The color ranges from gray through red, green and purple to black. The grays and black are due to the presence of more or less carbonaceous material in the original rock, the carbon compounds having changed to graphite. The reds and purple are due to the presence of iron oxides, and the green to the presence of chlorite.
While the particles of slate are so small as to be indistinguishable to the unaided eye, the use of thin sections under the microscope shows that slate is composed mostly of quartz and mica, with a wide range of accessory minerals, like chlorite, feldspar, magnetite, hematite, pyrite, calcite, graphite, etc.
According to their chief constituents slates may be distinguished as argillaceous-slate or argillite, bituminous-slate, calcareous-slate, siliceous-slate, etc.
Slate will be found here and there in the metamorphic areas of New England, the Piedmont Plateau, the Lake Superior region, and in many places in the west.
[Phyllite]
[Pl. 66]
Phyllite is a thinly cleavable, finely micaceous rock of uniform composition, which is intermediate between slate and mica schist. In this case the flakes of mica are large enough to be distinguishable to the eye, but most of the rest of the material can only be identified with the aid of a microscope. It is mostly quartz and sericite. Phyllite represents a degree of metamorphism greater than for slate, but less than for schist; and it may grade into either of these other rocks. Garnets, pyrite, etc., may be present as accessory minerals. The color ranges from nearly white to black, and it is likely to occur in the same places as do slates.
[Marble]
[Pl. 66]
This is a broad term, and includes all those rocks composed essentially of calcium carbonate (limestones) or its mixture with magnesium carbonate (dolomite), which are crystalline, or of granular structure, as a result of metamorphism. It takes less heat to metamorphose a limestone, and for this reason the marbles have a more crystalline structure than most metamorphic rocks; and they do not have the tendency to split or cleave which is so characteristic of most metamorphic rocks. It is only when there is a large amount of mica present that the typical schistosity appears. Commercially the term marble is used to include true marble and also those limestones which will take a high polish; but in this book, and geologically speaking, no rock is a marble unless it has crystalline structure.
Marbles range widely in color according to their impurities. Pure marble is white. Carbonaceous material in the antecedent limestone is changed to graphite in the metamorphic process, and makes the marble black, but appears usually in streaks or spots, rather than in any uniform color. An all black “marble” is usually a limestone. The presence of iron colors the marble red or pink. Chlorite makes it green, etc.
Various accessory minerals are common in marbles, such as mica, pyroxene, amphibole, grossularite among the garnets, magnetite, spinel, pyrite, etc., through a long list.