Rock Around the World
Here's a fun citizen science project. Find a rock, note it's location and send it to Arizona State University to have it analyzed. The website not only will analyze your rock, but it is full of great information about minerals and rocks.
_A. All minerals are made of crystals. (Solid crystalline substances in which the particles are arranged in a particular, repeating 3-dimensional pattern) (see Crystals A.2.)
B. There are about 4000 - 10,000 different minerals
1. ~100 of these are commonly found
2. 10-20 (mostly silicates) make up 90% of the Earth’s crust
a. Feldspar & quartz are most common
Note: Quartz (silicon dioxide) is technically both a silicate (compound that contains silicon) and an oxide (compound that contains oxygen). In your mineral book it's listed as an oxide, but it's really both.
3. Remaining 10% made up of carbonates, sulfides, chlorides & halides
C. Each mineral is made of ONE type of particle forms in ONE type of crystal system
1.Orpiment & realgar are 2 different minerals
a.Both are monoclinic, but they are made of 2 different particles (As2S3 & AsS)
2. Pyrite and marcasite are 2 different minerals.
a. Both composed of FeS2, but pyrite is cubic, whereas marcasite is orthorhombic
3. Carbon may crystalize in the cubic system as the mineral, diamond, or in the hexagonal system as the mineral, graphite.
D. Minerals are inorganic or abiotic, (meaning that living things do not produce them)
1. Minerals that are made by living things are called ‘biominerals’.
a. Ex. shells, bone, stromatolites
Orpiment & Realgar are 2 different minerals
Pyrite & Marcasite are 2 different minerals
Guide to physical characteristics
Click on the Mineralogy for Kids link to find examples of the physical properties listed in your notes (and more).
The document on the right is another way to review the material that is also described below. I gave this to you as a handout, too.
E. A complete description of the external shape of a mineral or crystal is called its "habit"
1. Describes visible (or macroscopic) characteristics
2. The form of a crystal can also be the name of it's habit (but not always)
a. Ex. cubic form and cubic habit of pyrite
3. There are many, many terms that are used to describe a mineral’s habit. Some of them are as follows:
a. cubic – looks like a cube
b. fibrous – looks like strands or thread-like fibers
c. dendritic – looks like plantlike branches
d. acicular – needle-shaped, often radiating
e. bladed – flat and long
f. concentric – forming roughly spherical, round layers around a common center
g. pyramidal – forming a point like a pyramid
h. massive – looks like a lump of rock, meaning that the mineral does not give any hint of the microscopic crystal system it was made of
4. The habit of a particular mineral may or may not look like the crystal system it was formed by. The same mineral formed from the same crystal system can produce habits that look very different, depending on the conditions it was formed under.
a. Pyrite (FeS2 formed in the cubic crystal system) may have a cubic form & habit, but it may also look like an octahedron, massive, twinned, granular, etc.
b. Quartz (SiO2 in the hexagonal crystal system). It can form beautiful rock crystals (prismatic), agate (concentric), moss agate (dendritic), rose quartz (massive) and purple amethyst (pyramidal), ALL of which are produced in the hexagonal crystal system.
5. The habit is only one way to help identify a mineral
Pyrite has many habits
Quartz has many habits, too.
F. Minerals are identified by a series of physical characteristics, including:
1. Color: Usually the first and easiest characteristic to identify, but not always the best
a. Some minerals have characteristic colors (blue of azurite, yellow of sulfur), but others do not (fluorite occurs in all colors)
b. Trace elements or contaminants can give color to a mineral (i.e., amethyst is a form of quartz that has trace amounts of iron that give amethyst its purple color)
Trace contaminants affect color
2. Transparency: the way in which light passes through
a. Transparent: allows objects to be seen through mineral
b. Translucent: allows light to pass through, but object not clearly seen
c. Opaque: no light passes through mineral
Click on the images to enlarge them.
5. Hardness: its resistance to being scratched
a. Mohs scale of hardness goes from 1 – 10, where one is softest and 10 is hardest
b. Minerals with higher Mohs numbers will scratch those with the same or lower Mohs numbers, but not vice versa.
i. Talc is 1 and may easily be scratched by a fingernail (2.5)
ii. Calcite is 3 and may be scratched by a copper penny (3.5)
5. Moh's Scale of Harndess
6. Cleavage: ability of a mineral to break along its planes. Some ways include:
a. Makes a smooth surface
b. Different crystal systems cleave in different ways
i. Ex. halite: cubic crystal system cleaves in cubes
ii. Mica: (muscovite, lepidolite, biotite or phlogopite): monoclinic cleaves in sheets
Fracture & Cleavage
When you break a mineral, sometimes they break and create nice "faces" (called cleavage) and sometimes they don't break along flat planes.
The 1st video demonstrates how you can hit a mineral and create cleavage or fracture.
The second video is a teacher who gives a lot more in-depth discussion of fracture and cleavage. You may watch this for interest and more information, but you don't have to know everything in the second video.
Click on the images to enlarge them.
8. Other physical properties
7. Fracture: how a mineral breaks other than along its cleavage planes. Some ways include:
a. Conchoidal: shell-like fracture
b. Uneven: rough, uneven
c. Hackly: sharp edges and jagged points