METAMORPHIC ROCKS/Classification, Nomenclature and Formation 391 the definition of ‘ultramafic rock’, see comment to Question in the flow chart When the rock is mafic; answer Question with ‘yes’ and proceed down the flow chart You will then have the choice to either use an igneous name prefixed by ‘meta-’ (Question 3) or call your rock a ‘granofels’ (Question 6) iii The rock is an impactite Even if it is metamorphic, first have a look at the paper by Stoeffler and Grieve about impactites, on the homepage of SCMR at http://bgs.ac.uk/SCMR Question 5: Does the Rock Fit Any of the Specific Rock Names Listed in Table 1a, or Does it Make Sense to Use a Name Listed in Table 1b? i A name for a metamorphic rock should at best characterize its modal mineral contents as well as its structure Whereas the structure can easily be described by prefixing specific rock names with schistose, gneissose, or granofelsic, indication of the minerals which are present in a metamorphic rock and of their relative amounts is less easy to perform Therefore, the specific rock names listed in Table 1a, which are defined by mineral content and which are well known, are important and have, therefore, to be given preference over the the equivalent structural root names, schist, gneiss, or granofels As an example: a rock containing the essential minerals plagioclase and amphibole in the amounts defined in the definition of amphibolite (Table 1a) should always be called amphibolite, and never an amphibole-plagioclase gneiss The names in Table 1a should be prefixed by those major minerals, which are not essential minerals (see comment to Question in the flow chart) ii The specific rock names in Table 1b are defined using structural or other features or properties The list is by far from complete If such special features or properties should be highlighted in the name rather than mineral content, such rock names may be used, giving them preference over the the equivalent structural root names, schist, gneiss or granofels As an example: during field observations the term mylonite may be applied to a fault rock which is derived from a biotite-quartz-plagioclase gneiss In a hand specimen collection of a museum, the same rock may be called a very fine grained biotite-quartz-plagioclase schist, with or without mentioning that it originates from a fault zone It is obvious that the parameter ‘perfection of schistosity’ used for the subdivision into schist, gneiss, and granofels is only a qualitative one Mainly rocks at the transition ‘well developed’ and ‘poorly developed schistosity’ will be qualified by some colleagues as schists, and by others as gneisses Fortunately, no severe complications are expected to arise from this ii Mentioning the mineral contents of metamorphicrocks when applying structural root names: The main information about a metamorphic rock comes from its mode which is semiquantitatively fixed This is done by arranging the names of the major minerals in order of increasing modal abundance and hyphenated amongst themselves in front of the structural root name or, if not present in each individual sample of a group of rocks, behind the root name, preceeded by ‘Ỉ’ and separated by a comma, or if not co-existing, by a hyphen, respectively It is very important that all major minerals are prefixed with reference to a single rock sample or all those major minerals consistently occurring in each specimen of a group of rocks Ambiguities would arise, if this rule was not obeyed The presence of minor minerals (present in a volume of 5% by volume) may be accounted for by prefixing them, hyphenated with the suffix ‘-bearing’, either in front of the major minerals or behind the structural root name When not present in each sample of a set of rocks, a ‘Ỉ’ is set in front of the respective mineral or the paragenesis of minerals SCMR is preparing a list of abbreviations of mineral names containing the abbreviations already published, as well as additional ones These abbreviations may preferentially be used when prefixing minerals in front of rock names Formation of Metamorphic Rocks Introduction Questions 6–8: In petrology, the term metamorphism refers to a change in a rock’s mineralogy, structure and/or composition during geological processes that occur predominantly in a solid state under conditions between diagenesis and large-scale melting Most metamorphic rocks retain some of their parental heritage such as chemical composition and to some extent primary structural features such as bedding The process of metamorphism is driven by changes in physical and/or chemical conditions, usually as a result of large-scale geological processes (plate tectonics) i (i) Schistosity as a parameter for the subdivision of remaining rocks into schist, gneiss, and granofels: Temperature Increasing temperature promotes recrystallization of fine-grained rocks as well as