amphibolite, schist, gneiss, marble, calcsilicate rock

Wed Apr 25 14:03:49 2001

A comment by J. Wright Horton, Jr. about

responses to SLTT metamorphic classification questions

by Stephen M. Richard

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
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

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
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”?

Context of this discussion

This page is part of a discussion of responses to SLTT metamorphic classification questions:
(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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