Ring Silicates (Disilicates, Ring Silicates, Orthosilicates)

Lab 9! :)

Disilicates (sorosilicates)

Isolated double tetrahedra

-Si/Al:O ratio of 2:7

-Epidote group of minerals

Ring Silicates (Cyclosilicates)

-Tetrahedra linked into rings

-Si/Al ratio of 1:3

-3-member rings and 6-remember rings varieties

-Large gaps in crystal structure enable incompatible elements to fit into these crystals in some cases

Orthosilicates (Nesosilicates)

“island” silicates

Isolated tetrahedra connected only by interstitial cations

-Al₂SiO₅ group of minerals

-Garnet group of minerals

Epidote (clinozoisite)- monoclinic

Ca2Al2 (Al, Fe)[Si2 O7][SiO4]O(OH)

Disilicate (Sorosilicate)

Hand Sample 

• Pistachio-green color typically but also yellowish or black

• H ~6 to 7 

• Massive to grainy aggregate  typical 

• 1 plane perfect cleavage {001}

  • Fine grain nature may make this difficult to observe

  • Minor uneven fracture & and brittle tenacity

PPL 

• Strong pale green/yellow pleochroism, grainy crystal habit, 1-D @ 90 cleavage sometimes visible, & low relief

XPL 

• 2^nd to 3^rd to order birefringence & variable extinction

Vesuvianite-Tetragonal

Ca₁₉(Al,Mg)₁₃B_0-5Si₁₈O₆₈(OH,O,F)₁₀

Disilicate (Sorosilicates)

Hand Sample 

• Yellow to greenish brown color typical

• Stubby tetragonal prisms 

• H ~6 to 7

• Most commonly conchoidal fracture

  • Cleavage on {110} but very poor

Beryl-Hexagonal

Al₂Be₃Si₆O₁₈

Ring Silicates (Cyclosilicates)

 Hand Sample 

• Can be any colors but typically some shade of green, blue, yellow, or white

  • Aquamarine is blue 

  • Emerald is green

• Hexagonal prisms (harder than Apatite)

• H ~7.5 to 8

• 1-D basal {001} cleavage

  • Minor conchoidal to uneven fracture & brittle tenacity

Tourmaline-Trigonal

Ring silicate (Cycosilicate)

WX₃Y₆ [Si₆O₁₈](BO₃)₃ (O,OH,F)₄

• Class of minerals

  • Schorl

  • Dravite

  • Elbaite

Hand Sample

• Most commonly black color but can be any color of the rainbow based on impurities

• H ~ 7 

• Rounded trigonal prisms w/ striated faces

• Very poor cleavage & some conchoidal/uneven fracture

• Often found in granitic pegmatites

Garnet- Isometric

Solid solution series mineral

Orthosilicates (nesosilicates)

X₃Y₂(SiO₄)₃

• X can be Ca, Mg, Fe, Mn, or other divalent cations

• Y can be Al, Fe, Cr, or other trivalent cations

• Pyralspite Group

  • Almandine

  • Pyrope

  • Spessartine

• Grandite Group

  • Andradite

  • Grossular

  • Hydrogrossular

Hand Sample 

• H ~6.5 to 7, typically dark red in color but can be pretty much any color, dodecahedral crystal habit, minor conchoidal fracture

PPL 

Round habit, high relief, no cleavage, high relief fractured, no pleochroism, often inclusion-rich

XPL 

Isotropic (extinct in 360 )

Kyanite (aluminosilicate)- Triclinic

Al₂SiO₅

Orthosilicates (nesosilicates)

Hand Sample 

• Blue color most common but it can be white or rarely red

• metamorphic rock associate

• splintery cleavage w/two planes parting @~ 90°

• elongate bladed crystals

• H ranging from 5 to 7 depending on orientation of crystal

PPL 

• high relief, tabular grains with strong cleavage,  kink banding common, found in metamorphic rocks, ~ 90° parting.

XPL 

• 1^st to low 2^nd order birefringence, often twinned, inclined extinction

  
  

Andalusite (aluminosilicate)- Orthorhombic

Al₂SiO₅

Orthosilicates (nesosilicates)

Hand Sample 

• Typically yellowish green to gray in color

• Good cleavage parallel to prism faces

• H ~6.5 to 7.5

• Euhedral grains 

• Often exhibits dark carbonaceous cross-shaped inclusions (chiastolite variety)

• Medium-grade metamorphic rocks association

Sillimanite (aluminosilicate)- Orthorhombic

Al₂SiO₅

Orthosilicates (nesosilicates)

Hand Sample 

• Typically white to off-white to tan in color but can be a variety of colors, 

• H between ~5 & 7 due to orientation

• Slender to fibrous crystals 

• Associated w/ high T metamorphic rocks

PPL 

• High relief & slender prismatic crystals typical

XPL 

• 1^st order birefringence (pale yellow) & parallel extinction

Staurolite- Monoclinic

Al₉Fe₂ [(Si,Al)O₄]₄O₆ (O,OH)₂

Orthosilicates (nesosilicates)

Hand Sample 

• Typically red-brown to black

• metamorphic rocks associated 

• H  ~7 to 7.5

• Cross shaped penetration twins

  
  

PPL 

• pleochroic brown-yellow, inclusions rich, tabular to wedge shaped habit if euhedral

XPL 

• 1^st order (yellow to bluish) birefringence

Olivine- Orthorhombic

(Mg,Fe)₂SiO₄

Orthosilicates (nesosilicates)

Hand Sample 

• Most often green in color but can be brownish

• Grainy aggregate masses 

• Mantle xenoliths & phenocrysts in basalt association

• H of 6.5-7

• Conchoidal fracture  

  • very poor cleavage 

PPL 

• High relief, rounded habit, no pleochroism, no cleavage

XPL 

• 2^nd to 3^rd order birefringence, often found in aggregate masses most easily visible in CPL

Zircon-Tetragonal

ZrSiO₄

Orthosilicates (nesosilicates)

Hand Sample

• Typically far too small to see – 100 micron-sized

• Typically yellow to reddish brown 

• H ~7.5

• Very poor cleavage

  • Conchoidal fracture and brittle tenacity

• Vitreous to adamantine luster

• Hf isotopes make zircon highly useful in age dating

  
  

Topaz- Orthorhombic

Al₂SiO₄(F,OH)₂

Orthosilicates (nesosilicates)

• Associated with rhyolites 

• Clear to yellow in color

• H ~ 8

• 1-D Basal {001} cleavage (1 plane)

  • Minor subconchoidal to uneven fracture & brittle tenacity

• Cleavage & crystal system makes it distinguishable from quartz