Habit

1
Habit
Shape and appearance of mineral growth
2
Acicular

• slender, needle-like crystals tourmaline,
  hornblende, arsenopyrite, rutile, apatite,
  sillimanite

3
Capillary and filiform

• hair-like or thread-like native Au, Ag, Cu
• Very thin

4
Bladed

• elongated crystals flattened like a knife blade
  kyanite

5
Dendritic

• arborescent, in slender divergent branches,
  somewhat plantlike--native metals, pyrolusite

6
Radiating

• divergent zeolite tremolite talc
• pyrolusite, tourmaline

7
Drusy

• surface covered with a layer of small
  crystals--sugar like calcite, quartz,
  sphalerite, pyrite

8
Dodecahedral

• Also commonly seen in Garnets, as in this lab
• 12 sided crystal growth.
• NOTE Rhombohedral, Cubic, etc. are also
  geometric habits, i.e. how the crystal grows.

9
Fibrous

• aspect ratio 101
• chrysotile asbestos

10
Globular and colloform

• radiating individuals forming small spherical
  groups
• examples include zeolites, quartz, malachite,
  goethite, pyrolusite, hematite

11
Botryoidal

• bunch of grapes, example pyrolusite

12
Reniform

• kidney like, examples hematite, malachite

13
Foliated

• easily separable into plates or leaves
• Micas are the ultimate example

14
Micaceous

• similar to foliated but splits into very thin
  sheets muscovite, biotite, chlorite

15
Tabular or lamellar

• flat and plate-like barite, dolomite

16
Granular

• composed of many individual grains of similar
  size olivine, garnet

17
Prismatic or columnar

• elongated crystals with identical faces parallel
  to a common direction tourmaline, hornblende,
  apatite

18
Equant Crystal Habit

• Squashed, pinnacoids (terminating face)
  dominant over prisms

19
Types of crystals

• Euhedral
• Subhedral
• Anhedral
• Amorphous

20
Crystal Growth is not always perfect
21
Twinning

• Crystals that are related to one another by some
  geometric relation.

22
How does it happen?

• Errors occur during the crystallization of a
  mineral
• This error can be caused by random flaws in
  nature.
• During growth phases
• High temperature phase to a low temperature phase
• Siamese twins
• never random and follows certain defined rules
  called twin laws.

23
How do we recognize them?

• recognized by penetration angles or notches in
  the crystal
• Contact
• Planes that forms the boundary between the twins
• Penetration
• Two crystals grew into/out of each other
• Simple or complex
• 2 or more than 2

24
Simple
25
Penetrative
26
Polysynthetic (multiple)

• e.g. Plagioclase

27
Cyclic
28
Parting

• Occurs when a mineral breaks along a twin plane

29
Hopper Crystals
30
Dendrites