See:neutral stress
An instrument used to determine the porosity of a rock sample by comparing
the bulk volume of the sample with the aggregate volume of the pore spaces
between the grains. Porosimeters are of various designs, some using
liquids and some using gases, at known pressures, to find the volume of
openings. AGI
a. The ratio, P, expressed as a percentage of the volume, Vp, of the pore
space in a rock to the volume, Vr, of the rock, the latter volume
including rock material plus the pore space; P = 100 Vp/Vr.
Holmes, 2
b. The amount of void space in a reservoir usually expressed as percent
voids per bulk volume. Absolute porosity refers to the total amount of
pore space in a reservoir, regardless of whether or not that space is
accessible to fluid penetration. Effective porosity refers to the amount
of connected pore spaces; i.e., the space available to fluid penetration.
Syn:total porosity
Evolved by Professor H. Briggs in 1931 to express the conductance of a
waste to air leakage, per foot length of the roadway per foot width of the
leakage zone. Roberts, 1
Containing voids, pores, cells, interstices, and other openings, which may
or may not interconnect. See also:porosity
Any assemblage of rock material that, as a result of fracturing, faulting,
mode of deposition, etc., contains a high percentage of voids, pores, and
other openings. Long
Nonpolarizable electrode consisting of a metal bar immersed in a saturated
electrolytic solution which is contained in a porous pot.
Schieferdecker
A native alloy of argentiferous gold with palladium, the palladium content
varying up to 10%. From Porpez, Brazil. Syn:palladium gold
An obsolete term synonymous with porphyry. The term was originally used to
distinguish porphyries that contain plagioclase phenocrysts from those
that contain alkali feldspar phenocrysts. AGI
a. Said of the texture of an igneous rock in which larger crystals
(phenocrysts) are set in a finer-grained groundmass, which may be
crystalline or glassy or both. Also, said of a rock with such texture, or
of the mineral forming the phenocrysts. AGI
b. Pertaining to or resembling porphyry. AGI
Volcanic glass having microcrystalline phenocrysts.
See:porphyritic
A pseudoporphyritic crystal in a rock produced by metamorphic
recrystallization. Adj: porphyroblastic. Syn:metacrystal;
pseudophenocryst. AGI
a. Pertaining to the texture of a recrystallized metamorphic rock having
large idioblasts of minerals possessing high form energy (e.g., garnet,
andalusite) in a finer-grained crystalloblastic matrix. AGI
b. See:pseudoporphyritic
A rock fragment contained in mylonite.
See:mortar structure
Said of the texture of a diabase porphyry having phenocrysts of
plagioclase and augite in a ground mass of plagioclase laths and augite.
Said of or pertaining to a blastoporphyritic or sometimes porphyroblastic
metamorphic rock of igneous origin, or a feldspathic metasedimentary rock
having the appearance of a porphyry. It occurs in the lower grades of
regional metamorphism. AGI
An igneous rock of any composition that contains conspicuous phenocrysts
in a fine-grained groundmass; a porphyritic igneous rock. The term (from a
Greek word for a purple dye) was first applied to a purple-red rock
quarried in Egypt and characterized by phenocrysts of alkali feldspar. The
rock name descriptive of the groundmass composition usually precedes the
term; e.g., diorite porphyry. Obsolete syn: porphyrite. AGI
A large body of rock, typically porphyry, that contains disseminated
chalcopyrite and other sulfide minerals. Such deposits are mined in bulk
on a large scale, generally in open pits, for copper and byproduct
molybdenum. Most deposits are 3 to 8 km across, and of low grade (less
than 1% Cu). They are always associated with intermediate to felsic
hypabyssal porphyritic intrusive rocks. Distribution of sulfide minerals
changes outward from dissemination to veinlets and veins. Supergene
enrichment has been very important at most deposits, as without it the
grade would be too low to permit mining. AGI
a. A deposit in which minerals of copper, molybdenum, gold, or less
commonly tungsten and tin, are disseminated or occur in a stockwork of
small veinlets within a large mass of hydrothermally altered igneous rock.
The host rock is commonly an intrusive porphyry, but other rocks intruded
by a porphyry can also be hosts for ore minerals.
b. A deposit, usually of copper, molybdenum, or tin, in igneous rock of
any composition that contains larger crystals in a fine-grained
groundmass. SME, 1