Olivine, Cordierite & Feldspar

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Olivine
Cordierite
Feldspar
Olivine (Peridot) What is olivine (peridot)?
Where is it found?
What is olivine?
 
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)
 

Olivine Images


See a movie illustrating the olivine structure!

Origin:

Olivine is obtained from many geographic localities

Geological

  • Olivine is found in many different rocks:
    • Basalts and "peridotitic" (peridot-rich) rocks
      • rocks that contain little quartz
    • Mantle rocks
      • the mantle is composed mostly of olivine
      • "dunite" is a rock made almost entirely of olivine
    • Volcanic rocks, like those found at Kilbourne Hole
    • 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
Cordierite (Iolite ) What is cordierite?
Where is it found?

What is cordierite?

COMPOSITION Mg-Al(Fe) silicate
HARDNESS 7-7.5
CRYSTAL SYSTEM Orthorhombic
PLEOCHROISM Strong
FRACTURE Conchoidal
COLOR Blue (resembles quartz and amethyst)
SPECIFIC GRAVITY 2.58-2.66
INCLUSIONS Many!

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

See a movie illustrating the cordierite structure!

Where is it found?

Cordierite is found in contact and regionally metamorphosed rocks in localities like Burma, Brazil, Sri Lanka, India etc.

Feldspar

What are feldspars?
Where are they formed?
Characteristics that make feldspars interesting as gems
Feldspars are the most abundant minerals in the Earth"s crust. They are silicate minerals and are referred to as framework silicates because the tetrahedra (of Si and Al four coordinated by oxygen) are linked together in three dimensions to make a framework. Note the tetrahedra are connected through sharing of all oxygen atoms in the coordination polyhedra (see movies below).

Plagioclase feldspars - sodium, calcium, aluminum silicates

Learn more about how to identify plagioclase


Alkali feldspars - sodium, potassium, aluminum silicates

Learn more about how to identify microcline
Learn more about how to identify orthoclase

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

Images of feldspar

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

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.

Other interesting characteristics

Alkali feldspars Plagioclase feldspars
  • commonly twinned, twin planes produce parallel striations on mineral's surface
  • gem varieties: Labradorite: - dark grey color, irridescent - play of color is referred to as 'labradorescence'.
  • some wavelengths are amplified, and some are cancelled
  • the net result is that different spacings and orientations produce rainbow-like effects
  • irridescence is mostly blue, but often with rainbow-like appearance
  • rainbow moonstones
  • Sunstone and Aventurine have inclusions of Cu in the feldspar


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