Glossary of petrological names and terms found on the Isle of Skye

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From: Bell, B.R. and Harris, J.W. An excursion guide to the geology of the Isle of Skye : Geological Society of Glasgow, 1986. © 1986 B.R. Bell & J.W. Harris. All rights reserved.

Appendix 1 Glossary of petrological names and terms[edit]

Terms in italics are defined elsewhere in Appendix 1.

Igneous[edit]

ACID/ACIDIC. Igneous (and metamorphic) rocks with more than 65 wt.% SiO2 and dominated by light-coloured minerals. e.g. granite, granophyre, felsite.

ADAMELLITE. Coarse-grained, acidic. An intrusive rock dominated by quartz and feldspar. Alkali feldspar and plagioclase occur in approximately equal amounts. Accessory mica, amphibole, pyroxene and Fe-Ti oxides.

ADCUMULATE. Cumulus crystals which have continued to grow and are unzoned. Contains lower-than-normal amounts of intercumulus material.

AGGLOMERATE. A coarse-grained (> 64mm) pyroclastic rock, with rounded to sub-angular fragments set in a fine-grained matrix of the same material. cf. breccia.

ALKALINE. Rich in alkali-elements (Na and K) and/or alkali-rich minerals.

ALKALI OLIVINE BASALT. Fine-grained, basic. A critically silica-undersaturated basalt, containing normative nepheline.

ALKALI OLIVINE DOLERITE. Medium-grained, basic. A critically silica-undersaturated dolerite, containing normative nepheline.

ALLIVALITE. Coarse-grained, ultrabasic. Contains calcic plagioclase dominating over olivine, with very little pyroxene.

ANDESINITE. Coarse-grained, basic. A rock composed predominantly of calcic plagioclase (andesine).

ASH. A fine-grained (<2mm) pyroclastic rock. When consolidated, referred to as a tuff.

AUGITE-ANDESITE. Fine-grained, intermediate. Phenocrysts of plagioclase (labradorite) and augite set in a glassy groundmass.

BASALT. Fine-grained, basic. Contains plagioclase (labradorite) and clinopyroxene, with or without olivine, orthopyroxene, nepheline, and quartz. Varieties: alkali olivine basalt, tholeiitic basalt.

BASIC. Igneous rocks with 45–52 wt.% SiO2 and dominated by dark-coloured minerals (plus plagioclase). e.g. gabbro, dolerite, basalt.

BEINN DEARG TYPE DYKES. Defined in Section (9C) of Chapter 9.

BENMOREITE. Fine-grained, intermediate. Silica-saturated to -undersaturated, between mugearite and trachyte in the alkali olivine basalt fractionation series.

BRECCIA. A coarse-grained (>64mm) pyroclastic rock containing angular fragments in a fine-grained matrix of the same material. cf. agglomerate.

CRINANITE. Medium-grained, alkaline basic. An olivine-bearing teschenite.

CRINANITIC. Pertaining to a crinanite.

CUMULATE. A coarse-grained (usually basic or ultrabasic) accumulative rock consisting of touching crystals formed by some type of fractionation process (for example, crystal settling or in situ congelation crystallisation).

CUMULUS. Pertaining to a cumulate.

DIORITE. Coarse-grained, intermediate. An intrusive rock dominated by amphibole and sodic plagioclase. Pyroxene and/or quartz may also be present.

DIORITIC GLAMAIGITE. A homogeneous form of glamaigite. A coarse-grained, mixed-magma rock of intermediate composition.

DOLERITE. Medium-grained, basic. Contains calcic plagioclase (labradorite) and clinopyroxene, typically in an ophitic intergrowth, with or without olivine, orthopyroxene, nepheline, and quartz.

DUNITE. Coarse-grained, ultrabasic. A variety of peridotite, composed almost entirely of olivine. Accessory chromite.

EUCRITE. Coarse-grained, basic-ultrabasic. A variety of gabbro containing calcic plagioclase (An70–90) and clinopyroxene, with or without olivine.

FAIRY BRIDGE MAGMA TYPE. Defined in Section (9C) of Chapter 9.

FELDSPATHIC PERIDOTITE. Coarse-grained, ultrabasic. A variety of peridotite rich in calcic plagioclase (20–30%).

FELSITE. Fine-grained, acidic. A pale-coloured rock dominated by quartz and alkali feldspar, with or without phenocrysts of these minerals.

FERRODIORITE. Coarse-grained, intermediate-basic. A diorite with modal plagioclase (An30–50) and Fe-rich ferromagnesian minerals.

GABBRO. Coarse-grained, basic. Contains calcic plagioclase (labradorite) and clinopyroxene, with or without olivine, orthopyroxene, nepheline, and quartz.

GLAMAIGITE. A coarse- to medium-grained, mixed-magma rock consisting of dark patches of marscoite (itself a mixed-magma rock) set in a lighter groundmass of similar composition.

GRANITE. Coarse-grained, acidic. An intrusive rock dominated by quartz and feldspar. More than two-thirds of the feldspar is alkali feldspar. Accessory mica, amphibole, pyroxene, olivine, and Fe-Ti oxides.

GRANOPHYRE. Coarse-grained, acidic. A variety of granite in which the quartz and alkali feldspar have crystallised in an irregular, often microscopic, intergrowth.

HAWAIITE. Fine-grained, intermediate-basic. Silica-undersaturated, between alkali olivine basalt and mugearite in the alkali olivine basalt fractionation series.

HETERADCUMULATE. A cumulate where the intercumulus (interstitial) crystals are of the same composition as, but are distinct from, the cumulus crystals. cf. adcumulate.

HYALOCLASTITE. A glassy volcanic deposit, consisting of shattered fragments of rapidly-chilled intrusive or extrusive material, formed by interaction with water, ice, or water-saturated sediments.

IGNIMBRITE. A fine-grained pyroclastic rock formed by the consolidation (and sometimes welding) of ash flows and nuees ardentes.

INTERCUMULUS. The spaces between cumulus crystals.

INTERMEDIATE. Igneous rocks with 52–65 wt.% SiO2, between acidic and basic. e.g. diorite.

LAMPROPHYRE. Medium-grained, alkaline acid-intermediate-basic. Dark-coloured rocks containing phenocrysts of mafic minerals (typically biotite, hornblende and pyroxene) set in a fine-grained groundmass of the same minerals, plus feldspars and/or feldspathoids.

LAPILLI-TUFF. A pyroclastic rock consisting of lapilli-sized (2–64mm) fragments in a matrix of tuff.

LEIDLEITE. Glassy, acidic-intermediate. A glassy rock containing microlites (crystals only just large enough to show polarization colours) of calcic plagioclase and pyroxene. Accessory apatite and Fe-Ti oxides.

LOW-Fe INTERMEDIATES. Fine-grained, intermediate. Fractionated rocks, similar to mugearite and benmoreite, but relatively depleted in iron.

MARSCOITE. A medium-grained, mixed-magma rock of intermediate composition containing xenocrysts of andesine, alkali feldspar and quartz, set in a groundmass dominated by the same minerals.

MESOCUMULATE. A cumulate containing a small amount of intercumulus material. Between an adcumulate and an orthocumulate.

METALUMINOUS. A rock in which the molecular proportion of Al2O3 is greater than that of K2O + Na2O, but less than K2O + Na2O + CaO.

MINETTE. Medium-grained, alkaline acidic. A lamprophyre containing biotite phenocrysts in a groundmass of alkali feldspar and biotite.

MOLECULAR PROPORTION. The ratio of the weight percentage of an oxide, to its molecular weight.

MUGEARITE. Fine-grained, intermediate. Between hawaiite and benmoreite in the alkali olivine basalt fractionation series.

NORMATIVE. Pertaining to a mineral which, according to a set of predetermined rules applied to the major-element chemistry of the rock, should be present. In practice, this is not always the case.

ORTHOCUMULATE. A cumulate plus the crystallisation products of the intercumulus liquid.

PEGMATITE. A very coarse-grained (>3cm) rock with interlocking crystals. The minerals involved are often late-stage, hydrous phases.

PERALKALINE. A rock in which the molecular proportion of Al2O3 is less than that of K2O + Na2O.

PERALUMINOUS. A rock in which the molecular proportion of Al2O3 is greater than that of K2O + Na2O + CaO.

PERIDOTITE. Coarse-grained, ultrabasic. A rock rich in olivine, with or without other mafic minerals (pyroxenes, amphiboles, micas) and almost devoid of plagioclase. A peridotite composed only of olivine (with accessory chromite) is a dunite.

PICRITE. Medium- to coarse-grained, ultrabasic. A rock rich in olivine (plus minor pyroxene, amphiblole and mica), plus 5–10% plagioclase. Where little or no plagioclase is present, the name peridotite is used.

PICRODOLERITE. Medium-grained, basic-ultrabasic. Intermediate between picrite and dolerite.

PITCHSTONE. Glassy, acidic-intermediate. A volcanic rock with a dull, waxy (pitch-like) lustre. Contains crystallites (crystals not large enough to show polarization colours) typically of feldspar, set in an often-devitrified groundmass.

PORPHYRY. A porphyritic rock with obvious phenocrysts.

PRESHAL MHOR MAGMA TYPE. Defined in Section (3D) of Chapter 3 and Section (9C) of Chapter 9.

PYROCLASTIC ROCK. A clastic (fragmental) rock formed by volcanic explosion. e.g. tuff, lapilli-tuff, agglomerate, breccia.

RHYOLITE. Glassy to fine-grained, acidic. A flow-banded, typically porphyritic (with phenocrysts of quartz and/or alkali feldspar) lava.

SMLS. Skye Main Lava Series. Defined in Section (3D) of Chapter 3.

SMLS MAGMA TYPE. Defined in Section (3D) of Chapter 3.

SUBALUMINOUS. A rock in which the molecular proportion of Al2O3 is approximately equal to that of K2O + Na2O + CaO.

TACHYLITE. Glassy, basic. A basic glass.

TACHYLITIC. Pertaining to a tachylite.

TESCHENITE. Medium-grained, alkaline basic. An analcime-bearing dolerite.

TESCHENITIC. Pertaining to a teschenite.

THOLEIITE. Fine-grained, basic. A silica-saturated basalt, containing Ca-poor pyroxene, Ca-rich pyroxene and calcic plagioclase. Characterised by normative hypersthene.

THOLEIITIC BASALT. See tholeiite.

TRACHYTE. Fine-grained, alkaline acidic. An alkali feldspar -dominated rock, with lesser amounts of sodic plagioclase, plus the mafic minerals pyroxene, amphibole and biotite mica. The alkali feldspar crystals typically show a sub-parallel, flow-banded arrangement.

TRANSITIONAL BASALT. A basalt with a composition close to the Critical Plane of Silica Undersaturation of Yoder and Tilley (1962). Essentially, transitional between alkali olivine basalt and tholeiite.

TROCTOLITE. Coarse-grained, basic. A variety of gabbro containing calcic plagioclase (labradorite) and olivine, with little or no pyroxene.

TUFF. A pyroclastic rock with fragments typically <2mm.

ULTRABASIC. Igneous rocks with less than 45 wt.% SiO2 and dominated by dark-coloured minerals. e.g. peridotite, picrite.

VOGESITE. Medium-grained, alkaline acidic. A lamprophyre containing hornblende phenocrysts in a groundmass of alkali feldspar and hornblende.

Metamorphic[edit]

AMPHIBOLITE. A contact- or regional-metamorphosed basic rock dominated by amphibole.

BUCHITE. A hornfels in which partial fusion has taken place. Formed by extreme thermal metamorphism.

GNEISS. A banded, high-grade, regional metamorphic rock.

GRANULITE. A rock consisting of interlocking crystals, all approximately the same size, and with little or no visible preferred orientation.

GRANULITIC. Pertaining to a granulite.

HORNFELS. A fine-grained rock formed by contact metamorphism and lacking any obvious fabric.

MARBLE. A contact- or regional-metamorphosed carbonate-rich rock.

METABASITE. A metamorphosed (recrystallised) basic rock.

MYLONITE. An intensely deformed rock, with a streaky or banded appearance, associated with thrusts.

MYLONITIC. Pertaining to mylonites.

PELITE. A metamorphosed siltstone or mudstone.

PELITIC. Pertaining to, or characteristic of, a pelite.

PHYLLONITE. A banded rock, less deformed than a mylonite, associated with thrusts.

PHYLLONITIC. Pertaining to phyllonites.

PSAMMITE. A metamorphosed, impure sandstone.

PSAMMITIC. Pertaining to, or characteristic of, a psammite.

PYROXENE HORNFELS. A high-grade contact metamorphic rock formed at low pressures in the inner parts of contact metamorphic aureoles.

QUARTZITE. A regional- or contact-metamorphosed pure sandstone (cf. psammite).

SCHIST. A foliated, crystalline, regional metamorphic rock, with a definite fabric and typically containing visible minerals such as mica.

SERPENTINITE. A rock composed of serpentine-group minerals, formed by the metamorphism of ultrabasic rocks.

SKARN. A rock formed by the metamorphism and metasomatism of carbonate-rich rocks.

Sedimentary[edit]

ALGAL STROMATOLITE LIMESTONE. A limestone consisting of structures formed by micro-organisms (algae) which, as they grow, trap, bind and/or cause precipitation of sediment.

ALLUVIUM. Material transported by a river.

ARENACEOUS. Pertaining to, or characteristic of, an arenite.

ARENITE. A relatively pure, cemented sandstone.

ARGILLACEOUS. Pertaining to, or characteristic of, a rock dominated by clay-size particles. e.g. shale.

ARKOSE. A feldspar-rich (at least 25%) sandstone or grit.

CALCAREOUS. Said of a rock which contains calcium carbonate.

CARBONATE. A sedimentary rock dominated by carbonate minerals. e.g. limestone.

CEMENTSTONE. A carbonate-cemented, silty clay.

CHAMOSITE OOLITE. A rock dominated by ooliths (small (0.5–1mm), round, accretionary bodies) of the chlorite-group mineral, chamosite. A variety of bedded iron ore.

CHERT. A hard, micro- or crypto-crystalline rock of quartz and/or amorphous silica (opal).

CLASTIC. Said of a rock composed of fragments of pre-existing rocks which have been subjected to weathering, erosion and transportation. cf. argillaceous, arenaceous and rudaceous rocks: conglomerate, grit, sandstone, siltstone, shale, and mudstone.

CONGLOMERATE. A coarse-grained, clastic rock containing rounded to sub-angular fragments >4mm.

DIATOMITE. A deposit consisting of the remains of diatoms. See Section (11F) of Chapter 11.

DOGGER. An irregular concretion or nodule, of diagenetic origin, within a sedimentary rock.

DOLOSTONE. A carbonate-rich rock dominated by dolomite.

GRIT. A clastic rock with angular fragments, 2–4mm.

JASPER. A variety of chert containing iron oxide impurities.

LIGNITE. A low-grade, brownish-black coal, intermediate between peat and sub-bituminous coal.

LIMESTONE. A chemical/clastic rock with more than 50% calcium carbonate.

MARL. A calcareous mudstone.

MICRITIC LIMESTONE. A limestone with greater than 90% carbonate mud (lithified), with crystals <30 microns in diameter.

MUDSTONE. A non-fissile (non-laminated), clastic rock dominated by approximately equal amounts of clay and silt.

OOLITIC LIMESTONE. A limestone dominated by ooliths (small (0.5–1mm), round, accretionary bodies) of, commonly, calcite or dolomite.

PEAT. An unconsolidated deposit of semi-carbonised vegetable matter.

RUDACEOUS. Pertaining to, or characteristic of, a rock dominated by coarse-grained particles. e.g. grit, conglomerate.

SANDSTONE. A medium-grained, clastic rock composed of rounded to sub-angular fragments of sand-size.

SHALE. A fissile (laminated), clastic sedimentary rock dominated by approximately equal amounts of clay and silt.

SILTSTONE. A clastic rock composed of fragments of silt-size, intermediate between shale and sandstone.

References[edit]

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