Scapolite (Gr.
asairos, rod, stone), is a group of rock-forming
silicate mineralsThe silicate minerals make up the largest and most important class of rock-forming minerals, comprising approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
composed of
aluminiumAluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13. It is not soluble in water under normal circumstances....
,
calciumCalcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust...
and
sodiumSodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
silicateA silicate is a compound containing an ion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF6]2−, but the silicate species that are encountered most often...
with
chlorineChlorine Chlorine Chlorine ( , from the Greek word 'χλωρóς' (khlôros, meaning 'pale green'), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17 (formerly VII, VIIa, or VIIb). As the chloride ion, which is part of common salt and...
,
carbonateIn chemistry, a carbonate is a salt or ester of carbonic acid, characterized by the presence of the carbonate ion, or a carbonate functional group O=C2....
and
sulfateIn inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
. The variations in composition of the different members of the group may be expressed by the isomorphous mixture of the following:
CaCalcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust...
4(
SiSilicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon...
, Al)
12OOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
24(
CCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
O
3,
SSulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals...
O
4) and
NaSodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
4(Al, Si)
l2O
24ClChlorine Chlorine Chlorine ( , from the Greek word 'χλωρóς' (khlôros, meaning 'pale green'), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17 (formerly VII, VIIa, or VIIb). As the chloride ion, which is part of common salt and...
, which are referred to as the
meioniteMeionite is a silicate mineral belonging to the scapolite group with the formula Ca4Al6Si6O24CO3. Some samples may also contain a sulfate group. It was first discovered in 1801 on Mt Somma, Vesuvius, Italy....
(Me) and marialite (Ma) endmembers respectively. The tetragonal
crystalA crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography...
s are hemihedral with parallel faces (like
scheeliteScheelite is a calcium tungstate mineral with the chemical formula CaWO4. It is an important ore of tungsten. Well-formed crystals are sought by collectors and are occasionally fashioned into gemstones when suitably free of flaws...
), and at times of considerable size. They are distinct and usually have the form of square columns, some cleavages parallel to the prism-faces. Crystals are usually white or greyish-white and opaque, though meionite is found as colorless glassy crystals in the ejected
limestoneLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record...
blocks of Monte Somma, Vesuvius. The hardness is 5 - 6, and the specific gravity varies with the chemical composition between 2.7 (meionite) and 2.5 (marialite). The scapolites are especially liable to alteration by
weatheringWeathering is the decomposition of Earth's rocks, soils and minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice,...
processes, with the development of
micaThe mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic with a tendency towards pseudo-hexagonal crystals and are similar in chemical composition...
, kaolin, etc., and this is the cause of the usual opacity of the crystals. Owing to this alteration, and to the variations in composition, numerous varieties have been distinguished by special names. Scapolite is commonly a mineral of
metamorphicMetamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Both mineralogical, chemical and crystallographic changes can occur during this process.Three types...
origin, occurring usually in crystalline
marbleMarble is a non foliated metamorphic rock resulting from the metamorphism of limestone, composed mostly of calcite . It is extensively used for sculpture, as a building material, and in many other applications...
s, but also with
pyroxeneThe pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
in
schistThe schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced...
s and
gneissGneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. Gneissic rocks are usually medium to coarse foliated and largely recrystallized but do not carry large...
es. The long slender prisms abundant in the crystalline marbles and schists in the
PyreneesThe Pyrenees are a range of mountains in southwest Europe that form a natural border between France and Spain...
are known as dipyre or couzeranite. Large crystals of common scapolite (wernerite) are found in the
apatiteApatite is a group of phosphate minerals, usually referring to hydroxyapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH
−, F
−, Cl
− or...
deposits in the neighborhood of Bamle near Brevik in
NorwayNorway , officially the Kingdom of Norway, is a country in Northern Europe occupying the western portion of the Scandinavian Peninsula, as well as Jan Mayen and the Arctic archipelago of Svalbard under the Spitsbergen Treaty...
, and have resulted from the alteration of the
plagioclasePlagioclase is a very important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
feldsparFeldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
of a
gabbroGabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
.
Scapolite bearing rocks
According to their genesis the scapolite rocks fall naturally into four groups.
1. The scapolite limestones and contact metamorphic rocks. As silicates rich in
calciumCalcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust...
, it is to be expected that these minerals will be found where impure limestones have been crystallized by contact with an igneous
magmaMagma [from Greek μάγμα, paste] is molten rock that is found beneath the surface of the Earth, and may also exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma often collects in a magma chamber inside a volcano...
. Even meionite (the variety richest in soda) occurs in this association, being principally obtained in small crystals lining cavities in ejected blocks of crystalline limestone at Vesuvius and the craters of the Eifel in Germany. Scapolite and wernerite are far more common at the contacts of limestone with intrusive masses. The minerals which accompany them are
calciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 470°C, and vaterite is even less stable....
,
epidoteEpidote is a calcium aluminium iron sorosilicate mineral, Ca
2Al
2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry...
,
vesuvianiteVesuvianite, also known as idocrase is a green, brown, yellow, or blue silicate mineral. Vesuvianite occurs as tetragonal crystals in skarn deposits and limestones that have been subjected to contact metamorphism...
,
garnetThe garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
,
wollastoniteWollastonite is a calcium inosilicate mineral that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolostone is subjected to high temperature and pressure sometimes in the presence of silica-bearing fluids...
,
diopsideDiopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the...
and
amphiboleAmphibole defines an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain SiO
4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles crystallize into two crystal systems,...
. The scapolites are colorless, flesh-colored, grey or greenish; occasionally they are nearly black from the presence of very small enclosures of
graphiticThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek γραφειν : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead...
material. They are not in very perfect crystals, though sometimes incomplete octagonal sections are visible; the tetragonal cleavage, strong double refraction and uniaxial interference figure distinguish them readily from other minerals. Commonly they weather to micaceous aggregates, but sometimes an isotropic substance of unknown nature is seen replacing them. In crystalline limestones and calc-silicate rocks they occur in small and usually inconspicuous grains mingled with the other components of the rock. Large, nearly idiomorphic crystals are sometimes found in
argillaceous rocksArgillaceous minerals appear silvery upon optical reflection and are minerals containing substantial amounts of clay-like components . Argillaceous components are fine-grained aluminosilicates, and more particularly clay minerals such as kaolinite, montmorillonite-smectite, illite, and chlorite...
(altered calcareous
shaleShale is a fine-grained, clastic sedimentary rock composed of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable...
s) which have suffered thermal metamorphism. In the Pyrenees there are extensive outcrops of limestone which are penetrated by igneous rocks described as ophites (varieties of
diabaseDiabase or Dolerite is a mafic, holocrystalline, intrusive igneous rock equivalent to volcanic basalt or plutonic gabbro. In North American usage the term diabase refers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock and diabase refers to altered material....
) and iherzolites (
peridotiteA peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
s). At the contacts scapolite occurs in a great number of places, both in the limestones and in the calcareous shales which accompany them. In some of these rocks large crystals of one of the scapolite minerals (an inch or two in length) occur, usually as octagonal prisms with imperfect terminations. In others the mineral is found in small irregular grains. It is sometimes clear, but often crowded with minute enclosures of
augiteAugite is a single chain inosilicate mineral described chemically as SiO
3 or calcium magnesium iron silicate. The crystals are monoclinic and prismatic. Augite has two prominent prismatic cleavages, meeting at angles near 90°....
,
tourmalineTourmaline is a crystal silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classed as a semi-precious stone and the gem comes in a wide variety of colors...
,
biotiteBiotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K3AlSi3O102. More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the...
and other minerals, such as constitute the surrounding matrix. From these districts also a black variety is well known, filled with minute graphitic enclosures, often exceedingly small and rendering the mineral nearly opaque. The names couzeranite and dipyre are often given to this kind of scapolite. Apparently the presence of
chlorineChlorine Chlorine Chlorine ( , from the Greek word 'χλωρóς' (khlôros, meaning 'pale green'), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17 (formerly VII, VIIa, or VIIb). As the chloride ion, which is part of common salt and...
in small quantities, which may often be detected in limestones, to some extent determines the formation of the mineral.
2. In many
maficMafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term was derived by contracting "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
igneous rocks, such as gabbro and diabase, scapolite replaces feldspar by a secondary or
metasomaticMetasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
process. Some Norwegian scapolite-gabbros (or
dioriteDiorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
) examined microscopically furnish examples of every stage of the process. The chemical changes involved are really small, one of the most important being the assumption of a small amount of chlorine in the new molecule. Often the scapolite is seen spreading through the feldspar, portions being completely replaced, while others are still fresh and unaltered. The feldspar does not weather, but remains fresh, and the transformation resembles metamorphism rather than weathering. It is not a superficial process, but apparently takes place at some depth under pressure, and probably through the operation of solutions or vapours containing chlorides. The basic soda-lime feldspars (
labradoriteLabradorite , a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It is usually defined as having "%An" between 50 and 70. The specific gravity ranges from 2.71 to 2.74. The streak is white, like most silicates. The refractive index ranges from 1.555 to 1.575....
to
anorthiteAnorthite is a compositional variety of plagioclase feldspar. Plagioclase is an abundant mineral in the Earth's crust. The formula of pure anorthite is CaAl
2Si
2O
8.-Mineralogy :...
) are those which undergo this type of alteration. Many instances of scapolitization have been described from the ophites (diabases) of the Pyrenees. In the unaltered state these are ophitic and consist of pyroxene enclosing lath-shaped plagioclase feldspars; the pyroxene is often changed to uralite. When the feldspar is replaced by scapolite the new mineral is fresh and clear, enclosing often small grains of hornblende. Extensive recrystallization often goes on, and the ultimate product is a spotted rock with white rounded patches of scapolite surrounded by granular aggregates of clear green hornblende: in fact the original structure disappears.
3. In Norway scapolite-hornblende rocks have long been known at Oedegrden and other localities. They have been called spotted gabbros, but usually do not contain feldspar, the white spots being entirely scapolite while the dark matrix enveloping them is an aggregate of green or brownish hornblende. In many features they bear a close resemblance to the scapolitized ophites of the Pyrenees. It has been suggrsted that the conversion of their original feldspar (for there can be no doubt that they were once gabbros, consisting of plagioclase and pyroxene) into scapolite is due to the percolation of
chlorideThe chloride ion is formed when the element chlorine picks up one electron to form an anion Cl
−...
solutions along lines of weakness, or planes of solubility, filling cavities etched in the substance of the mineral. Subsequently the chlorides were absorbed, and pan passu the feldspar was transformed into scapolite. But it is found that in these gabbros there are veins of a chlorine-bearing apatite, which must have been deposited by gases or fluids ascending from below. This suggests that a pneumatolytic process has been at work, similar to that by which, around intrusions of
graniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their...
, veins rich in tourmaline have been formed, and the surrounding rocks at the same time permeated by that mineral. In the composition of the active gases a striking difference is shown, for those which emanate from the granites are mainly fluorine and boron, while those which come from the gabbro are principally chlorine and phosphorus. In one case the feldspar is replaced by quartz and white mica (in
greisenGreisen is a highly altered granitic rock or pegmatite. Greisen is formed by autogenic alteration of a granite and is a class of endoskarn.Greisens appear as highly altered rocks, partly coarse, crystalline granite, partly vuggy with miarolitic cavities, disseminated halide minerals such as...
) or quartz and tourmaline (in schorl rocks); in the other case scapolite is the principal new product. The analogy is a very close one, and this theory receives much support from the fact that in Canada (at various places in Ottawa and Ontario) there are numerous valuable apatite vein deposits. They lie in basic rocks such as gabbro and
pyroxenitePyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite and diopside, hypersthene, bronzite or enstatite. They are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both pyroxenes...
, and these in the neighborhood of the veins have been extensively scapolitized, like the spotted gabbros of Norway.
4. In many parts of the world metamorphic rocks of gneissose character occur containing scapolite as an essential constituent. Their origin is often obscure, but it is probable that they are of two kinds. One series is essentially igneous (orthogneisses); usually they contain pale green pyroxene, a variable amount of feldspar, sphene, and iron oxides. Quartz, rutile, green hornblende and biotite are often present, while garnet occurs sometimes;
hyperstheneHypersthene is a common rock-forming inosilicate mineral belonging to the group of orthorhombic pyroxenes. Many references have formally abandoned this term, preferring to categorise this mineral as enstatite or ferrosilite. It is found in igneous and some metamorphic rocks as well as in stony and...
is rare. They occur along with other types of pyroxene gneiss, hornblende gneiss,
amphiboliteAmphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
s, etc. In many of them there is no reason to doubt that the scapolite is a primary mineral. Other scapolite gneisses equally metamorphic in aspect and structure appear to be sedimentary rocks. Many of them contain calcite or are very rich in calc-silicates (
wollastoniteWollastonite is a calcium inosilicate mineral that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolostone is subjected to high temperature and pressure sometimes in the presence of silica-bearing fluids...
,
diopsideDiopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the...
, etc.), which suggests that they were originally impure limestones. The frequent association of this type with graphitic-schists and
andalusiteAndalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section....
-schists makes this correlation in every way probable. Biotite is a common mineral in these rocks, which often contain also much quartz and alkali felspar.