous luster and same
cleavage.
Cleavage is only
visible if:
1) the crystal size
is not
microscopic
2) the crystal has
broken up along
some of these
planes
Some minerals
have several
directions of
cleavage at
specific angles.
Clay minerals have sheets of strongly bonded
Mg, Al, Si and O. The sheets are weakly held
by van der Waals forces or hydrogen bonds.
Clays should have
perfect cleavage but
their crystals are too
small for our eye to
detect it.
Micas also have structures consisting of
sheets of strong bonds. Their crystals
are coarser (bigger) than those of clay
minerals, and can be cleaved in very thin,
translucent sheets.
... micaceous cleavage!
Some mineral descriptions mention
“cleavage”, others “parting”. Both are
patterns of planar breaks... The distinction
is in the cause of this pattern.
• cleavage is present at all scales, in all
specimens of a given mineral;
• parting does not occur in all specimens.
It depends on the growth conditions (more
on that later in the course).
Hardness:
it’s all relative...
Mohs
hardness
scale
This scale is
exponential.
1 - Talc
2 - Gypsum
FINGERNAIL : 2.5
3 - Calcite
PENNY : 3.5
4 - Fluorite
GLASS PLATE: 4.5
5 - Apatite
6 - Orthoclase
KNIFE or NAIL: 6
STREAK PLATE: 6.5
7 - Quartz
8 - Topaz
STEEL FILE: 7-8
9 - Corundum
10 - Diamond
How to test hardness on minerals...
1) Does apatite
scratch quartz?
Well, it left a
mark on it. But...
2) The mark can be
rubbed off. It was not
a scratch but powder
left on quartz by the
softer apatite.
Check the date when you pick that penny!
Pre-1997 Canadian pennies and pre-1982
U.S. pennies are nearly pure copper. Recent
pennies are mostly zinc (harder than
copper), thinly coated with copper to give
them colour.