Rock type: Phyllite slate.
Age: Ordovician (approx. 460 million years).
Otta slate represents what was originally a clay/siltstone formation which during the Caledonian period was deformed and metamorphosed with the foundation of regular cleavage strata.
Good slate deposits occur in tight fold structures. Sheet thickness varies between 8 and 80mm.
In the lower part of a slate formation the phyllite contains large crystals of needle-like hornblend (Pillarguri and H?gs?ter).
Pillarguri is exclusively quarried to the south of Ottadal, whilst H?gs?ter is quarried on the north side. In certain areas the slate can be found with a rusty surface.
This rust colour occurs in cleavage planes which have been exposed to erosion from water and to oxygen.
Petrografi c analysis
Phyllit
Density NS-EN 1936 2810 kg/m3
Water absorption NS-EN 13755 0,2 % by weight
Flexural strength NS-EN 12372 44,5 MP
Abrasion resistance NS-EN 14157(A) 25,0 mm
Slip resistance NS-EN 14231 64 SRV, dry
18 SRV, wet
Comment
Frost resistant
Mid-value after 48 frost/defrost cycles natural riven material
Tested on honed C600. Method A: Wide Wheel abrasion test Capon.
Honed C220
Honed C220
Mineralogy
Quartz 25 - 35 %
Mica 40 - 55 %
Chlorite10 - 15 %
Hornblende 0 - 5 %
Garnet 1 - 3 %
Carbonate 0 - 3 %
Iron oxides 1 - 7 %