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 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 %