2 Clastic vs. Non-clastic Sedimentary rocks in two major groupsClastic (detrital)Composed of fragments of silicate minerals (mostly quartz and clay)Name largely determined by the grain size (see handout)Non-clasticComposed of various minerals (usually just one)Name largely determined by the composition
3 Clastic (detrital) Coarse grained (> 2 mm) Conglomerate (rounded pieces)Breccia (angular pieces)Medium grained (<2 mm but still visible)Quartz sandstone (mostly quartz)Arkose (abundant feldspar – often pink)Greywacke (much clay – often dark in colour)Fine grained rocks (cannot see ind. grains)All “mudrocks”Siltstone (gritty), shale (splits easily), claystone (smooth, slippery feel)
4 Non-clastic All one mineral (usually) Often “crystalline” (visibly so or VERY smooth (with conchoidal fracture)Named according to mineral present (use hardness and reactivity to HCl)H > glass, no HCl reaction: mineral is chalcedony, rock is chertH ~ penny, no HCl reaction: mineral is dolomite, rock is limestone (dolomitic)H ~ penny, HCl reaction: mineral is calcite, rock is limestonePenny > H > fingernail, salty taste, no HCl reaction: mineral is halite, rock is Rock saltH < fingernail, no HCl reaction: mineral is gypsum, rock is Rock gypsum
5 Doing the lab – CaCO3? Test with HCl (acid). Fizz means calcite present.If it’s all calcite – rock is non-clasticBe careful to note if the rock is dominantly calcite or just partly (i.e., is the rock all calcite, a limestone, or just random parts, e.g., a calcite cemented sandstone)
6 Doing the lab – C or NCC: is the rock clastic; i.e., composed of discrete particles (grains) of silicate material (quartz, feldspar, clays, etc.)?NC: is the rock non-clastic; i.e., all one mineral, probably a non-silicate, and probably crystalline?
7 Doing the lab – Grain size Only applicable to clastic rocks (i.e. with non-clastic rocks say “not applicable” or “n.a.”)Coarse grained (c.g.): > 2 mmMedium grained (m.g.): vis. grains to 2 mmFine grained (f.g.): gritty on teethVery fine grained (v.f.g.): slippery to feel (probably with an earthy smell)HINT: The higher the clay content of clastic rock the darker the colour.
8 Doing the lab – Components What makes up the rockGrains? (Sand, silt, pebbles, …)Of what?A single mineral?Multiple components?…
9 SortingOnly applicable to clastic rocks (for non-clastic rocks say “n.a.”)Fine-grained (and v.f.g.) rocks are too fine to see the sorting. Say “too fine to see” or “n.d.”Two kinds of sortingCompositionalWell = composed of only one componentPoor = mix of several different componentsSizeWell = all grains are same sizePoor = grains are all sizesCompositional and Size sorting usually go hand-in-hand