This rock is made up of light grey sub-spherical ooids. 
Fine to medium sand grain size. 
A few lithic/biogenic fragments. 
Dark grey cement. 
Small hair-line white veins run through this rock.

Ooids made up of a greenish sheet silicate mineral called chamosite, partially altered to siderite and limonite. Concentric structure. Central cores of fine-grained material.

A layer formed of brown, transparent crystals, sometimes with rhombic cross-sections surrounds the peripheries of many ooids. This is siderite.

Some quartz grains.

The cement is made up mainly of carbonate (siderite), with a small quantity of a green-brown material, which has anomalous interference colours.

This rock is a grainstone (Dunham naming system) or oo-sparite (Folk naming system).

Deposited in a moderate energy, marine energy.

Chamosite is an Fe-rich chlorite, often greenish. It is a sheet silicate.

Limonite is amorphous or cryptocrystalline, and yellow-brown to brownish-black in colour. It comprises hydrated iron (III) oxides with a range of compositions.

Siderite is iron carbonate (FeCO₃). It is often difficult to distinguish siderite from calcite and dolomite optically.

oolitic ironstone
oolite

Two rock types, with an irregular contact between them.

1 – Very dark brown, microcrystalline rock. This fizzes with acid. It is a limestone.

2 – Grey and black, intermingled, microcrystalline silica. Brittle and well jointed. Some medium grained white quartz vein material.

1 – A fine to medium grained mosaic of calcite with cross-sections of foraminifera and other shell material.

2 – Microcrystalline silica, quartz forming a uniform mosaic texture, slightly coarser in some places than others. Scattered with grains of calcite and opaques, both typically 0.1 mm across. Criss-crossing calcite veins, some dark red-brown discolouration.

This rock has a complicated history. In essence the chert (2) has replaced the limestone (1). The irregular patches of limestone within the chert are probably relics of pre-existing limestone, but the calcite rhombs are related to the calcite veining and therefore later than the chert. Similarly the opaques, which cut across the structure of the chert, formed later than the chert.

The cubic form of the opaques is suggestive of pyrite, but we cannot be sure.

bedded chert

Very fine-grained rock with a prominent, well-developed cleavage. The dark red-purple colour is due to iron-staining. The light green smudge is a reduction spot.

Mud size grains.
Mainly indistinguishable clay minerals.
Some quartz, muscovite, opaques.

The protolith of this rock was a pelite (mudstone). Under low-grade, regional metamorphic conditions, crystals within the pelite were aligned, producing the slaty cleavage.

Iron staining 
Hematite(Fe₂O₃) → red (blood-red, cherry-red, brown-red) iron-staining 
Limonite (FeO(OH)·nH₂O) → yellow or brown iron-staining 
The red-purple colour of this rock is probably produced by hematite.

Reduction spot 
The dark red-purple of the majority of this rock is due to the presence of iron in the 3+ oxidation state (Fe³⁺). The light green colour of the reduction spot is instead due to the presence of iron in the 2+ oxidation state (Fe²⁺). Hence, reduction of the iron ions from Fe³⁺ to Fe²⁺ has taken place in the reduction spot.

slate

Almost entirely made up of fine sand sized quartz grains. Does not fizz with acid, so no carbonate present. Clean white rock, apart from pink-brown weathering. Dark carbonaceous marks are plant roots.

Subangular quartz grains, typically 0.25 mm across with sutured contacts.

The sutured contacts between the quartz grains indicate that this rock has been metamorphosed to a quartzite. This is a very well recrystallised rock. The protolith was a fine-grained quartz arenite (<15% matrix), but we cannot tell how angular or spherical the original grains were as they have been partially dissolved by pressure solution to form sutured contacts.

quartzite

(Mg,Fe)₂[SiO₄]

Orthosilicate

Crystal system: orthorhombic (+) (-)

Colour in hand specimen: pale green
Cleavage: none, instead it has characteristic curving cracks
Hardness: 7

Colour in plane polarised light: colourless
Relief: moderate 
Birefringence: 0.035; 2nd order pink

M100 Olivine 
North Carolina 
A mass of rather irregular crystals of olivine
Yellow-green colour
Vitreous lustre
No cleavage

L141 Gabbro 
L162 Basalt, porphyritic 
L172 Ultramafic cumulate

Olivine has two end-member compositions: forsterite, Mg₂[SiO₄], and fayalite, Fe₂[SiO₄]. The physical and optical properties detailed above refer to forsterite. Forsteritic olivine is a common constituent of mafic and ultra-mafic rocks.
Cracks are often filled with serpentinite alteration.

olivine
forsterite
fayallite

CaTi[SiO₄](O, OH, F)

Orthosilicate

Crystal system: monoclinic (+)

Colour in hand specimen: colourless, yellow, green, brown or black
Cleavage: one, good
Hardness: 5

Colour in plane polarised light: colourless, yellow or typically brown
Pleochroism: moderate if coloured
Relief: high
Birefringence: 0.100-0.192; >4th order
Extinction: inclined
Twinning: simple twins; occasionally lamellar twins

L101 Granite 
L120 Granodiorite

sphene
titanite

Advanced notes