Page 75 - 4571

buildings, clay minerals are extremely important to humans. One of the most common
clay minerals is kaolinite, which is used in the manufacture of fine china and as a
coating for high-gloss paper.
The dark (or ferromagnesian) silicates are those minerals containing ions of
iron and/or magnesium in their structure. Because of their iron content, ferromagnesian
silicates are dark in color and have a greater specific gravity, between 3.2 and 3.6, than
nonferromagnesian silicates. The most common dark silicate minerals are olivine, the
pyroxenes, the amphiboles, dark mica (biotite), and garnet.
Olivine is a family of high-temperature silicate minerals that are black to olive
green in color and have a glassy luster and a conchoidal fracture.
The pyroxenes are a group of complex minerals that are important components
in dark colored igneous rocks. The most common member of this group, augite, is a
black, opaque mineral with two directions of cleavage that meet at nearly a 90-degree
angle.
Hornblende is the most common member of another chemically complex group
of the dark silicates called amphiboles.
Biotite is the dark, iron-rich member of the mica family. Like other micas,
biotite possesses a sheet structure that gives it excellent cleavage in one direction.
Biotite also has a shiny black appearance that helps distinguish it from the other dark
ferromagnesian minerals.
Garnet is similar to olivine. Also like olivine, garnet has a glassy luster, lacks
cleavage, and exhibits conchoidal fracture. Although the colors of garnet are varied, this
mineral is most often brown to deep red.

Task 3. Look at Figure 13.1 and try to guess the names of the given
minerals. Which of them are light silicates? Which of them are considered to be
dark silicates? Provide as much information as possible about the minerals.

Task 4. Discuss the following:
1. The light (or nonferromagnesian) silicates.
2. The dark (or ferromagnesian) silicates.

Individual work

Task 1. Read the text and translate it into Ukrainian (in written form).
Build up a list of key terms to the text.

Important nonsilicate minerals
Although the nonsilicates make up only about 8 percent of Earth’s crust, some
minerals, such as gypsum, calcite, and halite, occur as constituents in sedimentary rocks
in significant amounts. Furthermore, many others are important economically. Some of
the most common nonsilicate minerals belong to one of three classes of minerals—the
carbonates, the sulfates and the halides. The two most common carbonate minerals are
calcite and dolomite. Calcite and dolomite are usually found together as the primary
constituents in the sedimentary rocks limestone and dolostone. When calcite is the
dominant mineral, the rock is called limestone, whereas dolostone results from a
predominance of dolomite. Limestone has many uses, including as road aggregate, as
building stone, and as the main ingredient in Portland cement.
Two other nonsilicate minerals frequently found in sedimentary rocks are halite
and gypsum. Both minerals are commonly found in thick layers that are the last vestiges
of ancient seas that have long since evaporated. Like limestone, both are important non-

74