Pyroxenes (and amphiboles), Tourmaline, & Garnet

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Pyroxenes (and Amphiboles) What are they?
Where are they found?
Varieties, etc.

What are pyroxenes?

  • 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
  • Their cleavages are at 90 degrees!

Pyroxene images

See a movie on a pyroxene I-beam structure!

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


    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)
Diopside (Calcic magnesian monoclinc pyroxene)

See a movie on a diopside structure!
Augite (Calcic clinopyroxene)
    • is another common pyroxene, whose composition is similar to diopside.
Jade (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 !
    • the color is due to the presence of Cr+++ - deep green color
    • formed by high pressure metamorphism of Na-rich rocks
    • famous localities include North Burma, Guatemala, Japan
    • nephrite is an amphibole not a pyroxene 
    • 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
    • Hardness 6-6.5
      • harder than steel, thus used in neolithic tools
    • S.G.: 2.95
    • "New Jade" = serpentine H = 5.5-6.0, S.G. = 2.5
    • "Styrian Jade" = chlorite

What is tourmaline?
Where is it found?
Other neat facts

What is tourmaline?

FRACTURE Conchoidal
CRYSTAL GROWTH Prismatic with triangular cross sections
  • 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 rangeof color!

Tourmaline Images

See a movie on a tourmaline structure!

Where is it found?

  • normally found in pegmatites and igneous and metamorphic rocks
  • Elba, Brazil, Ural Mountains, many US localities (inc. Wisconsin)

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"
  • 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

What are garnets?
What are their compositions and characteristics?
Where are they found?

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.
  • 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

Garnet Images

The structure is illustrated in this movie

Composition and names of varieties

All physical properties vary with composition.
There are two broad groups: Ca-garnets and Al-garnets.
  • Ca + a bunch of stuff (Ugrandite)
  • Al + a bunch of stuff (Pyralspite)
  • Most have compositions that involve complex mixtures of cations
    • Because of the compositonal variability, garnets may have almost any color!
    • Demantoid (= Cr-andradite): rare, vivid green garnet color (quite valuable)
      • colored by Cr / V (thus, Ca<Al,V) : it has more fire than diamond! (but it's much softer)
      • sometimes chatoyant
      • may contain "horsetail" inclusions (mostly from Urals)
    • Tsavorite: a green grossular
    • Spessartine and hessonite: orange colored garnets (Fe, Mn,Al)
      • hessonite is a variety of Grossular: green-yellow-brown
    • Pyrope: brown-red ("cape ruby")
      • Czechoslovakian source for much garnet used in 18th and 19th centuries
    • Almandine: violet-red
    • Uvarovite: emerald-green garnet
    • Rhodolite: purple
    • Malaia and color change garnets (V and Cr impurities)
    • purple, pink, and orange! garnets

    Where are they found?

    • found in a wide variety of rocks - igneous, metamorphic, and sedimentary

      •  Iron-rich almandine, the most common garnet, is widespread in metamorphic rocks such as schists and gneisses and in granitic igneous rocks.
      • The magnesium garnet, favored by high pressures of formation, is found in magnesium rich metamorphic rocks formed at great depth and may be an important mineral in the mantle of the Earth.
      • Spessartine is found in manganese-rich gneisses and in coarse grained, igneous rocks (pegmatites).
      • The ugrandites are rarer than the pyralspites.
      • Grossular, containing calcium and aluminum, is found in clay rich limestones that have been metamorphosed to marble and in contact metamorphic deposits, (skarns), formed when an igneous rock intrudes and reacts with limestone.
      • The calcium-iron garnet andradite and the rare calcium-chromium garnet uvarovite are also usually found in skarns.
    • also formed by regional metamorphism, especially of sedimentary rocks.

    Other information:

  • Garnets have been prized as gems for over 5000 yrs.

  • Previous Lecture:  Olivine, Cordierite and Feldspar

    Next Lecture:  Precious Stones: Lapis, Turquoise, Malachite and Azurite

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