Inclusions

 

last modified: Monday, 25-Apr-2011 13:45:41 CEST

Document status: incomplete

Quartz grows in many environments along with many different minerals. These minerals, as well as watery solutions and gas bubbles, can be enclosed by the growing crystal.
Many minerals that would otherwise be chemically altered or dissolved when the local conditions change are protected from aggressive chemical agents when they have been embedded inside quartz crystals. Likewise, fragile minerals that are very soft, crumbly or that occur as fibers or thin needles survive inside. Other partially embedded minerals might get dissolved at a later stage and leave ghostlike hollow forms behind.

There are basically three ways inclusions can "get into the crystal":

  • The minerals have formed before the quartz. The growing quartz crystals engulfs them and the original form of the included minerals is preserved. These inclusions are called protogenetic. Included fibers than run through the entire crystal at random orientations are typical examples.
  • Quartz crystals and the included minerals grow simultaneously. These inclusions are called syngenetic. The shape of the included minerals often deviates from the typical forms and habits that develop during unhindered growth. Crystals may be distorted beyond recognition and a non-destructive identification may pose a real problem even for a mineralogist.
    Sometimes inclusions cause the formation of phantoms. Here the quartz crystal might have been partially encrusted by another mineral when growth halted transiently and continued later, such a case could be considered a syngenetical formation that got overgrown.
  • Minerals can get into another mineral by exsolution. When the conditions during crystal growth allowed the incorporation of elements into the crystal lattice that are incompatible with the crystal structure at different temperatures or pressures, these elements may separate from the lattice to form new minerals once the conditions change. Such inclusions are called epigenetic. Very often these inclusions are specifically oriented with respect to the crystallographic axes of the main crystal. The best example of a quartz with epigenetic inclusions is rose quartz.

    • Since the included minerals, liquids, and gases are well protected from chemical alteration, quartz inclusions open a window to the past to the scientist. Many crystals carry inclusions, but quartz has a simple chemical composition and does not complicate the analysis of the included material too much and does not interfere with substances used in chemical tests.

    When the studied crystals are large and grew slowly - like rock crystals from alpine-type fissures - one can even observe systematic variations in the composition of the material that has been included during growth. In the central oldest part of the crystal, for example, the salt content of liquids might be higher, while the outer part of the crystal might contain more carbon dioxide.

    Inclusions can also be used to estimate the temperature at which the crystals formed.


     

    Amphibole-Group

    Amphiboles are an important group of minerals found in medium to high temperature environments, and some of them are rock-forming minerals. They form in silica-poor and silica rich rocks that are not completely void of water, mostly in metamorphic rocks and in basic to intermediate igneous rocks, but also during metasomatosis in skarn rocks. They are not uncommon in Alpine-type fissures, either as thin needles or as fibrous aggregates ("Bergleder"). Amphiboles are complex chain silicates with hydroxile (OH) groups of the general composition X0-1Y2Z5(Si,Al)8O22(OH,Cl,F)2, with X, Y and Z representing various metal cations. The chain-silicate structure is reflected in their often needly or fibrous appearence, and some amphiboles have been mined as asbestos.


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    Thin gray-green fibers of actinolite run through these rock crystals from the Tipling Mine in the Dhading-District, Nepal. While the tip only contains a few individual fibers, the base is filled with a dense fabric. Since there are no growth inhibitions found on the rock crystal and since the fibers run through the crystal at random directions, the fibers very likely predate the rock crystals and have been embedded in the quartz substance later. An example for an Alpine-type fissure environment occurrence of amphiboles.



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    Prase found in skarn rocks is most commonly colored by finely distributed fibers of actinolite. This single prase crystal from Russia, very likely from Dalnegorsk, shows the typical small sprouting quartz crystals all over the prism faces. One can see a few of the larger green actinolite needles at the tip of the crystal, while the base is filled with a dense fibrous fabric of actinolite.


     

    Anatase, Brookite, Rutile

    Titanium is not a rare metal, but it is finely distributed and only rarely found in large concentrations. The three polymorphs of its oxide, TiO2, Anatase, Brookite and Rutile are typically found in metamorphic rocks, where they formed from other titanium-bearing minerals during metamorphosis. Nice crystals can be found in alpine-type fissures, in particular in metamorphic rocks like mica shists. They can also be found in pegmatites. Rutile is quite common, anatase and brookite are much rarer. Rutile has the widest stability field and forms at moderate and high temperatures in hydrothermal environments, anatase is more common in low-temperature environments. Accordingly, rutile is more likely to be found as inclusion deeply inside quartz crystals or as needles that run through the entire crystals because their formation precedes that of the quartz crystals. Anatase tends to form at the lower temperatures of later stages in the development and thus often grows on the crystal faces. The three minerals occassionally occur together.

    Anatase


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    Anatase  is a typical mineral of alpine-type fissures in mica-rich gneiss and certain schists. Anatase occurs in a great number of crystal shapes and colors, but the most common and also basic form is that of a tetragonal bipyramid. Perhaps the best known anatase location is the Hardangervidda, a high plain in the mountains of southern Norway. Here large blue bipyramidal crystals with a metallic shine have been found together with rock crystals, very often grown on them, more rarely completely included. These specimen occur in alpine-type fissures in mica schists. The image shows blue anatase crystals partially embedded in a clear rock crystal from that location.



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    A nice anatase crystal sits on the back side of a Dauphiné habit rock crystal with chlorite inclusions. The anatase measures 14 mm. The specimen was found in the rubble below an old emptied alpine-type cleft in mica schist east of Storenuten mountain, north of the Ringedalsvatnet lake at Odda, Hordaland, Norway.


    Brookite


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    Gray  rutile needles emanate epitactically from a thin brown brookite crystal at the base of the left rock crystal on a specimen with white adular and anatase (the metallic-blue crystal right of the center). So all three titanium oxide polymorphs, anatase, brookite, and rutile occur on this specimen. The blue-gray patches above the rutile needles are a single small anatase crystal sitting on the quartz crystal (you see three patches because of the refraction of the light). There's another interesting inclusion inside the large rock crystal: at the left side you see the iridescent reflections of an included small quartz crystal. From an alpine-type cleft in mica schist east of Storenuten mountain, north of the Ringedalsvatnet lake at Odda, Hordaland, Norway.


    Rutile


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    Golden  rutile needles in a rock crystal from Novo Horizonte, Minas Gerais, Brazil



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    Epitactical intergrowth of rutile and hematite is very common, and if the rutile needles spread out from a single hematite crystal or a hematite rose, the result can be a six-rayed star. Here the rutile needles emanating from such an -albeit imperfect- star apparently spear a neighboring rock crystal, but the rock crystal grew after the rutile, of course. This specimen is also from Novo Horizonte, Minas Gerais, Brazil.



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    Thin silvery rutile needles in random orientations have been captured in this rock crystal from Diamantina in Minas Gerais, Brazil. They sit before a white phantom made of fine, sandy pocket detritus.


     

    Chlorite-Group

    Chlorite inclusions are very common in alpine-type environments, and generally occur in fissures and pockets inside igneous and metamorphic rocks, and in sedimentary rocks that are rich in clay minerals. "Chlorite" is actually the name for a group of phyllo-silicates (sheet-silicates), minerals of mica-like appearance, the most common of which is clinochlore, (Mg,Fe2+)5Al[(OH)8|AlSi3O10]. The name refers to the commonly green color, although chlorite minerals do not have to be green.

    Chlorite minerals form at low to moderate temperatures, often as a product of low- to medium-grade regional metamorphosis. Often quartz from alpine-type clefts has a chlorite "icing" on the crystal surface, giving them a rough and dull look, because the crystals started to grow at high temperatures, and when their growth slowed down at lower temperatures, chlorite formed in the pocket and settled on the crystal faces. Pockets in the central Alps are often completely filled with chlorite, which can be both a blessing or a curse for a Strahler looking for splendid crystals. The pocket clay protects the crystals from damage, but it may also cause a dull surface.


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    These clear smoky quartz crystals contain dense inclusions of green vermicular (worm-like) chlorite. In the right crystal one can see several phantoms made of chlorite. The crystals came from an alpine-type fissure in mica schist east of Storenuten mountain, north of the Ringedalsvatnet lake at Odda, southwest Norway.



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    The  needly crystals in this haystack aggregate grew inside chlorite pocket clay and are literally filled with chlorite. The specimen is from the Tipling Mine in the Dhading-District, Nepal. Dark green crystals from Nepal can be frequently seen on mineral fairs, sometimes partially, sometimes completely filled with chlorite.

    Crystals of very similar shape and intergrowth came from the Gliedergang valley, a side valley of the Pfitschtal in Southern Tyrol, Italy (->extraLapis No.22). Because of their bizarre look the locals call them "Teufel" (German for "devils").



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    A large amount of inclusions may interfere with the regular growth of a crystal and lead to an irregular or rough surface structure. From the Tipling Mine in the Dhading-District, Nepal.



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    An example for clear rock crystals that are partially filled with moss-like inclusions of chlorite together with feldspar crystals, also from the Tipling Mine in the Dhading-District, Nepal.



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    Since chlorite is denser than water, it will gather on the floor of a pocket if it remains undisturbed, by tectonic activity, for example. This rock crystal, also from the Tipling Mine, grew all through the chlorite ooze and continued to grow in the clear solution above the chlorite. Accordingly, the base of the crystal has a roughened surface on its prism faces.



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    A group of elongated, Dauphiné habit crystals with creme-colored pericline feldspar crystals on matrix. As in the two preceding photos, the chlorite inclusions are mostly confined to the lower part of the crystals, and the smaller crystals at the base are completely filled with it. Interestingly, the feldspar crystals show no chlorite inclusions. From the Tipling Mine in the Dhading-District, Nepal.



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    Chlorite  is often found as deeply green "pocket clay" in alpine-type clefts. The growth of quartz crystals from the central Alps often started before the formation of chlorite minerals took place at lower temperatures, so the crystals tend to have a clear core and chlorite only gets embedded at their surface. The precipitation of chlorite on the crystal interferes with its growth and causes rough and dull crystal faces. The image shows clear smoky quartz crystals peeking out of chlorite clay from an alpine-type cleft at the Seconda Muotta in the Val Giuv valley, Graubünden, Switzerland.



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    A  cinnamon colored and transparent smoky quartz has been covered by tiny green chlorite crystals at the end of its growth, giving it a silky shine. Rock collectors sometimes try to remove the chlorite layer on such crystals, but the result is only a boring and dull surface. From an alpine-type cleft at the Seconda Muotta in the Val Giuv valley, Graubünden, Switzerland.



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    A  very nice specimen of long chlorite-encrusted quartz crystals that sit on a large feldspar crystal. The chlorite has caused growth inhibition and growth resumed only on a few rhombohedral faces like the top-most left face. In addition to the chlorite, the mica mineral stilpnomelan covers parts of the specimen as dark green flakes with a golden shine. From a pegmatite pocket at the pegmatite mine Landsverk I at Evje, Aust-Adger, Norway.



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    A rock crystal from the Grimsel area, Bern, Switzerland, with inclusions of green chlorite and possibly related, but unknown mica minerals.



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    A haystack-like aggregate of elongated quartz crystals with chlorite inclusions. They resemble the crystals from Nepal and Southern Tyrol, except for their more prismatic habit, and like these they have probably been growing in chlorite clay. From the Lötschental, Wallis, Switzerland.



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    A very nice double terminated crystal with green chlorite phantoms underneath the rhombohedral faces and peculiar inclusions of yellow "limonite" and brown hematite in thin superficial sheets along the prism faces. It comes from a pocket in a skarn zone at Kato Vrondou, Kato Nevrokopi, Drama prefecture, Greece. Collection Anastasios Tsinidis.



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    These green crystals have been colored by very fine chlorite with a zonar distribution that is concentrated in phantoms along the edges at places. They sit on a matrix of black sphalerite and golden pyrite crystals and are accompanied by white calcite crystals. From the Unidad Naica Mine, Naica, Mun. de Saucillio, Chihuahua, Mexico.



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    This is a very nice chlorite phantom of a Muzo habit quartz crystal in a normal habit crystal (the specimen is also presented in the Crystal Habits and Growth Forms sections). The label that came with the specimen says that the white grains are inclusions of dolomite or ankerite, but it is very difficult to identify carbonate mineral inclusions in quartz by their shape. From Cerra Do Cabral, Minas Gerais, Brazil.


     

    Epidote 


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    Epidote  inclusions are not very common and are usually made of well-formed crystals, but this specimen seems to be an exception. Needle quartz has grown in a pocket inside an epidote rock. It is very faintly colored green, probably by finely distributed epidote at its base. Between the crystals there are numerous very thin shiny fibers of an unknown mineral that also run through the quartz crystals. From Rahjerd village, Markazi Province, Iran, a source of fine epidote crystals.


     

    Garnet-Group 

    Garnets are a group of minerals of the general formula X3Y2[SiO4]3 that commonly occur in typical crystals of the cubic system, namely as dodecahedra and icosahedra. Garnets are important rock forming minerals; they occur in metamorphic rocks of moderate to elevated pressures and temperatures, as well as a minor component in granitoid igneous rocks. Well-developed crystals of garnets are often found in in skarn rocks.

    Inclusions of garnets in quartz are not so common, because garnets are typical authigenic products that grow within the rock and not from watery solutions like most quartz crystals. Exceptions seem to be high temperature environments like pegmatites and miaroles.


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    Smoky  quartz with grossular garnet (often labeled spessartine) on a feldspar matrix, very likely from a pegmatite pocket, Longshan Mine, Guangdong Province, China.


     

    Iron Oxides and Hydroxides 

    Iron oxides and hydroxides are very common as inclusions and coatings. Some of these compounds are very stable under surface and near-surface conditions, and because iron is abundant, they are almost ubiquitous. The most important ones are:

    Magnetite, Fe3O4. At high temperatures it only forms and is only stable if the environment has a relatively low oxygen activity, otherwise it tends to be oxidized to hematite. At surface conditions it is very stable. Most people associate magnetite with the high-temperature environments of basic igneous intrusions and skarn rocks, but it does also form during sedimentary diagenesis. Its color is black. It is only rarely found as an inclusion in quartz crystals, but may be a compound in quartzites, giving them a blue-gray or gray color.
    Hematite, Fe2O3, forms in a large number of environments under oxidizing conditions, mostly at medium temperatures. It is black or red in fine aggregates and thin crystals. Inclusions in quartz may occur in finely dispersed forms, as small shiny "flakes" as well as large crystals.
    Limonite is not a mineral, but a mixture of various hydrous iron oxides, mainly goethite and lepidocrocite.
    Goethite, α-FeOOH, one of several polymorphs of FeOOH, is very common and is also the major compound of rust found on iron and steel. It is typical for low-temperature environments. Usually it is not found in pure form, but in limonite masses. Depending on the structure of the aggregates (masses, powders, fibrous aggregates) it is black, brown or yellow.
    Lepidocrocite, γ-FeOOH, another polymorph of FeOOH, is much rarer than goethite as an individual mineral. Its crystals are usually red and more platy. It is uncertain if it occurs as an inclusion in quartz (see discussion below).

    In many cases different iron oxides and hydroxides occur together, and if they are finely distributed, it is difficult to identify the individual components. Accordingly, the crystals may assume different colors (yellow, orange, brown, red, almost black) and even a patchy look.

    Goethite, Limonite


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    Quartz that contains more or less evenly distributed inclusions of iron oxides is commonly referred to as ferruginous quartz or eisenkiesel (German for "iron pebble", pronounced "i-zen keezle"). Many people call deeply yellow or orange quartz "citrine", but the yellow of natural citrine is not caused by inclusions of iron compounds, but trace elements (aluminum and sometimes possibly iron) built into the SiO2 crystal lattice, so "citrine" should be considered a misnomer. Goethite is a common inclusion in amethyst, which owes its color to iron color centers.

    Goethite inclusions can have a variety of shapes. In amethyst it often forms golden to brown needles which may form brum-shaped aggregates or spherulitic needle balls. Finely distributed, yellow to brown goethite in irregular shapes or thin layers are very common in low-temperature environments like limestones.

    The first four pictures show deeply colored ferruginous quartz from Hagen-Hohenlimburg, Germany. You might notice that the crystals are not yellow through and through, but instead just covered by a thin layer of goethite and later overgrown by clear quartz.



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    Well  crystallized goethite often forms broom-shaped aggregates of golden-brown to black needles with metallic luster in quartz crystals. In this quartz crystal that grew in a geode in volcanic rock, goethite needles peek out of the crystal's rhombohedral faces. Below you see a close-up image. The specimen is from Dienstweiler, Birkenfeld, Rheinland-Pfalz, Germany.


    Lepidocrocite

    Lepidocrocite is occasionally mentioned in the literature as red shiny flakes in amethyst of certain locations, e.g. Las Vigas, Mexico. The "beetle-leg type" inclusions found in many amethysts as well as the red inclusions in strawberry quartz that have initially been called lepidocrocite have been found to be hematite (White, 2000), and it is likely that most of the "lepidocrocite inclusions" in the literature are actually hematite. I don't know of any studies in which lepidocrocite has been positively identified.


    Hematite


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    Quartz  with more or less evenly distributed hematite inclusions is commonly called ferruginous quartz or eisenkiesel (German for "iron pebble", pronounced "i-zen keezle").

    Like goethite, hematite shows various shapes and aggregates in quartz crystals. It may occur as a fine-grained material in spherical or irregular red, orange or brown patches that resemble vermicular chlorite inclusions, or in fine-grained layers. Amethyst from various localities contains bright-red thin flakes of hematite. Often they look needle-like, but upon close inspection one can see that they are simply elongated irregular platy crystals. Such inclusions are sometimes called "beetle-legs". Occasionally, small black spheres made of platy hematite crystals can be found.

    The first image shows an aggregate of small double-terminated quartz crystals with patchy hematite inclusions from a limestone quarry at Hohenlimburg-Oege at Hagen, Germany.



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    This  image shows a ferruginous quartz containing an orange-red phantom made of hematite that precipitated on the crystal faces. The dark metallic spots at the left of the crystal are tiny hematite crystals, too. The specimen is from the Orange River in southern Namibia.



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    Many  amethysts from the Thunder Bay area in Ontario, Canada, contain inclusions of red hematite in the outmost layers of the crystals, like this specimen. The red "caps" on the violet crystals give them a very peculiar look. The hematite cap is not a continuous layer, but is made of small, circular, and flat inclusions.



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    Salmon-colored  rock crystals are faintly stained by finely distributed hematite that is included in a thin layer just under the crystal's surface. The crystals are a bit dull and not perfectly clear. The host rock is a fine-grained sandstone. From Sichuan, China.



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    Peculiar  irregular patches of red hematite and black manganese oxides are embedded less than a millimeter below the shiny surface of the rhombohedral faces of these crystals. The specimen has been illuminated from the back. Only the topmost layer of the crystals is clear, the rest cloudy and translucent. The crystals lack prism faces and have been removed from a wall of a vein that was completely covered by quartz crystals. The yellow and brown patches on the sides of the crystals are not inclusions but iron oxides that filled out cracks between the crystals. The specimen is from a limestone quarry at Kallenhardt, Sauerland, Germany.



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    Like chlorite, hematite may assume bizarre forms, like in these quartz crystals from Orange River, Namibia. The base of these crystals is smoky quartz, and the flashy pink patch to the left belongs to an amethyst crystal.



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    This crystal has only a thin superficial layer of fine hematite as an inclusion, and although the layer is not even complete, overall the crystal appears to have an even dark brown color, just like ferruginous quartz. The Tessin habit indicates that the crystal has grown at elevated temperatures. From a skarn zone at Kato Vrondou, Kato Nevrokopi basin, Drama prefecture, Greece. Collection Anastasios Tsinidis.



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    The picture shows a strawberry quartz from Chimkent, Kazakhstan. Its color is caused exclusively by the many tiny elongated flakes of a red mineral that has often been misidentified as lepidocrocite, but has found to be hematite (White, 2000). When polished, strawberry quartz has a nice metallic shine and thus is occasionally used as a gemstone (which makes crystals really expensive). The crystal faces of most specimen seem to be somewhat dull, and the quartz needs to be polished to show the nice shine. Good material has also been found in Chihuahua, Mexico (White, 2000).


     

    Pocket Clay and Sand


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    In tectonically active areas, like growing mountains, fine grained material like tiny flakes of mica, sand grains or clay can come off the walls of a pocket and cover the growing crystals. Depending on the type and amount of the material this can cause growth inhibition or promote crystal growth, but generally causes the formation of phantoms in the crystal. Cloudy, irregular inclusions are more unusual.

    The image shows black inclusions of mica and clay concentrated at the tips of the crystals. The clay is gray, but just as it gets darker when wet, it turns almost black inside the crystals. The tips of the two left-most crystals are rough and dull because of a cover of clay that caused growth inhibition. From Piz Regina, Lugnez, Graubünden, Switzerland.



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    This crystal contains multiple thin phantoms and irregular inclusions of fine-grained white quartz sand. The frontal crystal faces are shiny, while the faces on the back side are dull. From Diamantina, Minas Gerais, Brazil.


     

    Tourmaline-Group 


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    Smoky  quartz crystals on creme colored microcline feldspar and scattered schörl needles. From a granitic miarole in the western Erongo mountains, Namibia.


     

    Liquids 

    Almost all quartz crystals contain small amounts of fluids in inclusions, but usually the cavities are microscopically small. Many rock crystals have a cloudy or milky base because of myriads of small bubbles included. Even the quartz grains in granite contain fluids, although the granite has formed from an at least partially molten rock.

    Water is the most common inclusion, as most quartz crystals grew in a watery solution in a hydrothermal environment. Some quartz contains higher carbohydrates, like raw petrol and bitumen. But carbon dioxide, CO2, and methane, CH4, can be found as well. Although both are gases at normal pressure, they have been enclosed as liquids at very high pressures during crystal formation.

    A branch of mineralogy has specialized in studying fluid inclusions, and the favorite material is quartz, as it is chemically stable in a broad range of environments and fluids, gases and solid materials that have been enclosed during crystal growth are almost perfectly sealed and preserved. Studying the fluid inclusions helps to reconstruct the chemical and physical conditions during rock formation.

    Besides that, some fluid inclusions are fascinating even to people who do not collect minerals or know anything about them: those that are large and contain a small gas bubble that moves about when the crystal is turned are among the favorites on fairs (finally a stone that "does something"...).

    Watery inclusions are frequently found in skeleton quartz. They are trapped by quickly growing crystal faces that grow from the edges to the center.


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    A large negative crystal in an amethyst from Amatitlan, Guerrero, Mexico. Such liquid-filled cavities are enclosed by crystal faces and are thus shaped like a real crystal, so negative crystals are always oriented parallel to the host crystal. Often they are quite distorted and only rarely can one see negative crystals of such size and almost ideal shape.


    Further Information, Literature, Links

    There is a very comprehensive ->book by Jaroslav Hyrsl and Gerhard Niedermayr about quartz inclusions, written in English and German. Minerals known to appear as inclusions are systematically presented with many nice pictures.

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