porphyry, still alluring after all these years

[Tomas Lipps <tmlipps@zzzzzzzzzzzzz>]

"In a geologic sense, porphyry is not a rock type at all. . ."
a while ago there was a discussion thread about porphyry, I was too
preoccupied at the time to join in, otherwise I might have dug around
to find and share this paper written by my next door neighbour, John
Gillentine, and given to me after a discussion he and I had about
this  marvelous material.

I had in mind then to do a lengthy feature for STONEXUS Magazine
about porphyry projectively titled, "Still Alluring After All These
Years."  That never happened, at least not in to the extent imagined,
but I did manage to include an article about the Roman quarries of
Imperial Porphyry in southern Egypt that had been previously printed
in the 1998 November/December issue of a very interesting magazine,
"Saudi Aramco World"  here is the link:
http://www.saudiaramcoworld.com/issue/199806/via.porphyrites.htm  if
you're interested enough to check it out, be sure to click on the
link at the bottom of the page for the digital image archive.  there
are some excellent photographs there.

but I am happy to share John's rather scholarly elucidation on
Porphyry.  if it's too daunting technologically, there's a
description "in a nutshell" at the end.

enjoy.

Tomas

THE PETROLOGY OF PORPHYRY			By John Gillentine

Porphyry has long been used in works of art and as a building
material because of the intrinsic beauty of the stone.  But what
exactly is porphyry?  In a geologic sense, porphyry is not a rock
type at all, but is a textural term used to describe the size
distribution of mineral grains within an igneous rock.  Before
discussing the specifics of porphyry, however, it may be useful to
first review some nomenclature and a few basic geologic principles.

First Principles
All rocks are classified according to their formative mechanism into
one of three groups: igneous rocks of molten or magmatic origin
(e.g., granite and basalt); sedimentary rocks derived either from
chemical precipitation or from the erosion and re-deposition of
pre-existing rock (e.g., limestone and shale); and metamorphic rocks
derived from the deformation and recrystallization of pre-existing
rock through heat and/or pressure (e.g., marble and schist).  Igneous
rocks make up approximately 95% of the earth's crust, but are largely
hidden by a thin veneer of sedimentary and metamorphic rocks.

Igneous rocks are subdivided into two types, intrusive and extrusive.
Extrusive or volcanic igneous rocks reached the earth's surface in a
molten or partially molten state as a fluidized lava flow that poured
 from a fissure or vent, or as ejecta from an explosive eruption.
Volcanic rocks tend to cool and crystallize rapidly, producing
mineral grains that are typically small to microscopic.  If cooling
proceeds at a rate too rapid to allow growth of even microscopic
crystals, the resultant rock will consist entirely of glass (glass is
an extremely viscous liquid lacking an internal structure).
Intrusive igneous rocks, on the other hand, derive from
crystallization of magmatic material that did not reach the earth's
surface, and that intruded into the surrounding country rock along
existing bedding planes, joints or cracks, or by deformation and
cross-cutting of the country rock.  The term pluton refers to a
relatively large magma body that cooled and crystallized at depth in
a manner discordant with existing geological structure or
stratigraphy, and includes large volume intrusions (greater than 100
km2) called batholiths, and medium volume intrusions called stocks.
A dike is a small volume, tabular or sheet-like discordant intrusion
that cuts across the fabric of existing rock, and a sill is a tabular
shaped, concordant intrusion that squeezes between the bedding planes
or fabric of existing rock.  Intrusive igneous rocks typically cool
at rates slow enough to allow crystallizing minerals to grow to
relatively large sizes, giving the rock a medium to coarse-grained
appearance.

Volcanism and Magmatic Style
Most of the magma that reaches the earth's surface as extrusive rock
does so through one of the three major types of volcanoes--shield,
cinder cone and composite--that differ markedly in their size, shape
and product composition.  Shield volcanoes are broad, gently sloping
cones named for their flattened dome or shield-shaped profile and are
built from highly fluidized lava that solidified around a central
vent.  Cinder cones are constructed of loose rock fragments ejected
 from a central vent, most of which lands near the vent and serves to
build up the cone to its characteristic steep-sided peak.  Composite,
or stratovolcanoes are more or less symmetrical structures built from
alternating bands of lava flows and ejected volcanic rocks fragments,
and have side slopes that are intermediate in steepness when compared
to shield volcanoes and cinder cones.  Composite volcanoes may have
an extensive network of feeder dikes, sills and plugs that allow
magma to erupt from the flank of the cone.

Eruptions from shield volcanoes, such as those regularly seen on
Mauna Loa on the island of Hawaii, are relatively non-violent because
the low viscosity of their basaltic lava allows gases dissolved in
the magma to easily escape.  The two styles of flows that typify
shield volcanoes are given the Hawaiian names pahoehoe
("pah-hoy-hoy"), which describes a ropy or billowy surface character,
and aa ("ah-ah"), which describes a rubbly, jagged surface character.

Material ejected from cinder cones such as Cerro Negro in Nicaragua,
which are often also seen built on the flanks of larger shield
volcanoes, may be very similar in chemical composition to the
basaltic lava characteristic of shield volcanoes.  The explosive
nature of their eruptions reflects localized pockets of gas within
the magma chamber that forcefully escape to the surface.  Escaping
gases may carry pasty chunks of rapidly cooling magma called
pyroclasts or tephra.  Pyroclasts range in size from ash (less than 2
millimeters), to lapilli (2-64 millimeters), to angular blocks and
spindle-shaped bombs (greater than 64 millimeters).  Vesicles of
escaped gases are conspicuously preserved in material ejected from a
cinder cone, giving the rock its characteristic scoriaceous texture.

The gas content of shield volcanoes and cinder cones is typically low
and the lava sufficiently fluid that eruptions, although spectacular,
are seldom catastrophic.  This is not true of eruptions from
composite volcanoes.  Cataclysmic volcanic events recorded over the
course of human history are almost always associated with the
violent, explosive eruptions of composite volcanoes.  Mount St.
Helens in Washington, Mount Pinatubo in the Philippines, Mount
Vesuvius in Italy, Mount Pelee on the island of Martinique, and Mount
Fujiyama in Japan are well known examples of composite volcanoes
whose eruptions have claimed thousands of lives.  Part of this cost
in human suffering stems from the fact that composite cones are built
up over very long periods of time, with quiescent periods between
eruptions spanning hundreds to thousands of years.  The myth of the
"extinct" volcano is attributable in large part to the serene,
snow-covered peaks of composite volcanoes that may have slumbered for
the whole of human existence.  Communities flourish on the rich soil
of their flanks.

Magma beneath composite volcanoes is predominantly andesitic in
composition, though eruptions may produce rhyolite during one event
and basalt in another.  The rock type andesite takes its name from
the Andes Mountain Range of western South America, and is very often
found to exhibit a porphyritic texture.  If the temperature of the
andesite melt is considerably above the temperature at which the rock
solidifies, the fluidized lava may flow easily down the flanks of the
volcano.  If, on the other hand, the lava is viscous and the gas
pressure high, an explosive eruption may ensue that litters the
surrounding countryside with pyroclastic debris, particularly if
vents and fissures are clogged with partially solidified material.
Eruptions also tend to melt those beautiful white snowcaps, sending
torrents of mud and floodwater towards the fertile valleys below.

The Classification of Rocks
Regardless of type, all rocks (with the exception of coal and certain
volcanic glasses) consist of minerals, which in turn consist of
orderly arrays of atoms with specific crystal structures and chemical
compositions.  Although the word "mineral" has a variety of meanings
(even amongst geologists), the generally accepted definition given in
Klein and Hurlbut's classic Manual of Mineralogy1 is "a naturally
occurring homogeneous solid with a definite (but generally not fixed)
chemical composition and a highly ordered atomic arrangement.  It is
usually formed by inorganic processes."  Rocks are simply aggregates
of minerals.  In some cases, the chemical and mineralogical
composition of categorically different rocks may be quite similar,
differing only in the constituency of some of their accessory
minerals.  For example, granite and rhyolite have compositions that
are nearly identical, yet these two rock types have extremely
different magmatic histories and tell a very different geologic
story.  For this reason, as well as for the convenience of
identifying rocks in the field without the aid of laboratory
analysis, rocks are classified not only on the basis of their
mineralogical and chemical composition, but also on the
grain-to-grain relationships visible within the rock.  These
relationships are collectively referred to as texture, which
addresses the size, shape, arrangement and crystallinity of the
components of a rock.

There are four principle classification textures that occur in rocks
of magmatic origin: phaneritic, aphanitic, glassy and clastic2.
Phaneritic rocks have mineral grains that are sufficiently large to
be identifiable in hand sample, whereas aphanitic rocks have mineral
grains that are too small to be identified without the aid of a
microscope.  Glassy rocks (such as obsidian) typify lava flows and
shallow intrusions that lose heat so rapidly that atoms in the
silicate melt have insufficient opportunity to organize into the
regular geometric arrays of crystals.  Clastic rocks contain
aggregated clasts or broken fragments of pre-existing rocks, minerals
or glass bound together by newly crystallized minerals smaller in
size than the clasts.  Phaneritic and aphanitic rocks may be
equigranular, consisting of grains all about equal in size, or may be
inequiqranular, consisting of conspicuously larger mineral grains
called phenocrysts within a finer-grained matrix or groundmass.  The
textural term porphyritic--the subject of this issue--refers simply
to the presence of crystals of distinctly different size within the
same rock.  A rock can be porphyritic-aphanitic or
porphyritic-phaneritic, depending upon the grain size of the matrix.
A glassy rock with scattered crystals is described as vitrophyric; a
clastic rock with fragments of glassy material is described as
vitroclastic.  Any igneous rock may exhibit a porphyritic texture,
but it is more commonly seen in volcanic rocks such as rhyolite,
andesite and dacite.

Crystallization and the Origins of Texture
The textural relationships visible in a rock are determined by the
way in which individual minerals crystallize from the molten or
liquid state, which is a two-step process involving crystal
nucleation and crystal growth.  Nucleation occurs where ions come
together in a regular structural pattern to form the initial products
of crystallization, and crystal growth occurs through accretion of
additional ions to the surface of a nucleated crystal-something like
stacking ionic blocks to the first row of a block wall.  The relative
rates of nucleation and crystal growth are key controls on the size
and range in size of crystals within a rock.  If the rate of
nucleation is low and the crystal growth rate high, such as occurs in
most plutonic rocks, the resulting texture will include fewer, larger
crystals.  If, on the other hand, the nucleation rate is high and the
crystal growth rate low, the result is a higher number of small
crystals and a much finer-grained texture.

Crystallization may be homogeneously distributed throughout the melt,
such that nucleation sites are more or less equally spaced and
crystals grow at similar rates to approximately equal sizes, or
crystallization may be heterogeneously distributed throughout the
melt.  A porphyritic texture suggests heterogeneous crystallization
of a cooled or cooling magma body, and is believed by most geologists
to result either from differential cooling within the magma chamber
or from sequential growth of minerals in differing states of chemical
equilibrium.  The differential cooling model is the traditional
interpretation of porphyritic texture in which magma, existing in a
particular temperature/pressure regime, begins to nucleate and grow
mineral crystals of a particular type and composition.  A change to a
lower temperature/pressure regime, such as occurs through movement or
dislocation of the magma body during a volcanic event, results in
higher nucleation rates, lower crystal growth rates and later-formed
crystals that are more numerous but far smaller than early-formed
crystals.  This change in the T/P regime also shifts the chemical
equilibrium between the melt and the crystallizing minerals, such
that the later-formed, fine-grained components (the groundmass) will
differ not only texturally, but also chemically and mineralogically.

Another possible mechanism for the development of porphyritic texture
is the sequential growth model.  This is an in-situ, heterogeneous
crystallization process in which the composition of the magma
"evolves" as chemical constituents are selectively removed through
crystallization.  In a slowly cooling magma, early-nucleated minerals
crystallizing in a highly fluid, low viscosity environment will have
unlimited room to grow, and will tend to develop crystal faces and
interfacial angles characteristic of that mineral.  Later-nucleated
minerals, growing within the confines or "void spaces" left between
early-formed minerals, will have insufficient room to develop their
characteristic crystal habit and will be of limited size.

Norman. L. Bowen, an experimental petrologist with the Geophysical
Laboratory of the Carnegie Institution in Washington, D.C.,
determined in the early 1900's that specific minerals crystallize
 from a melt at specific temperatures.  In what has become known as
Bowen's Reaction Series, Bowen and his coworkers demonstrated that
those minerals with the highest melting temperatures--the
ferromagnesian minerals with low silica content but a relatively high
content of iron, magnesium and calcium--crystallize from a cooling
magma before those with lower melting temperatures.  Bowen also
demonstrated that magma of any composition could be derived from an
originally basaltic parent magma through a process of
differentiation.  For example, an early-formed olivine crystal may
react with the cooling residual melt to form pyroxene; the pyroxene
may react with the melt to form amphibole; and the amphibole may
react with the melt to form biotite.  Biotite is the last of the
ferromagnesian minerals to crystallize, and any magma remaining after
biotite crystallization will be depleted in iron and magnesium and
will be richer in silica, aluminum, sodium and potassium than the
original magma.  Late-stage crystallization products will therefore
consist of the relatively low-temperature minerals orthoclase (an
alkali feldspar) and quartz.  Rhyolite, the most abundant of the
silica-rich volcanic rock and one commonly observed with a
porphyritic texture, is light in color because of its low iron and
magnesium content.  Not all magma progresses entirely through the
reaction series, however, before its constituent elements are used up
and the composition of the rock becomes "fixed" with a
higher-temperature mineral assemblage.  Olivine-rich basalt is an
example of a common rock type having a mineral assemblage
characteristic of the high-temperature end of Bowen's Reaction Series.

The Naming of Rocks
There are several hundred families and species of minerals identified
so far, with new minerals identified and added to the inventory on a
continuing, if infrequent, basis.  Fortunately, there are only a
relatively few common or "rock forming" minerals used to classify
igneous rocks.  These are quartz and the feldspar, pyroxene, mica,
Fe-Ti oxide, olivine and amphibole groups.  The feldspars are by far
the most common mineral group in magmatic rocks, and there are very
few igneous rocks in which feldspar is absent.  In fact, most igneous
rocks contain over 50% feldspar.  Quartz may be a major constituent
in one rock type, yet be an accessory mineral in another.

Volcanic rocks are named on the basis of phenocryst mineralogy alone,
since the presence of glass and microcrystalline groundmasses makes
rigorous mineralogical evaluation impractical for field and
preliminary laboratory classification.  The following table may be
used as a guide for naming volcanic rocks based on phenocryst
mineralogy.  Note that phenocrysts may be macroscopic or microscopic
when used to name a rock.  For a rock to be considered porphyritic,
however, the phenocrysts must be visible with no more assistance than
that of a hand lens.

Naming Volcanic Rocks on the Basis of Phenocrysts (modified from
Best, 1982; where appropriate, names for equivalent coarse-grained
plutonic rocks are given in parentheses)
Rocks with quartz	phenocrysts of sanidine (an alkali feldspar)
and quartz +/- plagioclase; biotite or pyroxene generally less than
5%	Rhyolite
(Granite)
	phenocrysts of plagioclase and quartz; alkali feldspar
commonly absent; quartz may be scarce; hornblende, pyroxene  and
biotite all likely	Dacite
Rocks without quartz, feldspathoids, melilite or analcite
	phenocrysts of plagioclase and lesser alkali feldspar;
biotite or pyroxene present +/- scarce olivine	Trachyte
	phenocrysts of plagioclase and lesser alkali feldspar;
hornblende, biotite or pyroxene present +/- scarce olivine	Latite
	abundant phenocrysts of plagioclase, with or without
pyroxene, hornblende or biotite; +/- scarce olivine; alkali feldspar
absent	Andesite
(Diorite)
	phenocrysts of olivine, pyroxene and generally minor
plagioclase; (high alumina basalt may have abundant plagioclase)
	Basalt
(Gabbro)
Rocks with feldspathoids, melilite or analcite	alkali feldspar
abundant and greater than plagioclase; pyroxene, biotite and
amphiboles all possible	Phonolite
(Syenite)
	plagioclase abundant and greater than alkali feldspar;
clinopyroxene abundant; no olivine	Tephrite
	plagioclase abundant and greater than alkali feldspar;
clinopyroxene abundant; with olivine	Basanite
(Theralite)
	feldspathoids abundant; little or no feldspar; clinopyroxene
abundant +/- olivine	Nephelinite
(Ijolite)

The texture of a rock is typically used as a modifier in the rock
name, which generally also includes the dominant mineralogy.  For
example, a latite with macroscopic biotite phenocrysts may be termed
a porphyritic biotite latite, and an andesite with macroscopic
hornblende (an amphibole) may be termed a porphyritic hornblende
andesite.  Porphyry is a fairly common igneous texture, but
quarry-able deposits are relatively rare because volcanic activity
and its products are highly variable in their nature at both the
microscopic and geographic scales.

References

1	Klein, Cornelius, and Hurlbut, Cornelius S., Jr., 1977;
Manual of Mineralogy, 20th edition; John Wiley and Sons, New York, NY

2	Best, Myron G., 1982, Igneous and Metamorphic Petrology, W.H.
Freeman and Company, New York, NY

3 Plummer, Charles C., and McGeary, David, 1979; Physical Geology,
3rd edition; William C. Brown Publishers, Dubuque, IA

John Gilletine's "The Petrology of Porphyry"

is a scholarly elucidation on the nature, not only of porphyry, but
of igneous rock in general.  It offers valuable information. Those
lacking the initiative and endurance needed to navigate such densely
technical prose (a petrified forest) may be satisfied with this
sketch of the subject

IGNEOUS "fire-formed rocks"
CRYSTALLIZE from molten material:
*	MAGMA - below the Earth's surface
* 	LAVA - erupts onto the Earth's surface through a volcano or
crack (fissure)
lava cools more quickly because it is on the surface.

COOLING RATES influence the texture if the igneous rock:
*	Quick cooling = fine grains
*	Slow cooling = coarse grains

CLASSIFICATION of igneous rocks is based are on texture and composition.

PORPHYRITIC- Mixture of grain sizes caused by mixed cooling history;
slow cooling first, followed by a period of somewhat faster cooling.

TEXTURAL COMPONENTS:
*	PHENOCRYSTS - the large crystals
*	MATRIX or GROUNDMASS - the finer crystals surrounding the
large crystals. The groundmass may be either aphanitic or phaneritic.
*	Mixed grain sizes imply mixed cooling rates, the upward
movement of magma from a deeper (hotter) location of extremely slow
cooling, to either:
*	a much shallower (cooler) location with fast cooling
(porphyritic- aphanitic), or a somewhat shallower (slightly cooler)
location with continued fairly slow cooling (porphyritic-phaneritic).

*	GRANITE PORPHYRY or PORPHYRITIC GRANITE
(porphyritic-phaneritic) - phenocrysts usually potassium feldspar
*	ANDESITE PORPHYRY or PORPHYRITIC ANDESITE
(porphyritic-aphanitic) - phenocrysts usually hornblende
*	RHYOLITE PORPHYRY or PORPHYRITIC RHYOLITE (porphyritic-aphanitic)
(the imperial purple porphyry belongs to this category)

 from http://www.dc.peachnet.edu/~pgore/geology/geo101/igneous.htm
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