A comment by J. Wright Horton, Jr. about
To follow-up on Steve's summary of questionnaire results, the suggested definitions below are offered as templates for you to accept, revise, or replace. Each is accompanied by more of the discussion Steve initiated. Are these definitions acceptable? What would you change? Thanks! Wright Steve's question #1 follow-up AMPHIBOLITE - A metamorphic rock composed mainly of amphibole and plagioclase with amphibole comprising 35-90 % of the rock. It includes ortho-amphibolite (igneous protolith) and para-amphibolite (sedimentary protolith). Discussion: I didn’t find the 50% figure in Robertson, and many typical amphibolites have <50% amphibole. The Glossary of Geology (Jackson, 1997, 4th ed.) defines amphibolite as “A crystalloblastic rock consisting mainly of amphibole and plagioclase with little or no quartz. As the quartz content increases, the rock grades into hornblende-plagioclase gneiss.” According to Robertson (1999, p. 7) “Amphibolite facies metamafic rocks are traditionally termed amphibolites” and “Metamafic rocks contain between 35 and 90% mafic minerals.” Robertson’s next page defines amphibolite as ”rocks composed predominantly of hornblende and plagioclase…” According to Yardley (1991, p. 26-27), amphibolite is “An essentially bimineralic dark-green rock made up of hornblende and plagioclase. A wide range of minerals may occur as accessories. Most amphibolites are metabasites (ortho-amphibolites) but some may be metamorphosed calcareous sediments (para-amphibolites.).” Steve's question #4 follow-up SCHIST - A medium- to coarse-grained, foliated metamorphic rock having schistosity (as defined by Passchier and Trouw, 1996) so well developed that the rock can split into thin flakes or sheets [IUGS interprets “thin” as <1 cm, whereas Steve proposes 5 mm]. Compositional layering may or may not be present. Discussion: Robertson overlooked the main characteristic of schist, which is schistosity! Here are some definitions of schist: (a) “Metamorphic rock commonly of pelitic composition, with a well developed schistosity.” (Barker, 1998, p. 242). (b) “Characterised by parallel alignment of moderately coarse grains, usually clearly visible with the naked eye…This type of fabric is known as schistosity…” (Yardley, 1991, p. 22) (c) "A strongly foliated crystalline rock, formed by dynamic metamorphism, that can be readily split into thin flakes or slabs due to the well developed parallelism of more than 50% of the minerals present, particularly those of lamellar or platy habit, e.g. mica and hornblende. The mineral composition is not an essential factor in its definition..." (Jackson, 1997) [Do the last two sentences contradict each other?] (c) A metamorphic rock having “schistosity well developed”… “A schistosity is said to be well developed if inequant mineral grains or grain aggregates are present in a large amount and show a high degree of preferred orientation, either throughout the rock, or in narrowly spaced repetitive zones, such that the rock will split on a scale of less than one cm.” (IUGS SCMR flowchart) Schistosity is defined as (a) “The foliation in schist or other coarse-grained, crystalline rock due to the parallel alignment of platy mineral grains (mica) or inequant crystals of other minerals" (Jackson, 1997), or (b) a “secondary foliation defined by preferred orientation of inequant fabric elements in a medium to coarse grained rock. Individual foliation-defining elements are visible with the naked eye” (Passchier and Trouw, 1996). GNEISS - A medium- to coarse-grained, foliated metamorphic rock characterized by alternating, mineralogically distinct compositional layers parallel to foliation, such as quartz-feldspar layers segregated from micaceous or mafic layers. Schistosity may be present in some layers more than others but is not well developed throughout the rock. [*IUGS SCMR considers schistosity “well developed” if the rock splits into <1cm sheets whereas Steve proposes 5 mm] Discussion: Barker (1998, p. 235) defines gneiss as “A coarsely banded high-grade metamorphic rock consisting of alternating, mineralogically distinct (usually felsic and mafic) layers” (Barker, 1998, p. 235). Yardley (1991, p. 22) points out that “English and North American usage [of gneiss] emphasizes a tendency of different minerals to segregate into layers parallel to the schistosity, known as gneissic layering; typically quartz- and feldspar-rich layers segregate out from more micaceous or mafic layers. European usage of gneiss is for coarse, mica-poor, high-grade rocks, irrespective of fabric.” The Glossary of Geology ( Jackson, 1997) defines gneiss as "A foliated rock formed by regional metamorphism, in which bands or lenticles of granular minerals alternate with bands or lenticles in which minerals having flaky or elongate prismatic habits predominate. Generally less than 50% of the minerals show preferred parallel orientation. Although a gneiss is commonly feldspar- and quartz-rich, the mineral composition is not an essential factor in its definition." [Do the last two sentences contradict each other?] The same glossary defines gneissic banding as "A type of foliation in metamorphic rock defined by compositional banding. Typically, gneissic banding consists of alternating dark (mafic) and light(felsic or silicic) bands of rock." Passchier and Trouw (1996, p. 259) define "gneissic layering" simply as “compositional layering in a gneiss” and note that use of the term “gneissosity” is discouraged. Steve's question 6 follow-up MARBLE – “A metamorphic rock consisting predominantly of fine- to coarse-grained recrystallized calcite and/or dolomite, usually with a granoblastic, saccharoidal texture.“ (Jackson, 1997) Discussion: Can we accept this latest Glossary of Geology definition with insertion of “(>50%)” after the word “predominantly? CALCSILICATE ROCK – “A metamorphic rock consisting mainly of calcium-bearing silicates such as diopside or wollastonite, and formed by metamorphism of impure limestone or dolomite.” (Jackson, 1977) Discussion: Can we accept this Glossary of Geology definition with insertion of “(>50%)" after the word “mainly”?
(This page is the only comment that has been posted so far.)Further discussion of amphibolite, schist, gneiss, marble, calcsilicate rock (this page):
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