A three-amphibole assemblage from the Tallan Lake Sill, Peterborough County, Ontario
A three-amphibole assemblage from the Tallan Lake Sill, Peterborough County, Ontario
The Canadian Mineralogist (August 1980) 18, Part 3: 275-284
- amphibole group
- amphibolites
- anthophyllite
- Canada
- cell dimensions
- chain silicates
- chemical composition
- clinoamphibole
- composition
- cummingtonite
- Eastern Canada
- electron probe data
- gedrite
- intergrowths
- intrusions
- ions
- metamorphic rocks
- mineral assemblages
- mineralogy
- minerals
- Ontario
- orthoamphibole
- Peterborough County Ontario
- phase equilibria
- silicates
- sills
- Tallan Lake Sill
- tschermakitic hornblende
Textural relations indicate that gedrite, cummintonite, and hornblende form an equilibrium assemblage. The amphiboles are easily distinguished optically (cummingtonite colourless; gedrite pleochroic with alpha tan, beta pale grey, gamma medium grey; hornblende pleochroic with alpha tan, beta olive green, gamma bluish green). Electron microprobe and unit cell data for the minerals are:123SiO (sub 2) 172TiO (sub 2) 216Al (sub 2) O (sub 3) 269Fe as 318MnO 366MgO 413CaO 452Na (sub 2) O273FeO 000Cummingtonite 12253.88 1740.14 2261.77 27020.74 3220.46 36619.81 4180.80 4620.09000Gedrite 12243.46 1740.58 21814.18 27020.35 3220.49 36615.17 4180.78 4622.02000Hornblende 12243.89 1740.79 21814.42 27014.87 3220.29 36612.37 4189.14 4622.10 117K (sub 2) O 162total 224a(A) 300b(A) 372c(A) 459beta (o)000Cummingtonite 1180.01 15997.72 2169.51 28418.12 3605.32 427101.9000Gedrite 1180.05 15997.07 20818.696(5) 28417.913(4) 3605.278(2) 43590000Hornblende 1180.12 15997.99 2169.832(3) 28418.037(5) 3605.302(1) 427105.01(2) Space groups are C2/m (cummingtonite and hornblende), and Pnma (gedrite). Crystal structure refinement of the hornblende (R for 1098 observed reflections 4.8%) indicates an Fe (super 3+) /(Fe (super 2+) + Fe (super 3+) ) value of (super approx ) 0.15, and considerable Fe (super 2+) at the M4 site; this calcic amphibole is therefore a tschermakitic hornblende, with a substantial amount of the cummingtonite component in solid solution.