Identifying Metamorphic Rocks

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Unfoliated rocks produced by contact or regional metamorphism

Quartzite

Parent rock of quartzite is orthoquartzite, sandstone composed of quartz grains held together by quartz cement. Quartz grains in the sandstone recrystallise, forming interlocking quartz crystals. The quartz crystals are equidimensional so there can be no foliation. This texture is described as granoblastic. Any sedimentary structures or fossils in the parent sandstone are destroyed. The colour of quartzite is white or grey, unless there were other minerals in the original rock. For example, if any iron oxide was in the parent rock, there would be a pink colour.

Marble

Limestones are made essentially of one mineral, calcite, which is stable over a wide range of temperatures and pressures. As a result, metamorphism of limestone only causes the original calcite crystals to grow larger. Calcite grains and fossil fragments in the limestone parent rock recrystallise to form an interlocking mosaic of calcite crystals. The crystals are equi-dimensional, so there can be no foliation. Marble has a granoblastic texture, but the crystals of calcite make it look sugary in texture. Calcite will react will dilute HCl. Fossils are destroyed in metamorphism.

Marble from pure limestone is white. Impurities in the parent limestone give some marble a range of coloured streaks:

if there are clay minerals in the limestones, then a number of green or red minerals such as garnet may form

if there are sand grains present, a chemical reaction between calcite and quartz will produce wollastonite, which can be light green, pinkish, brown, red or yellow.

Rocks produced by contact metamorphism

Spotted rock

Because contact metamorphism involves only increased temperatures, it cannot produce foliation. During contact metamorphism, spots may form in the rock where heat has only partly recrystallised the rock. A spotted rock contains the same minerals as shale or slate. If slate is the parent rock of spotted slate, it will show foliation, but this was produced to pressure during regional metamorphism. The randomly orientated spots may contain biotie, andalusite and graphite, but they are usually to indistinct to the identified.

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