Review: silicate depolymerization - recall 2 of 4 bridging O = pyroxenes and pyroxenoids

ALSO ring silicates

Difference ? Effectively infinite versus limited chains.

CYCLOSILICATES HAVE STRUCTURES THAT FEATURE RINGS:

Rings of different sizes (3, 4, 6).

Simplest ring: 3-membered Si3 O9 - found in rare mineral benitoite (a Ba-Ti silicate)

4-membered Si4 O12 - found in axinite (also contains BO3 trianges)

6-membered Si6 O18 - found in the more common ring silicates:

• Beryl
• Tourmaline
• Cordierite

Beryl

Beryl (405K) HEXAGONAL

6-membered rings stacked parallel to the c-axis (see model) * note channels parallel to c-axis: may accommodate water, hydroxyl, Rb, Cs, Na, K, etc. Charge balance is achieved by manipulating Be and Al ratios.

common impurities are Fe2+, Fe3+ and Cr, Mn, etc (note all beryl is not green!).

Prismatic crystals with hexagonal cross section.

Composition : Be3 Al2 Si6 O18

PEGMATITES! and metamorphic rocks with high Be concentrations.

Major source of Be.

Tourmaline

A complex borosilicate!

Note structure contains similar 6-membered rings. Composition complex!! (see below!)

BO3 trianges with 3-fold symmetry (not 6-fold!)

Tourmaline (374K)

Li, Al, Mg also in 3-fold pattern.

Many names:

• Dravite: Na Mg3 Al6 B3 Si6 O27 (OH,F)4

  Note 3 BO3 + Si 6 O18 = B3 Si6 O27.

• Schorl: Na (Fe,Mn)3 Al6 B3 Si6 O27 (OH,F)4
• Elbaite: Na (Li,Al)3 Al6B3 Si6 O27 (OH,F)4

Variable color!

e.g., Mg-rich may be brown, Fe,Mn-rich may be black, Li-rich may be colorless; + Mn -> pink!. Fe2+-> pale green, Fe3+ -> yellow green; Fe and Mn-> blue.

Cordierite

Structure can be considered as derived from beryl. Orthorhombic or pseudohexagonal. Short, prismatic xals

cordierite (279K)

At high pressure, the polymorph is ISOSTRUCTURAL with beryl. Lower T, Al orders -> lower symmetry (e.g., 2 of 6 are Al, destroys the 6-fold!0

(Mg,Fe)2Al4 Si5 O18 n H2O - most are Mg-rich!, Mn-may replace Mg.

water, as well as Na, K, etc may be in channels.

SOROSILICATES

Contain the Si2 O7 structural unit! (bow tie)

Movie (376K)

• many examples, most are rare!

• Epidote group
• Vesuvianite
• Hemimorphite
• Lawsonite
• one structure of olivine (later)

Epidote group

• Clinozoisite,
• Zoisite
• Epidote
• Allanite.

Common retrograde alteration products! (pyroxene, amphibole, feldspar)

Epidote contains SiO4 and Si2 O7 groups. Structure is basically:

• Chains of edge sharing octahedra cross-linked by single and bow tie silicate units.
• Monoclinic - octahedra run parallel to b.
• Ca sits in 8-coordinated sites.

In epidote: Al, Fe in octahedral sites, in clinozoiste, these sites contain only Al.

Orthorhombic zoisite derived from clinozoisite by twinning, which doubles the unit cell.

Clinozoisite: Ca2 Al3 O (SiO4)(Si2O7) OH

epidote with Ca (Fe, Al)...

Allanite: contains Ce and other other lower atomic number lanthanides (LREE). Also radioactive elements sub. for Ca and Fe in the Ca,Al site.

Metamict!

Vesuvianite

(idocrase)

Ca10 (Mg,Fe)2 Al4 (SiO4)5 (Si2O7)2 (OH)4

generally tetragonal

metamorphosed limestones, etc. often brownish prismatic xals. Resembles Garnet.

Hemimorphite

Zn4 (Si2O7)(OH)2 H2O

oxidized zinc deposits

Lawsonite

Al-bearing octahedra linked by Si2O7 gropus with Ca and water in between.

esp. metamorphosed sediments experienced low T but high P!