Porphyries may be aphanites or phanerites, that is, the groundmass may have invisibly small crystals as in basalt, or crystals easily distinguishable with the eye, as in granite.
32.
In thin section it can be observed that the siliceous groundmass is derived from the innumerable skeletons of radiolarians ( maximal size 0.1 mm in diameter ) altered during diagenesis.
33.
The foremost problem is that in large ultramafic intrusions, assimilation of wall rocks tends to alter the chemistry of the melt as time progresses, so measuring groundmass compositions may fall short.
34.
The matrix consists of an extremely fine-grained groundmass with fine-to coarse-grained megacrysts of biotite, diopside, aegerine-augite, orthoclase, magnetite, apatite, and nepheline.
35.
A 1981 paper, " Faunal Remains from the Howard Baum Site, Ross County, Ohio ", discusses the finding of 1, 522 animal remains discovered in the groundmass at the site.
36.
Group I kimberlites exhibit a distinctive inequigranular texture caused by macrocrystic ( ) to megacrystic ( ) phenocrysts of olivine, pyrope, chromian diopside, magnesian ilmenite and phlogopite, in a fine to medium grained groundmass.
37.
The grain size of the magnetite and its degree of commingling with the silica groundmass determine the grind size to which the rock must be comminuted to enable efficient magnetic separation to provide a high purity magnetite concentrate.
38.
Two types of groundmass are generally recognized : the trachytic, composed mainly of long, narrow, subparallel rods of sanidine, and the orthophyric, consisting of small squarish or rectangular prisms of the same mineral.
39.
Porphyroid neomorphism occurs when a small number of large crystals form in the area of static groundmasses, which are as the name implies areas of the ground that are characterized by relatively insignificant and unsubstantial metamorphic changes.
40.
The distinctive characteristic of olivine lamproites is phlogopite macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to " tetraferriphlogopite " ( anomalously Al-poor phlogopite requiring Fe to enter the tetrahedral site ).
