Silicon Tetrahedra by Charina Cameron

• 4 oxygen atoms

• 1 silicon atom in the center

• Forms a 4-sided geometry

• Each side is like an equilateral triangle

• Creates a 4- ion

Some Silicate structures• Single tetrahedrons, single chains, double chains,

sheets, rings, and 3-D networks

SINGLE SILICATE TETRAHEDRA

• Garnet (red)

• Olivine (green)(Fe,Mg)SiO4

crystal

SINGLE CHAIN SILICATES

• Pyroxene Group [Fe,Mg,Ca,Al silicates]:

– Augite (note the near 90o angle of the two cleavages)

DOUBLE CHAIN SILICATES

• Amphibole Group [Na,Ca,Fe,Mg,Al silicates w/ H2O]:

– The mineral hornblende is common– Nnote the 156 degree angle of the two cleavages

SHEET SILICATES

• Muscovite mica [K,Al + H2O] Biotite mica [K,Mg,Fe,Al + H2O]

– Note the single perfect cleavage and elastic sheets

• Talc Kaolinite (clay)

[K,Al + H2O]

ZEOLITE RING SILICATES

• Naturalite from North Mountain Basalt Formation

FRAMEWORK SILICATES• Feldspar group:

– Most abundant mineral group– Orthoclase (K-feldspar)

– Albite (Na-Plagioclase)

– Labradorite (Ca-rich Plagioclase)

2 cleavages

note twinning lines

FRAMEWORK SILICATES• Quartz (SiO2), the most common mineral:

• Chalcedony (SiO2):– microcrystalline quartz– many var. (chert, jasper, flint)

amethyst

quartz crystal

milky quartzrose quartz

FRAMEWORK SILICATES

• Hexagonal quartz crystal

FRAMEWORK SILICATES

• Amethyst (quartz) geode from the Scots Bay Formation

FRAMEWORK SILICATES

• Amethyst (quartz) geode replacing small straight tree trunk surrounded by chert (from Scots Bay Fm)