Page 152 - Laboratory Manual in Physical Geology

Pyroclastic grains (tephra) from the 18 May 1980
eruption of Mount St. Helens, Washington

Volcanic ash Lapilli or cinders
(pyroclasts <2 mm) (pyroclasts 2–64 mm)

FIGURE 5.3 Pyroclastic grains (tephra). These samples of volcanic ash and lapilli (cinders) were ejected from Mount St. Helens,

then collected and photographed by D. Wieprecht. (Image courtesy of U.S. Geological Survey. Scale x1.)

Using the Mafic Color Index (MCI) ACTIVITY
The mafic color index (MCI) of an igneous rock is the
percentage of its green, dark gray, and black mafic (fer- 5.4 Glassy and Vesicular
romagnesian) mineral crystals. If the rock has no visible Textures of Igneous Rocks
mineral crystals, then the overall color of the rock is used to
estimate its mafic color index and corresponding compo- THINK What are igneous rock textures? How
sitional group. A white, pale gray, or pink rock has a felsic | is texture used to classify and interpret
MCI (0–15%) and compositonal group. A moderately About It igneous rocks?
medium-gray rock has an intermediate MCI (16–45%) and
compositional group. A very dark gray rock has a mafic OBJECTIVE Experiment with molten sugar to

MCI ( 46–85%) and compositional group. A black or produce glassy and vesicular textures, then apply your
dark green rock has a mafic MCI (above 85%) and knowledge to interpret rock samples.
compositional group.
If the rock has visible crystals, then you should use a PROCEDURES
Visual Estimation of Percent chart to estimate the mafic 1. Before you begin , read about Textures of Igneous
color index as closely as possible. Rocks below. Also, this is what you will need :
The mafic color index of an igneous rock is only an ____ Activity 5.4 Worksheet (p. 146 ) and pencil
approximation of the rock’s mineral composition, because ____ sugar
there are some exceptions to the generalization that “light- ____ materials provided in lab: hot plate, small
colored equals felsic” and “dark-colored equals mafic.” For metal sauce pan with handle or 500 mL
example, labradorite feldspar (felsic) can be dark gray to Pyrex™ beaker and tongs, water (~50 mL),
black. Luckily, it can be identified by its characteristic play safety goggles, aluminum foil, hand lens,
of iridescent colors that flash on and off as the mineral is sugar (~50 mL, 1/8 cup), and hot plate
rotated and reflects light. Olivine (mafic) is sometimes a ____ collection of numbered igneous rock
pale yellow-green color (instead of medium to dark green). samples
Volcanic glass (obsidian) is also an exception to the mafic
color index rules. Its dark color suggests that it is mafic 2. Then follow your instructor’s directions for
when, in fact, most obsidian has a very high weight percent- completing the worksheet.
age of silica and less than 15% ferromagnesian constituents.
(Ferromagnesian-rich obsidian does occur, but only rarely.)

Using Point Counting Textures of Igneous Rocks
Point counting is counting the number of times that Texture of an igneous rock is a description of its constitu-
each kind of mineral crystal occurs in a specified area ent parts and their sizes, shapes, and arrangement. You
of the sample, or along a line randomly drawn across must be able to identify the common textures of igneous
the sample, then calculating the relative percentage of rocks described below and understand how they form.
each mineral. Notice the list of textures and their origins in FIGURE 5.4 .

Igneous Rocks and Processes ■ 133