36.2.3 Colour

Rock colours should be described as recommended in BS EN ISO 14689-1 and in Table 7; further comments on the description of colour are given in 33.4.2.

36.2.4 Texture

The texture of a rock should be described in accordance with geological terminology.

NOTE 1 The texture of a rock refers to individual grains and their arrangement, which might show a preferred orientation. Terms frequently used include "porphyritic", "crystalline", "cryptocrystalline", "amorphous" and "glassy". Definitions of such terms are provided in standard geological dictionaries. Geologists often subdivide texture into texture (geometric aspects of particles or crystals) and fabric (arrangement of grains), but this is seldom appropriate in field descriptions for engineering purposes.

NOTE 2 Examination of the rock texture might require the use of a hand lens or the microscopic examination of a thin slice of the rock.

36.2.5 Grain size

COMMENTARY ON 36.2.5

Quantified grain size boundaries are only appropriate in sedimentary and derived metamorphic rocks. In other rocks the grain size distinctions are relative. Grain size refers to the average dimension of the minerals or rock fragments dominating the rock's behaviour; this usually means the groundmass, but sometimes separate descriptions of the cement and grains might also be necessary.

The grain size of the rock material should be described using the scheme in Table 27. The terms "fine", "medium" and "coarse grained" should be applied either to the whole range of grain sizes, e.g. "medium grained limestone", which means it has grains of 0,063 mm to 2 mm, or it can apply to a grain size subdivision, e.g. a "medium grained sandstone", meaning it is made up of grains 0,2 mm to 0,63 mm. Wherever necessary, the terminology used should be reported and explained to avoid any confusion.

Where a rock has bi-modal grain sizes, the grains or clasts and matrix or groundmass should be described separately and linked by quantified terms, as given in Clause 32, together with an indication of whether it is the matrix or groundmass that is likely to affect the engineering behaviour. The size should be estimated by eye, which may be aided by a hand lens in the assessment of fine-grained rocks.

NOTE The limit of unaided vision is approximately 0,06 mm.

Grain size		Bedded rocks (mostly sedimentary)
mm		Grain size description		CONGLOMERATE Rounded boulders cobbles and gravel cemented in a finer matrix Breccia Angular rock fragments in a finer matrix		At least 50% of grains are of carbonate				At least 50% of grains are of volcanic rock	SALINE ROCKS
	20–6.3							Calcirudite		Fragments of volcanic ejecta in a finer matrix. Rounded grains AGGLOM­ERATE Angular grains VOLCANIC BRECCIA	HALITE ANHYDRITE GYPSUM
	6.3–2
	2–0,63		Coarse	SANDSTONE Angular or rounded grains commonly cemented by clay, calcitic or iron minerals Quartzite Quartz grains and siliceous cement Arkose Many feldspar grains Greywacke Many rock chips				Calcarenite		Cemented volcanic ash TUFF
	0,63–0,2		Me-dium
	0,2–0,063		Fine
	0,063 -0,002				SILTSTONE Mostly silt	Calcar-eous		Calcisiltite		Fine-grained TUFF
	< 0,002				MUDSTONE	mud-stone		Calcilutite		Very fine-grained TUFF
Amorphous or cryptocrystal-line				Flint: occurs as bands of nodules in the Chalk Chert: occurs as nodules and beds in limestone and calcareous sandstone							COAL LIGNITE
				Granular cemented — except amorphous rocks
				SILICEOUS		CALCAREOUS				SILICEOUS	CARBON ACEOUS
SEDIMENTARY ROCKS Granular cemented rocks vary greatly in strength, some sandstones are stronger than many igneous rocks. Bedding might not show in hand specimens and is best seen in outcrop. Only sedimentary rocks, and some metamorphic rocks derived from them, contain fossils. Calcareous rocks contain calcite (calcium carbonate) which effervesces with dilute hydrochloric acid.

Igneous rocks: generally massive structure and crystalline texture						Metamorphic rocks
Grain size description		GRANITE1	DIORITE1,2	GABBRO1,2	Pyroxenite Peridotite	Foliated	Massive
COARSE						GNEISS Well-developed but often widely spaced foliation sometimes with schistose bands Migmatite Irregularly foliated: mixed schists and gneisses SCHIST Well-developed undulose foliation: generally much mica PHYLLITE Slightly undulose foliation; sometimes spotted SLATE Well-developed plane cleavage (foliation) MYLONITE Found in fault zones, mainly in igneous and metamorphic areas	MARBLE QUARTZITE GRANULITE HORNFELS AMPHIBO- LITE SERPENTINE
		These rocks are sometimes porphyritic and are then described, for example, as porphyritic granite
MEDIUM		MICRO-GRANITE1	MICRO-DIORITE1,2	DOLER-ITE3,4
		These rocks are sometimes porphyritic and are then described as porphyries
FINE		RHYO-LITE4,5	AN- DESITE4,5	BASALT5
		These rocks are sometime porphyritic and are then described as porphyries
Amorphous or crypto- crystalline		OBSIDIAN5	VOLCANIC GLASS
		COLOUR				CRYSTALLINE
		Pale		Dark
		ACID Much quartz	INTERME- DIATE Some quartz	BASIC Little or no quartz	ULTRA BASIC	SILICEOUS	Mainly SILICEOUS
IGNEOUS ROCKS Composed of closely interlocking mineral grains. Strong when fresh; not porous Mode of occurrence: 1. Batholiths, 2. Laccoliths, 3. Sills, 4. Dykes, 5. Lava flows, 6. Veins. METAMORPHIC ROCKS Generally classified according to fabric and mineralogy rather than grain size. Most metamorphic rocks are distinguished by foliation which might impart fissility. Foliation in gneisses is best observed in outcrop. Non-foliated metamorphics are difficult to recognize except by association. Most fresh metamorphic rocks are strong although perhaps fissile.

36.2.6 Rock name

COMMENTARY ON 36.2.6

The rock names given in Table 27 follow general geological practice, but are intended as a guide only; geological training is required for the satisfactory identification of rocks. Engineering properties cannot be inferred directly from the rock names in the table, but the use of a particular name usually indicates a likely range of characteristics to the reader. Combinations of rock names from Table 27 are possible, for instance in the siliceous sedimentary rocks "a sandy MUDSTONE", or combinations of clastic/calcareous/carbonaceous such as "sandy LIMESTONE" or "carbonaceous MUDSTONE".

The rock naming should be based on the scheme in Table 27. Where mineralogies other than those implicit in Table 27 are expected or encountered, the field description might be supplemented by laboratory testing to determine the mineralogy (e.g. elemental chemistry or petrographic examination).

The description of the carbonate content should be carried out in accordance with BS EN ISO 14689-1. A rock should be described as calcareous if the addition of HCI produces a clear but unsustained effervescence, as slightly calcareous if the effervescence is weak or sporadic, and as highly calcareous if the effervescence is strong and sustained. A descriptive term should only be used if the presence of carbonate content is detected.

NOTE The term carbonate free is not expected on field logs unless as a confirmation that the test has been carried out with a negative result.