Jersey Geology Trail


There are many different textures that make up the fabric (internal crystal mosaic) of Jersey rocks. These should not be confused with structures such as graded bedding or cleavage. A knowledge of what they look like and how they were formed helps to interpret the origin of the rock. They are recorded by Bishop & Bisson (1989) who cite their use by many other authors. These textural terms are not in one's common vocabulary, so brief meanings and implications are as follows (Allaby & Allaby, 1999).

Igneous rocks.

Gabbros & Diorites. (NW Igneous Complex). North-west granite; Bishop & Bisson, pp. 46 - 50).

Ophitic. Large crystals of augite partly or wholly surrounding plagioclase crystal (Gk. ophis = snake).


Augite (colour); feldpars (grey) (by kind permission of C.H.Donaldson ( MacKenzie, Donaldson & Guilford, 1982, p. 35).


Poikilitic. Any large crystal, surrounding /enclosing several smaller ones, having grown faster than them (Gk. poikilos = multicoloured).


Augite (yellow) surrounding feldspars (grey) (MacKenzie, Donaldson & Guilford, 1982, p. 33)


Poikiloblastic. As above in a metamorphic rock.

Ocelli. Areas of eye-shaped texture with one colour surrounding another (L. oculus = eye).

Granites. (NW Igneous Complex). North-west granite; Bishop & Bisson, pp. 53 - 58).

Perthitic. Alkali feldspars (orthoclase-K rich) with alternate layers - intergrowths - of orthoclase and plagioclase feldspar (Na rich-albite) which separated during cooling and unmixing; macro- to cryptocrystalline in size.


Microperthite; albite lamellae (l.gy) in large orthoclase (med.gy), upper centre-left (MacKenzie, Donaldson & Guilford, p.51).


Granophyric. Small scale graphic, radiating intergrowth of quartz and orthoclase or plagioclase, formed by late, simultaneous, rapid crystallisation of the minerals between earlier crystals.

Graphic. A large scale intergrowth of quartz and orthoclase looking like wedge shaped (cuneiform) writing.


Intergrown quartz and feldspar; granophyric (radiating) and graphic textures (MacKenzie, Donaldson & Guilford, 1982, p. 48)


Porphyritic. Large crystals (phenocrysts. Gk. phaino = shine, show) which occur in a fine crystalline groundmass as a result of earlier, slowly formed, larger crystals mixing with a later stage, faster cooling melt.


Porphyritic andesite (Germany; MacKenzie, Donaldson & Guilford, p. 16). NB zoned and twinned plagioclase feldspars similar to local andesites.


Crystal twinning. Twinning is a feature common to many single crystals where the crystal lattice is differently orientated in two or more parts of the crystal about a twin plane or twin axis, giving simple or multiple twins (Allaby & Allaby, 1999, p.138).


Simple twin in orthoclase feldspar.


Crystal zoning. This texture is found mainly in the plagioclase feldspars which exhibit a colour change from the core to the rim, reflecting a chemical change between the zones. In Jersey granites the colour changes from red-pink to white-yellow (see Allaby & Allaby, p.139).


Zoned feldspars, SW granite and zoned feldspars (porphyritic andesite), Japan.


Dykes.  (SE Igneous Complex). South-east granite; Bishop & Bisson, pp. 62 - 66).

Aphyric. Composed of an aphanitic (Gk. a = not; phaino = shine, show) very finely crystalline groundmass with no large crystals, formed by rapid cooling due to loss of heat near its crystallisation (liquidus) temperature.

Ocellar. Eye shaped texture with a patch of colour surrounded by another colour (L.oculus = eye).

Granoblastic. Mosaic of uniform, anhedral (no crystalline shape) crystals in a metamorphic rock.

Spherulitic. Composed of spheres or ellipses of radiating, fibrous crystals of quartz and orthoclase in a glassy or very finely crystalline felsitic matrix due to devitrification of quenched glassy igneous rocks (see Rhyolites and Mourant (1932). (Gk.sphaira = ball).


Radiating spherulite of fine intergrown needles of quartz & feldspar (alkali) (MacKenzie, Donaldson & Guilford, 1982, p.55) and local Rhyolite spherulite x1.


Fig. 3. Meeting of two simple spherulites x23.
Fig. 4. Same field; crossed nicols x26.
Fig. 5. Crystallisation between felspar arcs x23.
Fig. 6. Poikilitic structure x69.
Fig. 7. Small rhythmic spherulite x23.
Fig. 8. Small spherulite; crossed nicols x67.
Fig. 9. Complex spherulite; grey areas - felspar, quartz & iron oxide; white areas - quartz. x23.
(Reproduced from a reprint of a paper by A. E. Mourant, M.A., D. Phil. in the Mineralogical Magazine, 1932, Vol. XXlll. No. 139, p. 238 & Plate Xl).



Allaby, A. and Allaby, M. 1990. A Dictionary of Earth Sciences. 2nd. Ed. Oxford University Press.

Bishop, A. C. and Bisson, G. 1989. Classical areas of British geology: Jersey: description of 1:25,000 Channel Islands Sheet 2. (London: HMSO for British Geological Survey.)

MacKenzie, W. S., Donaldson, C. H. & Guildford, C. 1982. Atlas of igneous rocks and their textures. Longman.

Mourant, A. E. 1932. The spherulitic rhyolites of Jersey. Mineral. Mag. Vol. 23. pp. 227 - 238.