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