PYROXENES (and AMPHIBOLES)

• silicate minerals
• simplest formula: MgSiO3.
• silica tetrahedra form single chains
• not resistant to physical and chemical processes at the Earth's surface
  • break down to clay !!
  • (found in alluvial deposits ???)
  • even more facinating information on pyroxenes and amphiboles
• Hardness 6.5 - 7.0
• S.G.: 3.18

  See a movie on a pyroxene I-beam structure!
  

• THEIR CLEVAGES ARE AT 90 DEGREES!!!

(2) Where are they found?

• Pyroxenes are found in many rocks, often those crystallized from a melt (sometimes in metamorphic rocks)
• it may constitute ~ 100 % of some rocks
• but it's not as common in rocks in the Earth's crust as feldspars

(3) Varieties

1. Spodumene or Kunzite
   • a Lithium (Li)-bearing pyroxene
   • (Li is the lightest metal)
   • color is light pink / purple
   • some crystals are very large: Etta Mine in South Dakota mined a crystal that was 47' long and 90 tons! But gems are usually smaller.
   • usually found in pegmatites
   • irradiation causes them to go to yellow - green (unstable color)

2. Diopside (Calcic magnesian monoclinc pyroxene)
   • it's a common mineral, less commonly used in jewelry
   • asterism in diopside
   • a close-up of the "star"
   See a movie on a diopside structure!
   

   Augite (Calcic clinopyroxene)

   • is another common pyroxene, whose composition is similar to diopside.

3. Hiddenite:
   • Chromium (Cr) - rich pyroxene
   • strongly pleochroic !!
   • good cleavages makes gems difficult to cut

4. Jade: also known as Jadeite.
   • often used for carvings
   • sometimes jade is not actually jadeite but a mineral known as nephrite, which is an amphibole
   • sodium (Na) - Aluminum (Al) silicate
   • H= 6.5-7.0
   • S.G.: 3.4
   • monoclinic
   • almost never found as large crystals !
     • small, interlocking crystals
     • cleavage are at 90 degrees and are not seen in massive form
   • the color is due to the presence of Cr+++ - deep green color
     • Fe produces a paler green color
   • formed by high pressure metamorphism of Na-rich rocks
   • famous localities include Nth Burma, Guatemala, Japan

• nephrite is an AMPHIBOLE, not a pyroxene
  • amphiboles are double chain silicates
• unlike pyroxenes, amphiboles are HYDROUS minerals (contain hydrogen)
• Used for carvings in places like China (for more than 2000 yrs) and Central America (over 7000 yrs)
• CLEAVAGES ARE AT 60 degrees (not apparent in massive varieties)
• Composition: calcium (Ca) magnesium (Mg) silicate
• Green color is due to Fe
  • greatly resembles jadeite in color and structure
  • composed of many tiny interlocking crystals
• Hardness 6-6.5
  • harder than steel, thus used in neolithic tools
• S.G.: 2.95 ***(Jadeite = ?)

• "New Jade" = serpentine H = 5.5-6.0, S.G. = 2.5
• "Styrian Jade" = chlorite
  Both are sheet silicates

VIEW A COLLECTION OF PYROXENE IMAGES

TOURMALINE

TOPICS

1. What is tourmaline ?
2. Where is it found?
3. Other neat facts

(1) What is tourmaline?

See a movie on a tourmaline structure!

• Tourmaline is a complex Boron (B) bearing mineral
• Note that the ends of tourmaline crystals have different shapes.
• Tourmaline has some interesting properties: it's pyroelectric and piezoelectric:
  • pyroelectric: crystal + heat - develops charge:
  • Causes crystals to collect dust in display cases
  • Piezoelectric
  • apply pressure to opposite ends of the crystal - develops charge at opposite ends
• Structure:
  • (Na,Ca)(Li,Mg,Al)3(Al,Fe,Mn)6(BO3)3(Si6O18)(OH)4
• NOTE: THIS GEMSTONE DISPLAYS A TREMENDOUS RANGE OF COLOR !!!
  • often many colors in a single crystal
  • From blue to aqua, aqua to green, green to red, and red to colorless

(2) Where is it found?

• normally found in pegmatites and igneous and metamorphic rocks

• Elba, Brazil, Ural Mountains, many US localities (inc. Wisconsin)

(3) Other neat facts:

• strongly pleochroic; strongest color seen along prism length
• tourmalines are often color zoned along length of prisms
• you can also find cats eye tourmaline
• famous color zoning is known as "Watermelon tourmaline"
  • as a faceted gem
  • picture of an "inside-out" Watermelon tourmaline
  • Varieties:
    1. Elbaite: pink - green
    2. Schorl: black
    3. Dravite: brown
    4. Verdelite : green
    5. Rubellite: pink - red
    6. Achroite: white
    7. Indicolite: blue
• SYNTHESIS: limited and generally unsatisfactory
• TREATMENTS: turn dark shades to a paler color using heat
  • gentle heat only otherwise crystals lose water and the structure is destroyed
  • heat makes crystals more brittle and facet junctions abrade
• IRRADIATION: to produce popular pink and red, sometimes yellow or orange
  • effect is probably due to change in oxidation state of Fe or Mn.
• CHATOYANT TOURMALINE:
  • the tubes are parallel to length and sometimes filled with oil or epoxy

VIEW A COLLECTION OF TOURMALINE IMAGES

GARNET:

TOPICS

1. What are garnets?
2. What are their compositions and characteristics?
3. Where are they formed?

(1) What are garnets?

• Garnets are silicate minerals with diverse compositions. All garnets have almost identical atomic structures.

• generalized chemical formula of garnets is X3Y2 (SiO4)3,
  • X indicating a divalent cation, such as iron(Fe2+), magnesium (Mg2+), calcium (Ca2+) or manganese (Mn2+)
  • Y is a trivalent cation, such as aluminum (Al3+), iron (Fe3+), or chromium (Cr3+).
  • The SiO4 indicates silica tetrahedrons--a silicon ion surrounded by four oxygen ions.

  The other atoms are packed between the tetrahedrons.

  The structure is illustrated in this movie

  Garnets belong to the isometric crystal and commonly grown in a distinctive well developed crystal form is known as a dodecahedra (triangular-shaped faces)

• hardness: 6.5 - 7.5
• S.G. 3.58 - 4.32

  Because the atoms are tightly packed, garnets are relatively hard and dense.

• Graph of R.I. vs. S.G. for Garnets

(2)Composition and names of varieties