This means they are older than the groundmass. Aphanites are commonly porphyritic, having large crystals embedded in the fine groundmass or matrix. They consist essentially of olivine and augite with a brownish glassy groundmass. The most important criterion is the phenocryst species, followed by the groundmass mineralogy. Tholeiitic rocks may have a fine, glassy groundmass, as may other types of basalt. Phenocrysts below this level, but still larger than the groundmass crystals, are termed "microphenocrysts". The groundmass of these rocks is composed of plagioclase and quartz. Because their groundmass is generally amorphous in structure, fulgurites are classified as mineraloids. The primary crystallized and fast crystallized calcites form phenocrysts and fine-grained groundmass, respectively. Although the rocks have a seriate texture, the groundmass is hyalopilitic. In many trachytes, however, the phenocrysts are few and small, and the groundmass comparatively coarse. The groundmass contains plagioclase, pyroxene, silicon dioxide and oxides of iron and titanium. The groundmass comprises 80-85% plagioclase feldspar, with lesser quartz, biotite and magnetite.