Sorosilicates, Colors in Minerals (cont), and Deep Earth Minerals ESS212 January 20, 2006
Double tetrahedron Sorosilicate is defined by the Si 2 O 7 group. Three groups of minerals, commonly, Epidote Zoisite (important gem known as tanzanite) Clinozoisite
Sorosilicate structure Key feature is the Si 2 O 7 group (cyan) Still contains SiO 4 (blue) Both above are connected by AlO 6 units (vertical). Horizontally, weakly bonded by metal cations (red and green balls). Perfect cleavage in this direction (001) Unusual, chains are parallel to b axis
Common Sorosilicates The Epidote group is composed of minerals of the general formula Ca 2 Al 3 Si 3 O 12 (OH). Zoisite is orthorhombic with Z = 4 whereas clinozoisite and epidote are isostructuiral monoclinic and have Z = 2. Epidote is the name give to the green variety that contains Fe 3+ in substitution for Al in one of three octhedral sites. (Allanite is a rare earth rich variety that a trivalent REE in one of the Ca sites and charge compensation is achieved by Fe 2+ for Al. Handcockite has Pb and/or Sr substituting in one of the Ca sites).
Epidote structure: Monoclinic, cleavage 001
Tanzanite, different colors in different directions: Pleochroism Zoisite, same formula as clinozoisite, ie, iron less epidote
Clinozoisite Monoclinic like epidote: Ca 2 (Al, Fe) 3 (SiO 4 ) 3 (OH) CZ has no iron.
The Earth and other planets are layered.
Seismic record, globally averaged Key features include: Major transition at 2900 km depth separates stone from metal Outer core solid, inner core fluid From 670 km to 2900 km, average density of 4 g/cc no transitions apparaent At 400 and 670 km, two 5% jumps in mean density
Forsterite consists of isolated tetrahedra plus two types of octahedra At 13 GPa (corresponds to 400 km depth), Fo transforms to a denser form: a Sorosilicate!
Diamond Anvil Cells
Laser Heating of Minerals
Laser Heated Sample
At 400 km depth: Upper Mantle (Forsterite) to β phase P= 15GPa, T~1800 K Phases detected by Raman spectroscopy
Raman can pinpoint identification
High pressure sorosilicate High pressure phase of forsterite (but not fayalite!)
Pressure sequence: Fo Wadsleyite Ringwoodite PV
Clapeyron Slope dp/dt = S/ V Three sets of multi anvil determinations of Forsterite to β phase transition Uncertainty in slope = 50% Uncertainty in depth = +/ 20 km
Wadsleyite a sorosilicate: It is 5% denser than forsterite, has more edge shared MgO 6 octahedra, and fills more of the voids in the oxygen lattice. Also orthorhombic
Ringwoodite, spinel structure Has isolated tetrahedral silicon and octahedral magnesium. About 2 % denser than wadsleyite. Mg octahedra have more edge sharing Found in impacted meteorites and craters like this natural sample:
The β Form Does NOT Appear on the Iron Rich Side of Olivines
MgSiO3 Perovskite Consists of octahedra with Si and dodecahedra with Mg. Mean density is 4 g/cc up from 3.3 g/cc for forsterite Size mismatch between sites and cations causes the perfect cubic structure to be distorted to orthorhombic. A very large number of compounds crystallize in these symmetries (orthorhombic, tetragonal, cubic) due to the size difference between cations and sites
Move up in pressure Coordination number increases, Si from 4 to 6, M 2+ from 6 to 8 Edge sharing of polyhedra increases More of the interstices are filled Bond strengths become more uniform Minerals become more incompressible Thermal expansivity decreases Polymorphs increase in density
Colors in Minerals 2 Recap: transition metals due to their d electrons as major elements such as Fe in fayalite, almandine, andradite or Mn in rhodonite or Cr in uvarovite Or as dopants (natural impurities ): Cr 3+ emerald green, ruby red, green diopside V 3+ green in grossular garnet, purple in tanzanite (a zoisite, sorosilicate) Mn 3+ causes red and green colors, Mn 2+ pink Co 2+ pink to red Ni 2+ pale green Cu 2+ blue, in elbaite, azurite, malachite, aurichalcite Fe 2+ green to brown Fe 3+ yellow
Fe 2+ to Fe 3+ commonly found in minerals, absorbs red, thus minerals look blue or green (aquamarine) Ti 3+ to Ti 4+ is a source of deep blue in meteoritic rocks Fe 2+ to Ti 4+ produces brown in tourmalines and pyroxenes Kyanite and sapphire get their blue from both the first and third charge transfers Sometimes you find Mn 2+ to Ti 4+ causing a yellow color (tourmaline) Charge transfer
Irradiation Amethyst Sunlight will Fluorite bleach these Tourmaline Blue Calcite Colored diamonds Topaz, blue or brown
Physical Effects Interference from thin film and beads as in opal hematite