Ophitic and Subophitic Textures

While learning about igneous rocks, you often come across the terms ophitic and subophitic textures or fabrics.

What do they mean, and how do they form? I guess you are curious and would like to know. I am going to explain to you in detail.

However, to be on the same page, you should know texture in geology and Earth sciences doesn’t mean the surface feel, appearance, or consistency.

Instead, texture, fabric, or rock microstructure refers to sizes, shapes, and how grains relate, including their orientation. This includes even the voids present.

What is an ophitic texture?

Ophitic texture or fabric occurs when larger, poorly formed crystal pyroxene olivine or other mineral completely surrounds or engulfs a well-formed plagioclase lath.  

A lath is a crystal habit marked by thin, elongated crystals with moderate to narrow widths.

On the other hand, well-formed crystals have shapes and easy-to-recognize faces and are usually known as euhedral. The opposite is poorly formed or anhedral.

Ophitic texture is a variation of poikilitic texture. A poikilitic texture or fabric forms when a larger, poorly formed mineral surrounds many smaller, well-formed crystals of a different mineral. The smaller, surrounded crystals often have a random orientation.

Plagioclase laths in ophitic texture have a random orientation, i.e., point to various directions in space on a 3D framework. They don’t show any relationship with each other.

On the other hand, the surrounding olivine or pyroxenes seem to occupy space left between the laths, i.e., interstices.

In which rocks are ophitic textures common? It is common in rocks that have plagioclase and pyroxene or olivine. These rocks are mostly mafic, i.e., iron and magnesium-rich – like diabase, gabbro and some basalts.

In basalts, the ophitic texture is seen in those rocks that form in sills, dikes or at the interior of thick basalt flows.

Subophitic texture

Subophitic is a variation of ophitic texture. In subophitic texture or fabric, poorly-formed pyroxene, olivine, or other minerals only partially surround well-formed plagioclase laths.

Therefore, the plagioclase laths will touch each other. This impinging may result in sharp angles forming.

Compared to ophitic, pyroxene or olivine crystal size is comparable to plagioclase. Thus, they cannot completely surround them.

Subophitic texture occurs in diabase, gabbro and some basalts. Also, eucrites from the 4 Vesta asteroids have this fabric.

How do ophitic and subophitic textures form?

One possible way ophitic and subophitic textures plagioclase laths and pyroxene or olivine grow simultaneously. However, pyroxene and olivine will have a low nucleation (form inception embryo upon which crystals grow) rate, but their crystals grow fast.

Therefore, pyroxene or olivine will grow larger surrounding the slower-growing pyroxene laths.

Simultaneous crystal growth occurs in cotectic conditions, i.e., temperature, pressure and composition under which two or more minerals crystallize simultaneously without being resorbed from a single melt.

Alternatively, plagioclase could form earlier. This favors a well-formed crystal. Later, olivine or pyroxene will grow from the intercumulus phase (the melt between crystals). Again, they will surround the pyroxene laths, forming an ophitic texture.

However, if cooling happens too quickly, an intersertal texture with plagioclase laths in a glassy matrix will instead form.

Lastly, it is possible for ophitic texture to change to subophitic. An increase in the nucleation rate of surrounding pyroxene or olivine crystals or a cooling rate change is a likely cause.