May 21, 2014 - Nom Geo

Common rock descriptions – Geologist’s notes.

This is a quick look at a bunch of rocks and some notes on making common rock descriptions. You might find it useful as background reading for academic use or to aid identification. It has been done in the style of a quick rock identification exercise. By using physical properties to build up some evidence a systematic approach to rock identification can be made. Of course most of us know what a granite or a sandstone looks like, but how does a geologist actually make that determination? Remember that there has to be a scientific method. In this article I’ve named the rocks after making each set of observations.





Sample description: Hard, medium to high density rock of interlocking coarse crystalline minerals, predominantly light pink and translucent grey coarse 4-6mm size maximum grain size of 9mm, minor white, brown and black mineral species.

Average grain size 6mm;

Description of minerals in sample 1:

  • 1:Pink, Well formed euhedral, reflective vitreous, hard crystals to 9mm, simple twinning. Mineral = Orthoclase – 50-55% of the sample.
  • 2:Grey, translucent,well formed euhedral,late interlocking interstitial crystals to 6mm,  irregular, conchoidal fracture. Mineral = Quartz – 30-35% of the rock.
  • 3: Well formed, white 2:1 elongate oblong, 90˚ intersection to 3mm slightly embayed (early formed),vitreous lustre. Mineral = Plagioclase – 10% of the rock
  • 4:Irregular, black, low to medium strength,variable dull to vitreous lustre, irregular shapes to 3mm,associated with deep black  <1mm grains. Mineral = Biotite – less than 5% of the rock.
  • 5: Well formed, black rare,less than 1mm in size, evidence of cleavage. Mineral = hornblende – less than 1% (minor)
  • 6: Small glints of highly reflective vitreous or metallic lustre. Mineral =  Muscovite – minor occurrence.

Rock field identification;

Granite:Granite is a Felsic coarse grained igneous rock comprised of up to 65% alkali feldspar and at least 20% quartz. Chemically felsic rocks are predominantly made up Silicone Oxide (Quartz) and Aluminium Silicates minerals with a high Sodium or Potassium content. The pink colour is strongly associated wth Potassium.

Granite Formation;

  • This is a plutonic rock. It was crystallized by slow cooling in an underground magma chamber, allowing the growth of the coarse, well defined crystals which characterize this rock.
  • The source of the magma is partially melted or re-melted continental crust.
  • Its mineral assemblage is dominant in elements such as potassium and silica which, through partitioning, are preferentially concentrated within continental crust.Pink granite is strongly associated with re-melting of older rocks, especially sedimentary rocks which are high in potassium. Therefore it can also be called an S-type granite (after Chappel and White, 1982)
  • It is formed in large, often extensive bodies known as plutons, and can measure hundreds or even thousands of meters in vertical thickness. It is formed as a consequence of convergent subduction zone – plate boundary magmatic arcs.
  • Country rocks into which a granite has intruded show heat and stress related deformation.
  • Granites are a weathering source of clay minerals, chlorites, trace elemetns as well as quartz. It commonly contains  Phosphorus in the mineral Apatite.  Lead (Pb ), Thorium (Th) and Uranium (U) elements also occur in trace amounts and these are generally associated with crystals of the mineral zircon. These radioactive elements are also fluid mobile and through weathering by water will leach out into the surrounding formation. Zircon crystals are commonly too small to be seen with the naked eye however the darkish halos caused by alpha radiation are often visible ringing the crystals containing these elements.
  • There are many types of plutonic rocks generically known as granites. Most of these are not really granite but are actually rocks of other names such as syenite, grano-diorite, gabbro, rapakivi granite and so on.
  • In addition to S-type, (sedimentary), there are I-type (Igneous) and A-type granite under a classification proposed by Chappel and White, 1982.

Sample 2)





Description of sample 2; Dense, hard, black, aphanitic groundmass with fine white crystals forming a porphyritic texture. Elongate, euhedral white phenocrysts to less than 1mm occur throughout and are consistent with early formed plagioclase.

Very small,  white crystals which are visible might be formed from a late release of volatile fluids.


Rock Field Identification;


  • Basalt is formed from the rapid cooling of magma either erupted at the surface (submarine or terrestrial) or within dykes amongst country rock. The cooling is so rapid that most of the minerals do not have time to nucleate into crystals, and so set as a groundmass of volcanic glass.
  • Those early formed minerals which have nucleated exist as very small, rapidly cooled crystals. Granite, which has large grains, is the product of very slow cooling of magma.
  • The dark colour of the basalt groundmass is due to its higher content of darker(mafic) minerals containing iron and magnesium.     
  • Basalt is known to consist principally of pyroxene, and plagioclase with subordinate quantities (up to 10%) of magnetite and possibly olivine, however they are visible only in thin section with a microscope.
  • Small white crystals to 1/5mm but mostly 1/20mm may be early formed plagioclase phenocrysts which existed in the magma as early crystallizing components which locked in the element aluminium out of the melt.

Basalt Formation

Basalt is an igneous, plutonic intrusive flow into country rock and forms as a dyke or a laterally extensive sill. The magma source generally has a mantle component and has a higher temperature, and is less silicic that granite.

It also forms from surface eruptions of lava as a volcanic flow if terrestrial, or a pillow basalt if flowing into a sub-marine setting.

Whether plutonic or volcanic, it’s properties are characterized by rapid cooling and a iron/magnesium rich melt source.

Sample 3)


Description: Earthy grey – brown, dull, crypto-crystalline, hard, very low density rounded, igneous rock with a highly vesicular texture as seen in figure above. It has a very low density.

  • The vesicles increase the porosity of the rock, resulting in its low density and “froth like” appearance.
  •  This rock was formed from a siliceous, highly viscous magma which erupted in an explosive manner. This caused a rapid depressurization resulting in the instantaneous degassing of water vapour and other gas. The escaping water resulted in a sudden increase in viscosity, which instantly solidified thereby trapping the large number of gas bubbles within it. (Geonote: Water, even in very small quantities will reduce the melting temperature of any rock – hence the lrapid oss of the water solidifies the volcanic ejecta)
  • Thus, while porous this specimen is impermeable due to the unconnected nature of its pores which are actually preserved bubbles of gas.
  • The low density is primarily due to the vesicular porosity, but is also due to the absence of heavier elements such as iron.

Mineral recognition:

  • The light colour of the rock suggests that it is of a highly siliceous composition, however no discreet minerals are observed in the amorphous groundmass.
  • Small, randomly distributed white and dark vitreous crystals suggest that some crystallizing minerals existed in the siliceous magma/lava prior to eruption, and that these became entrained within the rock matrix. These accessory minerals have the appearance of quartz and mica.

Rock field Identification;


·        Pumice is a volcanic igneous rock formed from highly silicic (read “high viscosity”) lavas erupting explosively in volcanic arcs adjacent to tectonic plate boundaries.

Sample 4)

SANDSTONE (clastic)

Description; Clastic yellowish, medium density, angular to sub-angular grains.

 The size fraction as shown in figure above is of grains in the 1/4mm to 1/2mm size.

This is a medium sandstone. Sandstones are defined as sedimentary rocks made up of discreet grains in the 0.0625 to 2.0000 mm size range.

 Framework particles:

  • The angularity of the grains suggests that deposition of the original sediment was close to the source of the grains.
  • The uniform colour suggests that the grains are all from the same source, and are likely composed of quartz grains.
  • It can thus be described as a moderately to well sorted, sub-angular, medium quartz sandstone, or arenite.
  • The rock structure is dominantly clast supported, although weak, yellowed cement occurs as a coating between grains, suggesting an autogenic chemical process and the presence of an oxide coating.

Porosity and Permeability

  • The sandstone was tested for calcium content by adding droplets of vinegar. This produced no reaction, suggesting an absence of carbonates.
  • To test for effective porosity, that is permeability and porosity, water when added to a dry surface, soaked into the rock. This suggests that the rock has a permeable porosity and is therefore a potential reservoir rock.
  • Hand lens observation reveals that the rock has a high number of inter – granular pores spaces. Grain contacts show little deformation, and the grain orientation is random. As such the preserved porosity is likely of the primary type. (Geonote: Primary porosity is the preserved portion of the original inter-granular pores spaces which existed when the sediment was initially deposited as sand) 

Sample 5)


Common rock Description; Hard, medium density, fine sand grained brown groundmass of grains described as polymitic or of diverse mineralogy. Observation with a hand lens shows that each individual grain is itself be composed of diverse minerals.

Grain size:

  • This is a fine sandstone with a grain size of 1/10mm to 1/4mm( see scale figure 5). Sandstones are defined as sedimentary rocks made up of discreet grains in the 0.0625 to 2.000 mm size range.

Framework particles;

  • Silicic brown, yellow and black lithic and mineral grains. Evidence of oxide minerals and clay cement. Likely source of the grains is weathering from a nearby volcanic arc.


Porosity and permeability;

  • Water drop testing is inconclusive as there is minimal absorption. In contrast to sample R4 sandstone, above, it is not easy to dislodge individual grains when abraded. This suggests a well cemented structure. 

Rock field Identification:

Lithic Sandstone; Lithic Sandstone is composed of grains which are themselves composed of a number of smaller, diverse mineral types.This is because the individual grains did not have time to break down into clays and silt as they have been deposited a relatively short distance from the source. We call these grains “lithic” because the are in fact rock fragments. This is in contrast to clastic sandstones where each grain is comprised of a single mineral grain, usually made of chemically resistant, physically strong minerals such as quartz. Because of the presence of clay forming minerals lithic sandstones tend to have a poor porosity.

Sample 6):



Size and sorting:

  • Well rounded, randomly orientated silicic pebbles to 40mm supported within a  blackish clay matrix composed of abundant poorly sorted fine  1/10mm to 1/5mm angular grains.
  • Angular micaceous as well as lithic fragments, including quartz and felsic minerals which are of a volcanic origin.
  • Contains significant clay cement. The difference between the matrix materials and the large pebbles strongly infer mixing of different depositional conditions as the cement is of a different source to the pebbles. A possible explanation is that this may be as a result of an abrupt event such as a mass movement.
  • The surface of the cement is very hard, does not absorb water, and is inert against acetic acid (vinegar).
  • As shown in the photo above, the clay cement is closely moulded to the larger pebbles, suggesting that the matrix has no natural fluid pathways, and water testing shows nil permeability. Porosity is inferred to be absent, or if present, exists as isolated pores meaning that there is no effective porosity.

Rock field Identification:


Conglomerates are a product of high water energy stream deposits where well rounded river pebbles are covered in finer sediments.. Today this depositional process can be observed in braided rivers and mountain streams.

Sample 7)

Shale (banded siltstone)


Very fine grained medium to dark-grey banded clastic rock, with sub millimetre scale to sub centimetre scale laminar texture. Hard, sandy textured surface, although individual grains cannot be visually distinguished. Of medium density.

Grain size and formation:

  • Grain size is very fine and is less than 0.0625 mm meaning that individual grains cannot be discerned without magnification, inferring a siltstone or finer clay.
  • The formation of this sedimentary rock as suggested by its grain size is that of low energy deposition in marine or lacustrine environment.The dark laminar bands have a brown streak and there is likely a carbonaceous component.
  • The alternating dark – light bands represent episodic changes in depositional energy, the coarser light grey zones appearing to be composed of transported silt sized felsic and quartz grains.
  •  This indicates that there was still some flow transport of grains occurring, and a low level of water energy was still present.

Geotechnical Note:

  • The rock shows no parting planes and is very competent. It is not of the fissile type of shale that commonly occurs.
  • Bedding shows sub millimetre wavy bedding, to sub-centimetre carbonaceous banding.
  • This rock can also be categorized as a siltstone.

           Porosity and permeability

  • When tested with water the sample displayed nil permeability, but it may have a porous micro-texture. The close packing of fine grains reduces permeability. The fine texture may mean that clay minerals are present, and these will reduce the pore volume and inhibit permeability.

Rock field Identification;


Shale is the term for sedimentary rocks that are finer than sandstone and which contain a high clay content. While some geologists prefer to separate the description out into siltstone and claystone, shale nevertheless is formed by the deposition of very fine sediments. The depositonal environment could be a tidal flat or the lower marine continental shelf, a flood plain, a river mouth, a lake bed, a lower delta plain or any deposit of finer sediments that were carried further from their source than sand grains.

Sample 8);

Claystone (mudstone)

Grain size:

Aphanitic, very finely textured  dark red brown, medium density rock, of medium strength, exhibiting conchoidal fracture surfaces. Surface is smooth, and without the coarseness of shale and therefore has a finer texture.

  Red Brown mineral?

  •   The rock colour suggests a clay mineral composition which contains an iron oxide.
  • Small white accessory grains are attached to the rock and may be re-crystallised carbonates or salt.

Rock Field Idenification:


Clay is deposited in a number of ways. This claystone may have been formed as a result of deposition of clays on the sea floor or in a lake bed, a scenario which is supported by the presence of other grains.  Some of the other transport models are dust storms, flooding events and volcanic eruption outfalls. Geologically speaking volcanic clays are interesting because they allow isotopic dating of the horizon to be made and hence are usefull for dating sedimentary successions.In this case a closer look at the other minerals and a paleontological analysis (looking for any sign of fossils) would need to be made to confirm the depositional environment.

GEONOTE: Why are clay horizons useful geological marker beds?

  • The primary condition for a useful stratigraphical marker bed is that is possesses sufficient lateral continuity to cover the geological domain being investigated.
  • Distinctive clay formations are useful marker beds for exploration of outcrops and exploration boreholes because they are commonly the result of regional events (for example; dust storms, distal flood deposits and volcanic outfall deposits and so on) and hence are in many cases recognised as being laterally extensive. This will allow a correlation with the known stratigraphy of the area to be made at different locations.
  • A further condition is that it has a stratigraphical relevance to the depositional sequence of the area. That is, it must be formed of a depositional process that occupies a specific place in the historical depositional record. This precludes for example, late intrusive rocks which cut across the geological record and do not obey the laws of superpositioning.

Sample 9)



 White, medium strength rock showing evidence of crystallized fusing of grains. It has a coarse surface, and irregular fracture. It is of medium density.

Texture and Grain:

  • Coarse crystalline texture . Being of an all white  colour is an indication that it is mono-minerallic

Porosity and permeability:

  • The water test has produced nil permeability and no evidence of any porosity. This is consistent with its re-crystallized texture.

Acid reactivity:

  • Testing with vinegar produced a positive result. An observed reaction with vinegar by the bubbles that appear is evidence a Calcium Carbonate composition (CaCo3).

Rock field Identification:


Limestone is formed in a marine environment from the accumulation of calcium carbonate. Calcium Carbonate is either accumulated throught the growth of a coral reef, or in deeper waters from the depostion of the skeletal remains of plankton.

Sample 10:





  •  Black, low density, low strength brittle rock with alternating dull and vitreous bands, and a blocky fracture. Identified as coal.


  •  Possesses a much lower density than most rock types other than pumice,
  • This is due to its very high porosity and the absence of heavy elements

Bright and dull bands:

The bright and dull bands in the rock represent different types of Macerals. Macerals are the coal equivalent of petro-facies and is term that describes the original vegetation that was deposited, and then diagenetically turned into coal.

  •  Coal with a high reflectance, that is, bright, vitreous bands is formed from woody plant remains, such as branches, leaves and tree trunks. Its very high micro-porosity means that is has a low density, it is also low in impurities, (described as ash content).
  • Dull coal is derived of other types of macerals, such as algal spores, and if of a higher density will also contain a higher amount of non- combustible, mineral impurities.


Fracture network;

  • The blocky fractures are called cleat, and this is a type of jointing unique to coal.
  • Bright, vitreous coal has a higher frequency of cleat joints, up to one every millimetre.
  • Cleats represents the macro-permeability, where Darcy fluids flows can occur but it is the network of micro-pores in coal which represent the significant fraction of the pore volume.

The permeable character of a coal seam is therefore not measured by the connectivity of its pores, but is determined by its cleat spacing and aperture.

Coal is a rock type about which whole books are written due to the complexities with understanding its many properties.In broad terms, coal is characterized according to the type of organic matter and its maturity, or rank. However it does defy being “shoe-horned” into a single classification system and so is generally classified according to the needs of the reader. For example it might be initially described and classified by the field geologist according to its colour as determined by streak, density and the type and occurence of lustrous vitreous or dull sections which make up a seam. Structurally it possesses a crypto-porosity which is difficult to study, even in the lab, however cleating is macroscopically observable.

A rundown on coal is being prepared in a separate article. Below are some simple field observations and notes and my apologies for its simplicity.

This concludes my first post about common rock descriptions. I promise better is to come so please keep an eye out for further posts.

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