Here's why:

* Olivine is a mineral that forms in high-temperature, mafic (iron-rich) magmas.

* Quartz is a mineral that forms in lower-temperature, silica-rich (felsic) magmas.

Basaltic magmas are mafic, so they favor the formation of olivine. Quartz is unlikely to crystallize from these magmas because:

* Silica content: Basaltic magmas have a relatively low silica content, while quartz requires high silica content to form.

* Crystallization temperature: Olivine crystallizes at higher temperatures than quartz. As a basaltic magma cools, olivine forms first, leaving less silica available for quartz to crystallize.

Therefore, olivine and quartz rarely crystallize together from mafic basaltic magmas.