Contain the Si2O7 structural unit! (bow tie)
Common retrograde alteration product of pyroxene, amphibole, feldspar.
Epidote contains SiO4 and Si2O7 groups. Structure is basically:
In epidote: Al, Fe in octahedral sites; in clinozoisite, these sites contain only Al.
Clinozoisite: Ca2Al3O(SiO4)(Si2O7) OH
Orthorhombic zoisite (commonly simply called Zoisite) derived from clinozoisite by twinning, which doubles the unit cell. (See Fig. 13.31 p. 464) Epidote with Ca (Fe, Al)... up to 1/3 of the Al (all of the octahedral site not in the chain) = pistacite.
Allanite: contains Ce and other lower atomic number lanthanides (LREE). Also radioactive elements substitute for Ca and Fe in the Ca,Al site.
Allanite is commonly metamict!
(Also formerly known as Idocrase)
Ca10 (Mg,Fe)2 Al4 (SiO4)5 (Si2O7)2 (OH)4
generally tetragonal
metamorphosed limestones, etc.; often brownish prismatic xals.
Resembles garnet and the structure is closely related to that of grossular garnet.
Zn4 (Si2O7)(OH)2 •H2O
Found in oxidized zinc deposits.
Al-bearing octahedra linked by Si2O7 groups with Ca and water in between.
Found especially in metamorphosed sediments which have experienced low T but high P metamorphism - blue schists.
Rings of different sizes (3, 4, 6) all of which have Si:O ratios of 1:3.
Simplest ring: 3-membered Si3O9 - found in rare mineral benitoite (Ba Ti Si3 O9)
4-membered Si4O12 - no remotely common examples
6-membered Si6O18 - found in the more common ring silicates:
Beryl Movie (405K)
Hexagonal 6-membered rings stacked parallel to the c-axis (see model) - note channels parallel to c-axis: these 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!).
form prismatic crystals with hexagonal cross section.
Composition : Be3 Al2 Si6 O18 or perhaps we should think of it as: Al2 Be3 Si6 O18 based on the octahedral/tetrahedral distribution.
PEGMATITES and metamorphic rocks with high Be concentrations.
Historically a major source of Be which is an important light weight metal and used in alloys with copper.
Tourmaline Movie (374K)
A complex borosilicate with 3m symmetry (no center of symmetry).
Tourmaline is strongly piezoelectric (Why?) and used in pressure transducers.
Note structure contains similar 6-membered rings. Composition complex! (see below)
BO3 triangles with 3-fold symmetry (not 6-fold!)
Li, Al, Mg also in 3-fold pattern.
Many names:
Note 3 BO3 + Si6 O18 = B3 Si6 O27.
e.g., Mg-rich may be brown; Fe,Mn-rich may be black; Li-rich may be colorless; + Mn -> pink; Fe+2-> pale green; Fe+3 -> yellow green; Fe and Mn-> blue.
Structure can be considered as derived from beryl. Orthorhombic or pseudohexagonal. Short, prismatic xals
(Si,Al)6 O18 - 1/3 of the 6-membered rings contain Al, with Al replacing Be. Mg and Fe are in sites that would contain Al in beryl.
At high pressure, the polymorph is ISOSTRUCTURAL with beryl. At lower temperature, Al orders -> lower symmetry (e.g., 2 of 6 tetrahedra are occupied by Al, this destroys the 6-fold) symmetry.
(Mg,Fe)2Al4 Si5 O18 •n H2O - most are Mg-rich, Mn-may replace Mg.
Water, CO2, as well as Na, K, etc may be in channels.