Rock A:  Calcite

Rock A - Specimen # 1

Rock A - Specimen # 2

Calcite is a mineral, composed of crystals of calcium carbonate (CaCO2) which crystalizes out of solution when the concentration of calcium carbonate in the water exceeds saturation.  It is called an evaporite because this happens when sea water is evaporating, causing it to become more saline.

Different depositional environments can cause the crystal forms to look different.  Trace chemicals can make the crystals a wide variety of colors.
 
 

 

Rock B - Specimen # 1

Rock B - Specimen # 2

Limestone is a rock formed primarily from the mineral calcite.  It is formed when hard parts of organisms settle out of the ocean.  Because living organisims draw the calcium carbonate from the water and because the settling, it is called a bioclastic rock.

It is not a mineral because it has not overall crystaline structure, and because usually is a mixture of calcium carbonate with other substances.  For example, although Specimen # 2 is nearly 100% calcium carbonate, specimen #1 has a lot of shale mixed in.

Differences in color are caused by the composition of the mixture.  Differences in texture are caused by the turbulence of the water in which it forms.  Specimen #1 which formed in relatively turbulent water is coarse grained, because the fine grains remained in suspension.  Specimen # 2 is smooth and fine grained because it formed in quiet water, which allowed the finest particles to settle out.
 

 

Chattanooga Shale

Shale from the Mississippian Fort Payne Formation

Shale is formed from settling clay.  it is composed primarily of the minerals which make up the feldspar group (aluminum silicates).  Because it forms from settling particles, and living organisms play no part in its formation, it is called a clastic sedimentary rock.  Its most obvious characteristic is that it breaks into thin sheet.  (The term used is fissile).

Shale can be relatively pure, as in the case of the Chattanooga Shale, or it can be mixed with limestone as in the case of the Mississippian shale.

The color of shale is determined by trace elements.  The Chattanooga Shale gets its color from carbon (black), iron (rust colored), and sulphur (yellow)
 
 
 

Rock D - Specimen # 1

Rock D - Specimen # 2

Chert is composed of Silicon Dioxide (SiO2), which is also quartz or glass.  It can be formed in two ways.  Sometimes it precipitates out of the water.  Under other conditions, it is drawn out of the water by organisms which use silicon dioxide for their shells.  In the Ordovician Period this would be radiolarians and silicaceous sponges, the original creatures that live in glass houses.  If chert forms the first way it is considered a mineral.  It if forms the second way, it is considered a bioclastic rock.

Because it is composed of silicon dioxide it is very much harder than any of the other common rocks found in Nashville.  If find a rock in Nashville which will scratch a knife, you can be pretty sure that it's chert.

As with other rocks, the color is determined by trace elements.  Black chert is commonly known as flint and was used by native people to make tools and weapons.  Particularly beautiful pieces of chert are polished and used as gem stones under the name of agates.
 
 

Rock E - Specimen # 1

Rock E - Specimen # 2

Gypsum is our second mineral.  It is composed of Calcium Sulfate (CaSO4) and precipitates out of sea water when evaporation creates super-saline conditions.  Like calcite, the crystals can take several forms.  Most commonly it looks like snowballs as in specimen #1, but when if forms large sheets as in specimen #2 in its crystals form long parallel lines.  At other times it forms large transparent crystals called selenite. (We find these here, but I've given all of mine away.  The next time I find one, I'll scan it before it goes.)