• C:\Users\Sven\Documents\škola\Škola\Books\Atlas of ignous rock\kapak.jpg Mutual relations of crystals (ans amorphus material) The various pattern of crystal arrangement which can exist are convently introduces under the following headings: equigranular textures; inequigranular textures; orientes textures; intergrowth textures; radiate textures; overgrowth textures; banded textures; and cavity textures. Particular textures may be belong to more than one of these categories and some also belong to the categories of crystalinity, granularity and crystal shape. Thus certain of the textures introduced in this section have already been mentioned and reference is made to photographs of them in previous sections. EQUIGRANULAR TEXTURES Depending on the general shape of crystals, three textures can be distinguished in which crystals of the principal minerals in a rock are roughly uniform grain size: name synonymous definition euhedral granular panidiomorphic granular subhedral granular hypidiomorphic granular anhedral granular allotriomorhic granular bulk of crystals are euhedral and of uniform size bulk of crystals are subhedral and of uniform size bulk of the crystals are anhedral and of uniform size (granitic and granitoid textures apply to siliceous rock only) Boundaries between these categories are not sharply defined and consequently the terms applied very subjectively. Furthermore a rock may not fit neatly into a single category, thus one in which ~ 50% anhedral might best be described as having a mixed euhedral and anhedral granular texture. Euhedral granular hornblendite Subhedral granular gabbro Anhedral granular troctolite Granular granite Granular lherzolite • INEQUIGRANULAR TEXTURES This category includes seven kinds of texture: (a) seriate; (b) porphyritic; (c) glomerophyritic; (d) poikilitic; (e) ophitic; (f) subophitic; and (g) interstitial (intersertal and intergranualr). It is not uncommon for a single thin section to display more one of these textures. Seriate texture Crystals of the principal minerals show a continous range of sizes. Porhyritic texture Relatively large crystals (phenocrysts) are surrounded by finer-grained crystals of the groundmass. Glomeroporphyritic texture A variety of porphyritic in which the phenocrysts are bunched, or clustered, in aggregates or clots called glomerocrysts. (A minority of petrologists maintain taht the term applies only to monomoneralic clots and for polymineralic clots they use the term cumulophyric texture.) Glomerophyric is usually be reserved for clusters of equant crystals (Johannsen, 1931). (Synneous texture also describes crystal clots but includes the genetic implications that crystals ´swam together´and therefore best avoided. Glomeroporphyritic tholeiitic basalt Glomeroporhyritic hawaiite Poikilitic texture Relatively large crystlas of one mineral enclose numerous smaller crystals on one, or more, other minerals which are randomly oriented and generally, but not necesarily, uniformly distributed. The host crystal is known as an oikocrysts (or enslosing crystal) and enclosed crystal as chadacrysts. Although chadacrysts are generally equant, or nearly so, they need not be uniform in size; sometimes they display progressive change in size from the interior to the margin of an oikocrysts, indicating differences in extant of chodocrysts growth at the time of enclosure. It is not customary to apply poikilitic texture to the arrangement in which scarce minute crystals of accessory minerals are embeded in a crystal, nor to that in which the encloding mineral is approximately the same size as that included. Poikilitic enclousure of olivine crystals by augite Plagioclase chadacrysts enclosed by augite • Olivine gabbro containing poikilitic domains • Olivines enclosed by plagioclase oikocryst • Ophitic texture This is variant of poikilitic texture in which the randomly arranged chadacrysts are elongated and are wholly, or partly, enclosed by the oikocrysts. The commonest occurrence is of bladed crystals of plagioclase surrounded by sebequant augite crystals in dolerite (sometimes refferd to as doleritic texture); however the texture is not confined to dolerites, nor to plagioclase and augite as the participating minerals. Some petrologists distinguish the arrangement in which the elongate chadacrysts are completely enclosed (poikilophitic texture) from that in which they are partially enclosed and therefore penetrate the oikocrysts (subophitic texture). Poikilophitic texture could also be used when oikocrysts surround elongate chadacrysts of one mineral and equant chadacrysts of another. Fine-and medium-grained rocks made up of many small oikocrysts have a patchy appearance, sometimes described as ophimottled. Ophitic-textured alkali olivine dolerite Subophitic texture in olivine dolerite • Subophitic alkali olivine dolerite • Poikilophitic texture in olivine gabbro Ophiomottled texture in olivine basalt • Feldspar-olivine-phyric ophiomottled basalt • Interstitial textures Two varieties are recognized on the basis of the material occupying the angular spaces between feldspar laths: 1.Intersertal texture – glass or hypocrystalline material wholly, or partly, occupies the wedge-shaped interstices between plagioclase laths. The glass may be fresh or have been altered to plagonite, chlorite, analcite or clay minerals, or it may have devitrified. If a patch of glass is sufficiently large and continuous to enclose a number of plagioclases, some petrologists would describe the texture as hyalophitic. (See also hyalopilitic texture). 2. Intergranular texture – the spaces beween plagioclase laths are occupied by one, or more, grains of pyroxene (±olivine and opaque minerals). Unlike ophitic texture, adjacent interstices are not in optical continuity and hence are discrete small crystals. The feldspar may be in diverse, subradial or subparallel arrangement (see also pilotaxitic and felty textures). 3. As shown by some of the photographs illustrating these textures, a single thin section may contain both types of interstitial texture in separate, but contiguous, textural domains. Intersertal (hialophitic) texture in thoeliitic basalt • Intersertal texture in alkali dolerite • Intergranular dolerite • Intergranular olivine gabbro • Tholeitiic basalt with two types of interstitial texture • Intersertal, intergranular and subophitic textures in dolerite ORIENTED, ALIGNED AND DIRECTED TEXTURES Sevral classes of this textural type exist: (a) trachytic texture; (b) trachytoid texture; (c) parallel-growth texture; (d) comb texture; and (e) orbicular texture. Trachytic texture A subparallel arrangement of microcrystalline lath-shaped feldspars in the ground-mass of a holocrystalline or hypocrystalline rock. Note well the therm is not restricted in use to rock of trachyte composition. Some petrologists subdivide trachytic texture with microlite-sized feldspars into pilotaxitic texture and hyalopilitic texture, depending on whether the material between the feldspars is crystalline or glassy. Stristly, however, the microlites in these textures may be more or less aligned. (For a pilotaxitic texture in which the microlites are essentially randomly arranged the term felty texture exists.) Trachytoid texture A subparallel arrangement of tabular, bladed or prismatic crystals which are visiable to the naked eye (Holmes, 1921). While the term is usually applied to crystals feldspar, Johansen (1931) states that in may equally well be used for oriented crystals of any other mineral. The term flow and fluxion texture are sometimes used as synonyms for trachytic and trachytiod textures, however they should be avoided of their genetic implications. Trachytic texture in a trachyte Trachytic texture in trachyte • Hyalopilitic texture in rhyolitic pitchstone • Trachytoid diorite • Trachytoid gabbro • Olivines in trachytoid arrangement in olivine dolerite • Parallel-growth texture A single elongate skeletal crystal which in thin section appears to consist of a clot of crystals having the same elongation direction and the same optical orientation. In rocks trachytoid texture it is not uncommon for neighbouring parallel-growth crystals to be alignated. Comb texture ( comb layering) Elongate, possibly curved, branching crystals sharing the same direction of elongation. The crystals typically form a band, layer, or fringe with the elongation direction of the crystals inclined at 60-90° to the plane of the layering. (Synonyms are Willow-Lake layering and crescumulate layering, though the latter is a genetic term and, hence, shoud be avoided. Orbicular texture ( Orbicular layering) In connection with the group of textures being considered here, note that in some orbicules the concentric shells have elongate crystals aligned radially about the centre of the orbicule. Pyroxene comb layer in a thin lampropyre (fourchite) dyke Comb layers in dolerite dyke INTERGROWTH TEXTURES In thin section the junction between two crystals may appear as a stright line, a simple curve, or a complex curve; in the third case the crystals interdigitate or interlock, possibly so intimately that they appear1 to be embedded in one another. These interpenetrative patterns are all examples of intergrowth textures. Usually the crystals concerned are anhedral but one or both my be skeletal, dendritic or radiate. Seven varietes are distinguished here: (a) consertal texture; (b) micrographic texture; (c) granophyric texture; (d) myrmekitic texture; (e) intrafasciculate texture; lamellar and blebby intergrowths; and (g) symplectite texture. 1 The appearence of an interdigitating boundary between two crystals, A and B, depends on the extent of interpenetration and the direction in which the boundary is sectioned; some intersections may show the crystals meeteng in a complex curve; others may crystal A enclosed in B; others may show the converse; and yet others may show each enclosing the other. Consertal texture The boundary between two crystals involves interdigitations, ane hance appears to be notched or serrated in section (iddings, 1909; Niggli, 1954). Micrographic texture (or graphic, if visiable with the naked eye) A regular intergrowth of two minerals producing the appearance of cuneiform, semitic or runic writing. The best-known instance is of quartz and alkali feldspar, the quartz appearing as isolated wedges and rods in the feldspar. (a micrographic intergrowth pf quartz and alkali feldspar is also known as micropegmatitic texture.) Granophyric texture A variety of micrographic intergrowth of quartz and alkali feldspar which is either crudely radiate or is less regular than micrographic texture. Consertal texture in granodiorite Consertal intergrowth texture in gabbro • Graphic granite • Micrographic texture in aplite • Micrographic and granophyric textures in microgranite • Granopyric texture Mirmekitic texture Patches of plagioclase intergrown with vermicular quartz. The intergrowth is often wart-like in shape and is commonly to be found at the margin of a plagioclase crystal, where it penetrates an alkali feldspar crystal. The texture could be regarded as a variety of symplectite texture. Intrafasciculate texture Hollow, columnar plagioclase crystals filled with pyroxene. Interfasciculate texture in dolerite Lamellar and bleb-like intergrowths Parallel lamellae, or trains of blebs, of one mineral, and all of the same optical orientation, are enclosed in a single ´host´crystal of another mineral. Well-known examples involve lamellae or blebs of sodium-rich feldspar in a host of potassium-rich feldspar (perthitic texture); the converse (antiperthitic texture); and lamellae or blebs of one pyroxene in a host of another (e.g. Augite in orthopyroxene or vice versa, and pigeonite in augite or vice versa). Other examples include: ilmenite lamellae in (ulvöspinel-magnetite)solid solution crystals; metallic iron rods, and blebs in lunar plagioclases; plagioclase lamellae in pyroxene; and chrome-magnetite lamellae in olivine. Careful examination may reveal lamellae of more than one orientation and scale and sometimes even fine lamellae within coarse lamellae,i.e. multiple generations of lamellae. Lamellar and bleb-like intergrowths are often attributed to exsolution of the lamellae and blebs from the host crystal (i.e. solid-state reaction) and the genetic term exsolution texture is often therefore applied to them. However, laboratory experiments in which antiperthite formed from a melt as a result of co-crystallization of two feldspars, and others in which ilmenite lamellae formed in pyroxene during co-crystalizzation of the two phases from the melt, highlight the danger of uncritical use of the term exsolution texture. Microperthitic textures Antiperthitic texture in tonalitic gneiss • Lammellar intergrowths of two pyroxenes in gabbro • Bleb-like intergrowth of augite in orthopyroxene in olivine gabbro • Symplectite texture An intimate interwrowth of two minerals in which one mineral has a vermicular (wormlike) habit. Symplectite of iron ore and orthopyroxene Fayalite-quartz symplectite RADIATE TEXTURES Radiate textures are those in which elongate crystals diverge from a common nucleus. They are most frequently found in fine-grained rocks, but not exclusively; for example Fig. 34, 35, 36, 70 and 71 show large branching pyroxene, plagioclase and olivine crystals in fan-shaped radiate arrangements. A remarkably large number of terms exists to describe the various patterns, including: fan, plume, spray, bow-tie, spherical, sheaf-like, radiate, radial, axiolitic, spheeulitic and variolitic. All except the last three (which are defined and illustrates here), are of self-evident meaning. Spherulitic texture Spherulites are approximately apheroidal bodies in a rock: they are composed of an aggregate of fibrous crystals of one or more minerals radiating from a nucleus, with glass or crystals in between. The acicural crystals may be either single, simple fibres or each may brenches alon its length; any branches may or may not share the same optical orientations as their parents. The most common occurence of spherulitic texture is a radiate aggregate of acicular alkali feldspars with glass between them, though quartz or other minerals may be present, resulting in an intergrpwth texture. Should the spherulite have a hollow centre it is known as a hollow spherulite, and if it comprises a series of concentric, partially hollow shells, the term lithophysa is used. Axiolithes differ from spherulites in that radiating fibres extend from either end of a linear nucleus (i.e. from a small acicular crystal) rather than a point. They could be regarded as a variety of overgtowth texture, as indeed could those sherulites which grow about visible crystal s rather than on submicroscopic nuclei (e.g. Fig. 88). • Plagioclase spherulite in dolerite Spherulite in rhyolite • Compound spherulites in rhyolite • Variolitic texture A fan-like arrangement of divergent, often branching, fibres; ussually the fibres are olagioclase and the space between is occupied by glass or granules of pyroxene, olivine or iron ore. This texture differs from spherulitic in that no discrete spherical bodies are identifiable; in fact, each fan as seen in thin section is a slice through a conical bundle of acicular crystals. Variolitic olivine dolerite • Radiate intergrowth of plagioclase and augite in dolerite • Overgrowth textures This term applies to textures in which a single crystal has been overgrowth either by material of same composition, or by material of the same mineral species but different solid-solution composition, or by an unrelated mineral. There are three types: (a) skeletal and dendritic overgrowths; (b) corona texture; and (c) crystal zoning. Skeletal or dendritic overgrowths Porphyritic rocks with a glassy or very fine-grained groundmass may show delicate fibres or plates extending from the corners or edges of the phenocrysts. The overgrowth and the phenocryst need not be the same mineral. Overgrowth textures in rhyolitic pitchstone Corona texture A crystal of one mineral is surrounded by a rim, or a ‘mantle’, of one or more crystals of another mineral, e.g. olivine surrounded by orthopyroxene, or biotite surrounding hornblende. Such relationships are often presumed to result from incomplete reaction of the inner mineral with melt or fluid to produce the outer one and for this reason the equivalent genetic terms reaction rim and reaction corona are frequently used. The special term Rapakivi texture is used to describe an overgrowth by sodis plagioclase on large, usually round, potassium-feldspar crystal, and kelyphitic texture is used for a microcrystalline overgrowth of fibrous pyroxene or hornblende on olivine or garnet. Corona texture • Corona texture Rapakivi texture Crystal zoning One or more concentric bands in a single crystal are picked out by lines of inclusions (Fig 95) or by gradual or abrupt changes in solid-solution composition of the crystal. As regards the latter type of zoning, a large number of patterns are possible, the commoner ones being illustrated graphically and named below, using plagioclase as an example. Normal versus reserve zoning These terms specify the general trend of solid-solution composition from core to rim. ‘Normal’ indicates high temperature component→ low-temperature component (e.g. An-rich plagioclase → Ab-rich plagioclase, see Fig C) and ‘ reverse’ indicates the opposite. Continuous1 versus discontinuous1 zoning These terms indicate respectively a gradual or an abrupt change in composition. Fig C shows examples of continuous normal zoning and Fig D an example of discontinuous normal zoning. Continuous and discontinuous zoning may alternate (Fig E). 1 These terms are not the same as continuous reaction and discontinuous reaction of crystals with melt. Zonal arrangement of melt inclusions in plagioclase Fig. C Three examples of continuous normal zoning represented on a sketch graph. Fig. D Discontinuous normal zoning Fig. E combined continuous and discontinuous normal zoning Zoned plagioclase • Multiple zoning This term used for crystals having repeated discontinuous zones. If the zones show a rhythmic repetition of width, the pattern is known as a oscillatory zoning. The overall compositional trend of the multiple zoning may be normal or reverse or even (in which there is no general trend from core to rim). Individual zones may be of uniform or variable composition, such that the zoning pattern on a composition-distance graph is square wave, step-like, saw-tooth, curved saw-tooth, or some combination of these (see Fig H-J). However, these are details which only very careful and lengthy optical examination or electron-probe microanalyses would reveal. The reader should appreciate that the sketches in Fig C-J are all idealized and that in real crystals the oscillations will be less uniform; furthermore multiple or oscillatory zoning may only occupy part of a crystal, the remainder perhaps being homogenous or continuously zoned. Fig F Multiple, even zoning Fig G Oscillatory, even zoning Fig H Oscillatory, normal zoning: step-like Fig I Oscillatory, normal zoning: saw-tooth Fig J Oscillatory, normal zoning: curved saw-tooth Convolute zoning This is a varierty of multiple zoning in which some of the zones are erratic and have non-uniform thickness. Zoned plagioclases Zoned olivines Sector (or hourglass) zoning As seen in thin section, this ideally takes the form of four triangular segments (sectors) with a common apex (Fig K-b). Opposite sectors are chemically identical, whereas adjacent ones differ in composition (though possibly only slightly) and hence in optical properties. Each sector may be homogenous or show continuous or discontinuous or oscillatory, normal ore reverse or even zoning. In there dimensions the sectors are pyramid shaped (Fig K-a), and depending on the orientation of the crystal with respect to the plane of a thin section, a variety of patterns may be seen in thin section (Fig K-b to f). If the sector boundaries are curved, the pattern can resemble that of an hourglass (Fig K-g). Sector zoning is a common feature of pyroxenes in alkali-rich basic and ultrabasic rocks. It has also been in plagioclases in a few quickly cooled basalts. Sector-zoned augite Sector-zoned pyroxene Oscillatory- and sector-zoned, inclusion-beraing pyroxene Oscillatory- and sector-zoned pyroxene BANDED TEXTURES (BANDING) Textures of this involve two, or more, narrow (up to a few centimeters), subparallel bands in a rock which are distinguishable by difference in textures, and/or colour and/or mineral proportions. The term layering is also used by petrologists; while it includes banded texture, it is also used for larger scale stratifications. An example of banded texture due to textural differences in illustrated in Images-5, and 103 and 104 shoe examples resulting from extreme differences in mineral proportions. Olivine and chrome-spinel banding (or layering) Anorthosite-chromitite banding (or layering) Comb layering, orbicular texture, ocellar texture and eutaxitic Comb layering and orbicular texture are particularly exotic kinds of banding. In the latter, ‘orbs’ consist of concentric shells of rhythmically alternating mineral constitution. Within the shells the texture may either be granular or elongate crystals may be radially arranged. ‘Orbs’ may be reach a few tens of centimetres in diameter. A further variety of banded texture, eutaxitic, occurs in some tuffs and ignimbrites and consists of a regular alignment of flattened glassy fragments. Anorthosite-chromitite banding (or layering) Cavity textures These are a collection of textures witch feature either holes in the rock or likely former holes which are now partly or completely filled with crystals. Vesicular texture Round, ovoid, or elongate irregular holes (vesicles) formed by expansion of gas, in a magma. Amygdaloidal texture Former vesicles are here occupied, or partially occupied, by late-stage magmatic and/or post-magmatic minerals, such as carbonate, zeolites, quartz, chalcedony, analcite, chlorite, and/or, rarely, glasses of fine groundmass. The filled holes are known as amygdales or amygdules. Cavity textures - continuation Ocellar texture Certain spherical or ellipsoidal leucocratic patches enclosed in a more mafic host are known as ocelli (singular ocellus). Unlike amygdales, the minerals filling an ocellus can normally all be found in the host rock; they may include any of: nepheline, analcime, zeolites, calcite, leucite, potassium feldspar, sodium feldspar, quartz, chlorite, biotite, hornblende and pyroxene or even glass, and the minerals are commonly distributed in a zonal arrangement (Fig 109a). Often, platy and acicular crystals in the host bordering an ocellus are tengentially arranged (Fig 109b) but sometimes project into the ocellus. Ocelli are normally less than 5 mm in diameter but may reach 2 cm. Their origin has been ascribed on the one hand to separation of droplets of immiscible liquid from magma, and on the other hand to seepage of residual liquid or fluid into vesicles. Miarolitic texture These are irregularly shaped cavities (druses) in plutonic and hypabyssal rocks into which euhedral crystals of the rock project. Lythophysa (or satone-ball) This is term given to a sphere consisting of concentric shells with hollow interspaces. Vesicular feldspar-phyric basalt Vesicular trachyte Amygdaloidal basalt Ocellar texture Miarolitic (or drusy) cavity in granite