Mineral Information and Data:

SiO2 [CNMNC approved formula]

Named for Loring Coes, Jr. (1915-1973), American chemist, Norton Company, who first synthesized the phase.

IMA status : Approved Species 1960 ()
(8th edition) :  
  (75) Tectosilicate Si Tetrahedral Frameworks
  (75.01) SiO2 with [4] coordinated Si
(10th edition) :
  (04) Oxides and Hydroxides
  (04.D) Metal:Oxygen = 1:2 and similar
  (04.DA) With small cations: Silica family
Crystal system: Monoclinic System
Point group (H-M): 2/m — prismatic
Space group: B2/b
Unit cell: a = 7.143(2) Å, b = 12.383(3) Å, c = 7.143(2) Å β = 120°
Crystal Habit: As irregular to nearly rectangular grains, up to 3 mm.
Color: Colorless
Diaphaneity: Transparent
Luster: Vitreous
Hardness (Mohs): 7.5 - 8
Measured Density: 2.92 (calculated) g/cm3
Cleavage: {010} and {012}, suspected
Fracture: Sub-conchoidal
Polymorphism and Series: A high-pressure modification of SiO2. Quartz, tridymite, cristobalite, and stishovite are polymorphs
Geologic Setting: Formed at pressures in excess of 20 kbar, as fine-grained intergrowths with quartz and silica glass in impact metamorphosed rocks and tektites. A primary phase in grospydite (kyanite-garnet-clinopyroxene) xenoliths in kimberlite, and intergrown with quartz as inclusions in garnet and clinopyroxene within eclogites of high-grade metamorphic terranes.
Mineral Association: Quartz, stishovite, silica glass, kyanite, omphacite, garnet, sanidine, muscovite, talc, rutile, dolomite


Colorless coesite matrix with deep to light purple
to lilac colored, hexagonal ellenbergerite crystals to 0.75 mm.
Origin: Dora Maira Massif, Parigi, Martiniana Po, Cuneo Province, Piedmont,Italy. This locality is a very high pressure metamorphism of Mg-rich pelites, at depths over 100km, have created a very unusual pyrope-coesite-quartzite, now exposed in the Western Alps.
Sample size: 3 x 2 x 2 cm
Photo: Tom Loomis


(zoom-in of above)