1. 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

2. topaz
amethyst SiO2
“Prismatic” and “columnar” are used inter-changeably. They describe crystals that are elongated but with an equant cross section (may be 3-, 4-, 6-sided or even more complex).

3. Habits of elongated crystals are described as:
acicular
(needle-like)
prismatic
(angular cross section)
columnar
(rounded cross section)

4. The elongation of crystals can be extreme...
fibrous habit
(fiber-like)
If the fibers are flexible, the habit is called asbestiform (as with chrysotile).
natrolite

5. Crystals may grow as tightly packed aggregates
Crystals may grow as tightly packed aggregates. Bundles of fibrous crystals often develop smooth, rounded surfaces.
hematite
mammillary
botryoidal
reniform habits
jasper
malachite

6. Crystals that are elongated but with a distinctly flattened cross section, are said to be “bladed”.
cerussite PbCO3
kyanite AlSi2O5

7. psilomelane
Aggregates with a branching look have
a “dendritic”
habit.
silver, Ag
snow,
H2O
copper, Cu

8. Clusters of crystals can adopt many distinctive shapes.
These bladed crystals of gypsum show no preferred orientation.
But some clusters look strikingly like“rosettes”.

9. Cleavage: crystal breaking along planes
Three feldspars: different colours and composition (Na-, Ca- or K-silicates) but same hardness, vitreous luster and same cleavage.

10. Cleavage is only visible if:
the crystal size is not microscopic
the crystal has broken up along some of these planes
Some minerals have several directions of cleavage at specific angles.

11. Clay minerals have sheets of strongly bonded Mg, Al, Si and O
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.

12. Micas also have structures consisting of sheets of strong bonds
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!

13. Some mineral descriptions mention “cleavage”, others “parting”
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).

14. Hardness:
it’s all relative...

15. 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
Mohs hardness
scale
This scale is exponential.

16. 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.

17. 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.