Poicilitic or poikilitic texture is one of the igneous rock textures. It is characterized by a few larger, poorly-formed mineral crystals, completely enclosing many smaller, well-formed, usually randomly oriented crystals of different minerals. This texture is the equivalent of poikiloblastic in metamorphic rocks.
to visualize this texture, imagine a large, centimeter-sized mineral crystal enveloping several smaller other or guest minerals. However, variations of this texture occur. These include micropoikilitic, ophitic, subophitic, poikilophitic, hyalo-ophitic, or hyalopilitic. We will discuss each of these.
The larger mineral that hosts different minerals is known as an oikocryst, while the smaller enclosed mineral crystals are chadacrysts.
Poikilitic comes from the Greek word poikilos, which means mottled, spotted, or variegated. It describes the mottled appearance of this texture. You can imagine the luster-mottled appearance of the peridotites due to hornblende or pyroxene enclosing olivine minerals.
Igneous rocks of a wide range of composition can have poikilitic texture. Usually, it commonly occurs in monzonite, diabase (dolerite), basalts, gabbro, and peridotites (kimberlites, nepheline syenite, etc.).
Lastly, some minerals tend to show a tendency to engulf others. For instance, minerals like hornblende and biotite enclose feldspar more than iron oxides or sphene.
Note: Poikilitic texture definition may not include minute accessory minerals embedded in crystals or where enclosing minerals are nearly equal to surrounded.
Characteristics of oikocrysts and chadacrysts
Oikocrysts, the larger or host mineral enclosing smaller, different mineral crystals, are usually less well-formed or anhedral. Thus, they will not have well-formed crystal faces or cross-section shapes in thin sections.
Also, the faces of oikocrysts adapt or mold to the enclosed minerals. For instance, you can have pyroxene or amphibole on a centimeter scale molded around smaller olivine, plagioclase, chromite, etc.
Sometimes, oikocrysts may be large and interstitial. Such may be unrecognizable in a small area shown on a thin section. This can lead to wrong interpretation.
On the other hand, chadacrysts, the smaller, surrounded, or enclosed minerals, are usually well-formed, i.e., euhedral or subhedral (automorphic or idiomorphic).
Their well-formed crystal structure indicates they formed earlier. Furthermore, crystallization occurred with little interference from neighboring crystals or any disturbance from molten magma. However, this is not always the case.
As Mackenzie et al. (1991) note, chadacrysts are usually randomly oriented or scattered, i.e., they have different orientations inside the host mineral. Also, they are normally but not necessarily equal.
However, some may be subparallel or have a specific orientation. Others change in size progressively from the margin to the center of the rock body. Also, some chadacrysts may cluster in one area inside the host crystal.
An example of specific orientation is in orthoclase (KAlSi3O8) enclosing mica or plagioclase. The mica and plagioclase have a definite crystallographic orientation or relationship to their host mineral.
Lastly, since they have well-formed euhedral to subhedral crystals, chadacrysts are easy-to-recognize sharp faces. Also, you can easily identify them on a petrographic thin section.
How does poikilitic texture form?
How easily minerals nucleate and their subsequent crystal growth rates influence the formation of poikilitic textures.
Poikilitic textures commonly form when the enclosed mineral crystallizes before the host mineral. The enclosed minerals freely grow in the melt without restriction. This results in well-formed crystals before the anhedral enclosing crystals grow in the remaining available space.
For instance, nepheline may form large poikilitic phenocrysts from intercumulus liquid and enclose earlier cumulus phase minerals. Similarly, granitic rocks may have alkali feldspar oikocrysts (near-solidus) surrounding minerals precipitated at higher temperatures.
Another possible way poikilitic texture forms is through simultaneous crystallization and crystal growth. Here, the larger or host mineral has a lower rate of nucleation but a faster crystal growth rate, while the enclosed minerals have a higher nucleation rate but grow slowly.
A final possible way is where a few nuclei of host minerals form and grow a bit before abundant nucleation of enclosed minerals and growth of both phases.
The larger crystals grow faster for all these scenarios but don’t nucleate well. In contrast, the enclosed crystals nucleate faster but grow slowly.
Ophitic texture
Ophitic texture is a specific kind of poikilitic texture. It occurs where larger, poorly formed pyroxene (or sometimes olivine) completely encloses well-formed plagioclase laths.
Small subhedral to euhedral plagioclase crystal laths (slender, elongated commonly in radiation masses) are embedded in pyroxenes, olivine, or other minerals in ophitic texture. The main distinction from poikilitic is that the chadacrysts (different enclosed minerals) are plagioclase laths.
Ophitic comes from the Greek word ophis, which means snake-like. It describes the mottling noticed in rocks with this texture.
Plagioclase laths form a three-dimensional network, i.e., randomly oriented in various directions. Thus, they don’t have any cryptographic relation. In contrast, the surrounding pyroxene commonly fills in interstices.
Gill (2010) notes that the ophitic texture is common in gabbros, diabase, and some basalts. In basalts, ophitic texture occurs in thick sills, dikes, and the interior of thick lava flows, including flood basalt. Also, it is common in dolerite or diabase, where crystals of augite completely enclose smaller laths of plagioclase feldspar.
How does it form? According to Blatt and Tracy (2006), ophitic and subophitic textures represent the simultaneous intergrowth of plagioclase and pyroxene. Differences in crystallization properties result in pyroxenes enclosing fully or partially plagioclase.
Furthermore, Winter (2014) states that ophitic texture happens from a cotectic crystallization of these minerals.
The larger pyroxene (clinopyroxenes, usually augite) indicates that clinopyroxenes nucleate less, forming fewer crystals. However, they grow faster to envelop the smaller plagioclase, which nucleates abundantly and grows slowly.
Therefore, the intuition that plagioclase grew fast may not always be true in this or ophitic texture.
Subophitic
Subophitic texture occurs when pyroxene partially encloses plagioclase laths, not completely. It is a variation of the ophitic texture and forms more or less in the same manner as the ophitic texture.
The partially enclosed plagioclase laths touch or impinge on each other. Also, they may have sharp angles.
Additionally, pyroxene crystals are nearly the same size as plagioclase laths. So, they can only partially enclose them.
Subophitic texture is common in dolerite or diabase, basaltic rocks like tholeiite or eucrites, and gabbroic rocks.
Sometimes grains can change from ophitic to subophitic. A slightly faster nucleation and cooling rate may explain grain changes from ophitic to subophitic texture, especially in basaltic rocks. This texture change will occur as you move towards the margins of these basaltic rocks.
However, a high-speed cooling rate will result in an intersertal texture where the plagioclase laths will be embedded in glass or cryptocrystalline grains.
Poikilophitic texture
It is a variation of poikilitic texture where more than one mineral is enclosed. For instance, in gabbro, it is not uncommon for augite to contain plagioclase and olivine crystals.
Hyalo-ophitic and hyalopilitic texture
Hyalo-ophitic and hyalopilitic are variations of ophitic texture in which groundmass is substantial to plentifully glassy.
In hyalo-ophitic texture, plentiful glass surrounds plagioclase microlites with less pyroxene.
On the other hand, the hyalopilitic texture will have glass surrounding with tiny, plagioclase microlite crystals. These microlites embedded in glass are often subparallel but may be random.
Micropoikilitic
Micropoikilitic texture is a poikilitic texture that is only visible under the microscope. It is not visible by a hand lens or the naked eye.
Poikilitic texture interpretation
Usually, the poikilitic texture is commonly interpreted to mean that the enclosed mineral formed and ceased crystallizing earlier than the enclosing. However, this is true but not always the case. There are other possible scenarios.
According to Best (2013), pyroxenes enclosed in larger plagioclase may mean that:
- The pyroxene may have crystallized later from melt inclusions trapped within skeletal plagioclase. This happens after plagioclase stops crystallizing.
- It is also possible that the pyroxenes crystallized later from melt-filled cracks within plagioclase after it grew.
- Poikilitic plagioclase may be a xenocryst or restite grain from metamorphic rock in subcrustal rock where magma was generated by partial melting.
Lastly, while interpreting poikilitic texture, oikocrysts may be interstitial and unrecognizable in a small area shown on a thin section.
Enclosed minerals may or may not vary in size spatially
Enclosed minerals may show no spatial size variation, while others will show an increase from the core to the rim. How can you interpret these two observations?
A possible explanation for this spatial variation is that minerals that crystallized close to the nucleus of the enclosing mineral were enclosed early. This limited their growth. However, those far away had time to grow before being incorporated.
This variation in plagioclase size from core to rim occurs in the Skaergaard intrusion in Greenland. It further supports simultaneous crystallization notes Winter (2014).
On the other hand, if enclosed crystals grew earlier, their size would not be very much.
Frequently asked questions
The host (oikocrysts) is the larger crystal or phenocryst that encloses the different, smaller, or guest minerals (different). So, the host envelopes the guest or different smaller minerals.
The porphyritic texture is where large grains (phenocrysts) are surrounded by groundmass or finer-grained matrix. In contrast, a larger crystal completely encloses smaller, different minerals in poikilitic textures.
A poikilitic texture thin section will reveal larger crystals surrounding smaller, different crystal that is well-formed and has sharp, easy-to-recognize faces. To conduct a thin section, you will get a thin slice of the poikilitic textured rock and view it using a polarizing petrographic microscope, electronic microprobe, or microscope.
Ophite rock refers to dolerites whose ophitic texture remains intact, but deuteric alteration has changed pyroxene to uralite, dark green-colored fibrous amphibole.
Poikilitic shergottites (previously, lherzolitic shergottite) is one of the three rocks formed on planet Mars (Martian meteorite) with a poikilitic texture. It has large pyroxene crystals enclosing very small olivine. Shergottites are mafic to ultramafic igneous rock.
For instance, the Northwest Africa (NWA) 6342 poikilitic shergottites have “rounded Mg-rich olivine chadacrysts enclosed by large low-Ca pyroxene oikocrysts” (Kizovski et al., 2019)
Definitions and glossary
- Chadacrysts or guests refer to the enclosed crystals (guests) of a different mineral in a poikilitic texture. These crystals often have different orientations and are randomly scattered inside the enclosing crystal.
- Deuteric alteration refers to a change in mineralogical and texture that occurs when the igneous rock is at the final crystallization stage of molten rock.
- Oikocrysts: Refers to the larger mineral crystal, often a phenocryst, that encloses smaller, different minerals in a poikilitic texture. So, they enclose crystals in poikilitic fabric and may have minerals common in mafic and ultramafic rocks such as pyroxenes or plagioclase (common ones) and hornblende or olivine. But these minerals can also be felsic.
References
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
- Blatt, H., Tracy, R. J., & Owens, B. E. (2006). Petrology: Igneous, sedimentary, and metamorphic (3rd ed.). W.H. Freeman and Company.
- Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology. Pearson Education.
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
- MacKenzie, W. S., Donaldson, C. H., & Guilford, C. (1991). Atlas of igneous rocks and their textures. Wiley.