The orientation of the mica flakes accounts for the pronounced layering of most schists. All of the flakes are parallel and when folded and crumpled they give the rock its structure. Where the muscovite is most abundant, breakage along planes rich in mica produces smooth, shiny surfaces which may give the rock a slippery appearance. The layers rich in mica may show in a rather perfect manner the small-scale folding of the schist.
Albite and quartz are also important constituents of the schists. Together they are more abundant than mica in the average schist on Mount Mansfield but because they are not so “showy” they are more easily overlooked. Both minerals are white and granular. Quartz, which consists of silica or SiO₂, is the principal constituent of most beach sands. In the typical mica schist, quartz is glassy in appearance and occurs as small rounded or irregular grains without plane surfaces. Albite is a variety of plagioclase feldspar having the composition of NaAlSi₃O₈. It has a white chalky appearance and occurs in small equidimensional grains bounded by flat surfaces which break along plane surfaces that reflect light in certain positions. On the west side of Mount Mansfield some of the rocks contain so much albite and so little mica that the rock is granular in appearance.
Chlorite, variety pennine, is an important mineral constituent of some of the schist. Chlorite is characterized by its green color and small amounts are responsible for giving a greenish cast to much of the schist. Like mica it occurs as thin sheets which reflect the folding of the schist.
Garnet and magnetite are locally abundant minerals in the schist. Garnet occurs as pink to red grains ranging from pin point to pea size. Most of the grains are rounded although a few occur as equidimensional crystals which have twelve equally developed faces. Garnet has a semitransparent, glassy appearance and is harder than a knife blade. Magnetite occurs as bluish-black metallic masses with about the same size range as the garnet. Although most are rounded masses, crystal faces are developed on some. Perfectly developed crystals occur as octahedrons which is the form consisting of eight faces, as two four-sided pyramids with their bases together. The larger grains of magnetite have sufficient magnetic power to attract or deflect a compass needle. The garnet and magnetite usually do not occur together, but each may form localized concentrations as lenses or layers in the schist.
In smaller amounts, usually visible only with a hand lens or microscope, the schist also contains a green mineral called epidote, a white mineral apatite, and an elongated black mineral called tourmaline. Locally, as on the Nose Dive ski run above the Toll Road on Mount Mansfield, slender needles of tourmaline are visible in the schist.
When a piece of rock is sawed and ground to a thickness of 0.03 millimeters, many of the minerals that appear opaque are found to be transparent. By their color and their optical properties the minerals can be accurately identified. On the basis of the amount of each present the mineral and chemical composition of the rock can be determined. [Figure 2] shows the appearance of a thin section of the mica-albite-quartz schist from the Forehead of Mount Mansfield. The parallel orientation of the mineral grains is apparent even though the photomicrograph represents a very small area of the schist.
Figure 2. Photograph taken through a microscope at a thin section of the schist. The white and gray grains are quartz and albite and the dark colored elongate grains are either muscovite or chlorite. The magnification is about one hundred times normal size. Even at this scale, the layering of the minerals is clearly visible.
Other varieties of the schist occur less abundantly in the area. These contain the same minerals as the mica-albite-quartz schist but in different proportions. If the mica is most abundant, as it is locally on Mount Mansfield, the schist may be smooth or highly crinkled and have a very shiny appearance. If the albite is most abundant the schist is more uniform and granular in appearance and the rocks are more massive. Small scale folding is usually absent. Such albite schists occur on the west side of Mount Mansfield, particularly along the lower part of the Maple Ridge Trail and in the cliffs south of the Forehead along the Long Trail.
If the quartz is most abundant, but mica and albite are present in considerable quantities, the rock may have a granular, layered appearance. Locally some of the rocks consist almost entirely of quartz and are classified as quartzite. These rocks have a dense, fine-grained, sugary appearance and generally are gray to bluish gray in color. They are hard rocks and often form minor ledges in cliff exposures or are the resistant rock at the top of small waterfalls in some of the creeks. Most of the quartzite in the area occurs in narrow layers less than a foot thick. Although these layers cannot be traced, they are most abundant on the east side of Mount Mansfield at various localities about one-third of the way up the mountain.