Rock Classification Scheme (BGS).pdf - The Water, Sanitation and ...

VarioliticSpherulitica fan-like arrangement of divergent, oftenbranching fibres; usually the fibres are plagioclaseand the space between is occupiedby glass or granules of pyroxene, olivine oriron oxide.spheroidal bodies (spherulites) in a rock thatare composed of an aggregate of fibrouscrystals of one or more minerals radiating froma nucleus, with glass or crystals in between.The most common occurrence of spherulitictexture is a radiate aggregate of acicular alkalifeldspars with glass between them, thoughquartz or other minerals may be present.7.2.6 QUALIFIERS TO DESCRIBE CAVITY TEXTURESThe following definitions are taken from MacKenzie et al.(1982).Vesicular round, ovoid or elongate irregular holesformed by gas expansion.Amygdaloidal former holes occupied by late-stage orpost-magmatic minerals, for examplecarbonate, zeolites, and quartz.Miarolitic irregularly shaped cavities (druses) inplutonic and near surface rocks intowhich euhedral crystals of the rockproject.Lithophysic a sphere consisting of concentric shellswith hollow interspaces.7.2.7 SPECIAL QUALIFIERS FOR PYROCLASTIC ROCKSTextural qualifiers may be added to pyroclastic rock rootnames. Some of the more common ones are defined below,however it must be stressed that these should be includedin a rock name only where such textures are considered tobe an essential characteristic or are an important aspect ofthe particular study for which the rocks are being collected.WeldedEutaxiticParataxiticused when glass shards in a pyroclasticdeposit have been sufficiently plastic (hightemperature) after settling to sinter togetherduring compaction or flow.denotes a planar foliation texture, formedprincipally by the welding compaction ofpumice and glass shards. Pumice fragmentsthat have become elongate and lenticular inthis way are termed ‘fiammé’.denotes an extreme form of eutaxitic texturein which highly flattened and often stretchedfiammé produce a linear fabric as opposed toa planar fabric.7.2.8 QUALIFIERS TO INDICATE METAMORPHIC OVERPRINTINGThe term meta should be used with an igneous root name toindicate that an igneous rock has been metamorphosed, forexample metagabbro, but only when there is sufficient informationfor the original rock type to be ascertained andassigned to a particular QAPF field. This is the border zonewith metamorphic rocks and the rocks may not be strictly‘igneous or igneous looking’. Where there is insufficient informationto be certain of the igneous protolith they should beclassified according to the scheme for metamorphic rocksusing terms such as ortho-amphibolite. Definitions of theseterms are given in the metamorphic rock classification scheme.Textural terms for metamorphic rocks, such as foliated, hornfelsed,schistose nd gneissose, may be used as qualifiers forigneous rocks which display these features, provided the rockshave a recognisably igneous protolith. Where metamorphicqualifier terms are used, the ‘meta’ prefix on the igneous rootname should be retained, for example schistose metabasalt.Where this creates a name with a double vowel a hyphenshould be used, for example meta-andesite.7.3 Qualifiers based on colourQualifiers can be used to describe the colour of a rock, forexample pink granite, but in many cases this will be partof the general description of a rock. This is inherentlyrather subjective but it is difficult to incorporate a MunsellColour Index into a rock name.The colour of a rock is not the same thing as the colourindex M’, a modal parameter (see Section 3.1 Determinationof modal parameters) which defines the proportion of darkand light coloured minerals.Range of M’leucocratic 0–35mesocratic 35–65melanocratic 65–90ultramafic 90–100These absolute values are independent of the prefixes‘leuco’ and ‘mela’ which are used to denote relatively lightand dark variants of rocks. For these prefixes the thresholdvalues of M’ vary from rock group to rock group. Figures33 and 34 show the limits within which each of these termsmay be applied in the various rock groups. Shortenedforms for colour index qualifier terms can be used andlinked directly to root names, for example leucogranite,melagabbro.8 ROCK NAMES THAT DO NOT CONFORMWITH THE SCHEMESome igneous rocks cannot be described adequately usingthe root-name-and-qualifier approach adopted for most ofthis scheme. Some of these, for example lamprophyres,have been treated elsewhere. The remainder fall into threegroups: (i) those which are more usefully named accordingto their field association; (ii) those which are more easilynamed according to the process by which they formed; (iii)those which are ‘sack’ terms for rocks which are difficultto classify in the field.8.1 Rock names based on field association8.1. 1 PEGMATITE AND PEGMATITICMany rocks described traditionally as ‘pegmatite’ or as‘pegmatitic’ probably crystallised from a fluid-dominatedmedia rather than from a melt-dominated media. In otherwords, these rocks occupy an area of overlap between‘vein and ore rocks’ (which are the product of hydrothermalprocesses) and igneous rocks (which are the product ofigneous processes). However, as most pegmatites and pegmatiticrocks are closely associated both spatially andgenetically with igneous rocks, they are included here. Werecommend that the terms be used in the following way.The term pegmatite should be used only for veinscomprised of coarse or very coarse crystals. Such rocksshould be named by listing the component minerals asqualifiers in front of the term pegmatite, for examplebiotite-quartz-feldspar pegmatite. Following the conventiondescribed in Section 7.1 Qualifiers based on mineralogicalcriteria, the mineral qualifiers should be listed in order of20