6.5 Evaporites and Other Non-Clastics

Evaporites

These are minerals formed by precipitation out of solution as ions become more concentrated when water evaporates.

The least soluble compounds are precipitated first, so calcium carbonate is first precipitated out of seawater, followed by calcium sulphate and sodium chloride as the waters become more concentrated. Potassium and magnesium chlorides will only precipitate once seawater has become very concentrated.

Discuss the order of compounds that precipitate out of seawater

Gypsum and Anhydrite

The most commonly encountered evaporite minerals in sedimentary rocks

Halite

It may occur as thick crystalline beds or as individual crystals that have a distinctive cubic symmetry, sometimes with a stepped crystal face

Sylvite

An important source of industrial potash that occurs associated with halite and is interpreted as the product of extreme evaporation of marine waters.

Epsomite

Evaporite that is used for production of Epsom salt

Chert

fine-grained silica-rich sedimentary rocks made up of silt-sized interlocking quartz crystals and chalcedony

Chalcedony

a form of silica made up of radiating fibers a few tens to hundreds of microns long

Siliceous ooze

accumulation of siliceous skeletons of microscopic organisms on the floors of seas and lakes

Diatoms

main component of siliceous ooze in lacustrine environment

Radiolaria

the more common component of siliceous ooze in marine conditions

Flint

Refers to chert that formed in the Cretaceous

Jasper

Chert with hematite

Phosphorites

Rocks with concentrations of phosphate (5% to 35% P2O5)

Francolite

calcium phosphate (carbonate hydroxyl fluorapatite)

Coprolites

fossilized feces of fish or animals

Ironstone

sedimentary rocks that contain at least 15% iron in the form of oxides, hydroxides, carbonate, sulfides, or silicates

Banded Iron Formation

an example of a type of sedimentary rock for which there is no equivalent forming today

Algoma-type BIF and Lake Superior-type BIF

The 2 episodes of banded iron formation (BIF)

Algoma-type BIF

Which formed first between the 2 episodes of BIF

Algoma-type is associated with volcanic arcs while Lake Superior-type is formed on stable continental shelves

What is the origin of each of the 2 episodes

The Algoma-type is composed of alternating layers of magnetite and chert, while the Lake Superior-type is composed of alternating layers of hematite and chert

What is the composition of the 2 types of BIF

Magnetite

a black mineral which occurs as an accessory mineral in igneous rocks and as detrital grains in sediments

Hematite

the most common oxide, bright red to black in color, occurring as weathering or alteration product in a wide variety of sediments and sedimentary rocks

Goethite

widespread in sediments as yellow-brown mineral, which may be a primary deposit in sediments, or is a weathering product of other iron-rich minerals representing less oxidizing conditions than hematite

Limonite

soil made of hydrated iron oxide

Pyrite

common iron sulfide mineral that is found in igneous and metamorphic rocks as brassy cubic crystals but can also be found in any affinity

Ferromanganese deposits

black to dark brown in color and range from a few mm to many cm across as nodules or as extensive laminated crusts on hard substrates

manganese nodules

form at any depth, very slowly and are only found concentrated in deep oceans where the rate of deposition of any other sediment is even slower

A deposit is considered to be carbonaceous if it contains a proportion of organic material that is significantly higher than average (>2% for mudrock, >0.02% for limestone, >0.05% for sandstone).

When to consider a deposit if it is carbonaceous or not

Peats

forming at the present day in a wide range of climatic zones from subarctic boggy regions to mangrove swamps in the tropics and contain a range of plant types from mosses in cool upland areas to trees in lowland fens and swamps

Sapropel

remains of planktonic algae, spores and very fine detritus from larger plants that accumulates underwater in anaerobic conditions

Sapropelic coal

formed from sapropel

Humic coal

formed from in situ accumulation of woody plant material

If over two-thirds of a rock is solid organic matter it may be called a coal.

When to call a rock coal

Vitrain

bright, shiny black coal that usually breaks cubically and mostly consists of woody tissue.

Durain

black or grey in color, dull and rough coal that usually contains a lot of spore and detrital plant material.

Fusain

black, fibrous with a silky luster, friable and soft coal that represents fossil charcoal

Clarain

banded, layered coal that consists of alternations of the other three types

They occur in wetland were water was stagnant and acidic

Where do coal originate

intermediate speed

Level of energy for deposition of coal

Oil shales

mudrocks that contain a high proportion of organic material that can be driven off as a liquid or gas by heating

Tar sands

clastic sediments that are saturated with hydrocarbons and they are the exposed equivalents of subsurface oil reservoirs