13.7: What Is a Non-Foliated Metamorphic Rock?

What Is a Non-Foliated Metamorphic Rock?

Metamorphic rocks that lack foliation are referred to as non-foliated. This is primarily due to a lack of differential pressure involved during metamorphism. However, if the protolith rock is monomineralic (composed of one mineral type), such as limestone, dolostone, or sandstone, then a foliation will not develop even with differential pressure. Why? Calcite, dolomite and quartz sands are not platy minerals. These minerals will recrystallize into equant, coarse crystals, and the resulting metamorphic rock is named for its composition, not by foliation type.

Quartzite Links to an external site., is a metamorphic rock made almost entirely of quartz, for which the protolith was quartz sandstone (Figure 13.13). Because quartz is stable over a wide range of pressure and temperature conditions, few-to-no new minerals form in quartzite during metamorphism. Instead, the quartz grains recrystallize into a denser, harder rock than the original sandstone. If struck by a rock hammer, quartzite will commonly break right through the quartz grains, rather than around them as in quartz sandstone. Colors will vary, and a hardness test must be done to be sure the sample is quartzite. Quartzite is fairly tough and will sometimes be used decoratively in building stone, facades, or for kitchen or bathroom countertops.

Marble Links to an external site. is a metamorphic rock made entirely of calcite or dolomite, for which the protolith was limestone or dolostone (Figure 13.14). Marble may have bands of different colors which were deformed into convoluted folds while the rock was ductile Links to an external site.. Marble will typically be soft, and can be scratched with a steel nail, and if there is enough calcite present, will react (effervesce, fizz, bubble) when HCl is added. Marble is also used as decorative stone in buildings, or in countertops in homes. Italy has the famous Carrera Marble Links to an external site. mine, which many sculptors, including Michelangelo Links to an external site., used for their great works. Additionally, many older grave markers, particularly those that appear blank or heavily worn, are likely composed of marble.   

Anthracite coal Links to an external site. (Figure 13.15) is much harder than the coal varieties we discussed in the Sedimentary Rock chapter. It is black in color with a submetallic luster. Anthracite accounts for <1% of coal mined in the US with most mined from northeastern Pennsylvania; it is the highest grade of coal, with around 86%–97% carbon content, which makes anthracite a hotter burning coal. Under higher temperatures and pressures the protolith, bituminous coal, loses more volatiles (water vapor), but the carbon content is enriched. This metamorphism is not recrystallization, as coal is mostly organic remains; however, due to changes in temperature and pressure, bonds are still broken and reformed.

Table 13.2: Non-foliated metamorphic rock chart.

Metamorphic Rock	Dominant Composition	Metamorphic Environment & Grade	Protolith
Quartzite	Quartz Test: scratches glass (H>5.5)	Contact intermediate to high	Sandstone
Marble	Calcite or Dolomite Test: Soft, will not scratch a copper piece (H=3); may react with HCl	Contact low to high	Limestone, dolostone
Anthracite	Carbon Test: Light heft, black color with a submetallic luster	Burial high	Bituminous coal