Learn more about how to identify olivine
 
 

**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:

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

What is cordierite?

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?

Feldspar

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

• Gem quality orthoclase: yellow stones colored by Fe impurities
• Microcline
• Amazonite is a light green-blue form of microcline

 
• Moonstone is normally set as a cabachon in rings, pins, and pendants ; it exhibits "adularescence" or "schiller"

• 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