OLIVINE (Peridot)

TOPICS

1. What is olivine (peridot)?
2. Where is it found?

COMPOSITION	Mg2SiO4-Fe2SiO4
HARDNESS	6.5-7
CRYSTAL SYSTEM	Orthorhombic
COLOR	Yellow/pale green-black
FRACTURE	conchoidal
INCLUSIONS	Commonly gas bubbles or other minerals

Learn more about how to identify olivine

Mg2SiO4-------**-----------MgFeSiO4---------------------Fe2SiO4

pale green------**----------brown green---------------------black

**most common color and composition (yellow-green)

Therefore, the depth of color depends on Fe content

• olivine is a silicate mineral where the silicate tetrahedra are embedded in a sea of cations (Mg or Fe)

  See a movie illustrating the olivine structure!

(2) Origin:

Geological

• Olivine is found in many different rocks:
  1. Basalts and "peridotitic" (peridot-rich) rocks
     • rocks that contain little quartz
  2. Mantle rocks
     • the mantle is composed mostly of olivine
     • "dunite" is a rock made almost entirely of olivine
  3. Volcanic rocks, like those found at Kilbourne Hole
  4. Xenoliths (lumps of rock ripped up from a source area and carried up in magma)
• olivine has a high melting point and is an early crystallizing mineral
  • Mg-rich crystals crystallize first, then Fe-rich ones. Consequently, they tend to have Mg-rich (pale colored) cores and Fe-rich (browner) rims (i.e., they are color zoned). crystals are commonly zoned

Learn more about how to identify cordierite

• one of the most diagnostic properties of cordierite is that it is VERY pleochroic
  • the color is purple from one side, and clear when rotated to another side
• Cordierite is a RING silicate, similar to beryl

  See a movie illustrating the cordierite structure!

• "Bloodshot Iolite" contains many small inclusions
• it is also known as iolite and dichroite

(2) Where is it found?

FELDSPAR

See a movie on an albite structure!
See a movie on an anorthite structure!

Learn more about how to identify orthoclase

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Feldspars are very important rock-forming minerals and they are covered in more detail in other geology courses. Further information about their structures, compositions, and microstructures can be found here.

Potassium(K) Sodium(Na) aluminosilicate = alkali feldspar.

Examples include moonstone, amazonite, orthoclase

Other terms used for potassium-rich feldspars include: sanidine, orthoclase, microcline

Albite is a term used for sodium-rich feldspar.

Sodium(Na) Calcium(Ca) aluminosilicate = plagioclase.

One example is the mineral labradorite

HARDNESS	6-6.5
CRYSTAL SYSTEM	Monoclinic or triclinic
CLEAVAGE	2 good cleavages at 90 degrees
SPECIFIC GRAVITY	2.56-2.76

(2) Where are feldspars formed?

• they can be found in lots of different rock types
• Feldspars make up 50-60 % of crustal rocks
  • large crystals in rocks cooled slowly deep in the earth
  • common in igneous rocks
• Feldspars are very important and interesting minerals but less important gems. If you want to learn more about these common rock-forming minerals, there is lots of additional information here.

(3) Other interesting characteristics