The poikilitic texture is an igneous rock texture in which large mineral crystals engulf or completely enclose many smaller ones of a different mineral. This texture doesn’t include minute inclusions in minerals.
The smaller, enclosed, or engulfed crystals of a different mineral are known as chadacrysts. Chadacrysts are usually randomly oriented and well-formed with euhedral to subhedral crystals.
Euhedral crystals are well-formed crystals with sharp and easy-to-recognize faces. They grow in unimpeded or unconfined environments, indicating they likely form earlier.
However, some chadacrysts may show subparallel orientation, while others show varying orientations inward.
Also, their sizes may vary, such as becoming larger from core to rim. However, most will have comparable or nearly equal sizes.
On the other hand, the larger, enclosing mineral crystals are known as oikocrysts. Oikocrysts are generally fewer and poorly formed or anhedral. These minerals lack easy-to-recognize sharp crystal faces.
Also, they adapt or mold to the faces of the enclosed minerals. This suggests that the enclosed smaller minerals were present when they formed.
Usually, poikilitic texture creates a mottled or spotted appearance with large crystals containing speckles of another mineral. This spotted appearance is the basis of the name poikilitic, which comes from the Greek word poikilos = mottled or spotted.
Lastly, poikilophitic texture occurs when the larger mineral encloses two or more different minerals. For instance, olivine could enclose plagioclase and another different mineral.
Which rocks are poikilitic?
The poikilitic texture is typical in many igneous rocks, especially plutonic and subvolcanic. However, some extrusive or volcanic rocks can have this texture.
Some rocks often with poikilitic texture include monzonite, gabbro, peridotites, nepheline syenite, syenite, norites, diorite, monzonite, some basalts, etc.
How does it form?
Poikilitic texture forms in various ways. Factors that influence the formation of this texture are ease of nucleation and crystal growth rate.
It first forms when the smaller engulfed crystals form earlier at higher temperatures. This gives them space to grow well-formed crystals. Later, the intercumulus melt may form larger crystals of a different mineral surrounding the already-formed crystals.
For instance, alkali feldspar at near solidus temperature may crystallize, surrounding different minerals formed earlier at higher temperatures.
A second way is simultaneous crystallization, where the engulfed minerals nucleate more but grow slowly, and the larger engulfing crystal nucleates less but crystallizes fast.
This will result in many small crystals surrounded by fewer larger crystals, forming a poikilitic fabric.
Sometimes, crystals can be larger towards the outer rim since those near the nuclei of the larger mineral will have less time to grow compared to those at rims or outer places.
A third way is if the larger crystal or oikocrysts start growing, and later, many engulfed crystals nucleate. Then, the two will grow simultaneously. Again, the oikocrysts nucleate less and grow faster, while chadacrysts nucleate more but grow slowly.
Lastly, in some instances, the smaller surrounded crystal may grow later from a melt-trapped larger crystal skeletal structure. For instance, pyroxene can crystalize from a melt trapped within skeletal plagioclase.
Poikilitic texture variants
Common variants of poikilitic fabrics are ophitic and subophitic. Others are micropoikilitic, hylo-ophitic, and hyalopilitic.
Ophitic texture occurs when olivine, pyroxene, or other minerals completely surround plagioclase laths. If they only partially surround plagioclase laths, you have a subophitic texture.
What about micropoikilitic texture? It is a poikilitic fabric with tiny crystals only visible under the microscope. You cannot see this with an unaided eye.
The other variations are hyalo-ophitic and hyalopilitic. These textures occur where glass surrounds minute crystals of other minerals.
In hyalo-ophitic texture, glass surrounds plagioclase microlites together with a smaller amount of pyroxenes.
On the other hand, the hyalopilitic texture is where glass surrounds minute subparallel or randomly oriented tiny plagioclase crystals or microlites.
Some enclosed minerals are xenocryst.
In poikilitic texture, some of the enclosed minerals can be xenocrysts. Xenocrysts are foreign minerals that are not formed from crystallizing magma.
They can include grains from metamorphic rock that didn’t melt during partial melting of subcrustal rocks.
Sometimes, growing minerals can trap such xenocrysts.