Answers to Reading Guide - Chapter #1

Question 1.
 
Substance
natural
solid
homogeneous
definite formula
inorganic
highly 
ordered 
a true mineral?
sandstone
Y
Y
N
N
Y
N
N
cubic zirconia
N
Y
Y
ZrO2
Y
Y
N
obsidian
Y
Y
Y
N
Y
N
N
natural pearl
_
_
_
CaCO3
N
N
N
steel
-
_
_
no
_
_
N
lodestone
_
_
_
Fe3O4
_
_
Y
lava
_
-
-
-
_
-
N
iron
_
_
_
Fe
_
_
Y
magnesium
-
_
_
Mg
_
_
N
coal
_
_
-
-
-
-
N
asbestos
_
_
_
_
_
_
Y

2. A mineral is (strictly speaking) a crystalline solid (i.e. possessing internal three-dimensional periodicity) but it must also form in nature without direct intervention from living organisms. A specific term, "mineraloid", covers the naturally occurring inorganic substances (such as mercury or opal) that are amorphous. They occur among minerals and form by the same type of abiotic processes, but do not acquire the same highly ordered structure.

  Crystals are solid and necessarily possess internal three-dimensional periodicity (i.e. the regular repetition of an atomic pattern) but they include any highly ordered solid secreted or artificially  synthesized by living organisms. Therefore, crystals include minerals (defined as above) but not vice-versa.

3. a) The law of constancy of angles (enunciated by Steno in 1669) states that the angles between equivalent faces of the same substance, measured at the same temperature, are constant (p. 35).

b) Habit (p. 52) refers to the general shape of a crystal (fibrous, prismatic, platy) rather than to its specific morphology (dipyramid, rhombohedron, etc.).

5. a) Hauy's hypothesis (crystals are made of a large number of small, identical integral molecules) preceded the invention of the polarized microscope. That instrument would not have helped him particularly since the actual size of the small building blocks (i.e. the atomic pattern that is repeated over and over) in crystals is only a few angstroms (10-10 m) across. A typical polarizing microscope cannot resolve objects smaller than about 5 micrometers (10-6 m).

b) The italicized part of the definition "naturally occurring homogenenous solid with a definite chemical composition and a highly ordered arrangement, usually formed by inorganic processes" is the modern wording of Hauy's hypothesis.

c) A cube will always have its faces at 90o because its atomic pattern is repeated at equal distances along three perpendicular axes (isometric). Isometric units that are stacked to form six flat faces therefore always produce perpendicular faces (constancy of angles). On the other hand, more layers of blocks might be added to some faces than on others, resulting in faces that would differ in area and shape. The cube would be distorted, and such variations would be referred to as different "habits" (e.g., platy or acicular).

6.a) A goniometer is a device used to measure interfacial angles (i.e. the angle between faces on a crystal).

b) Two types of goniometers are the contact goniometer and a reflecting goniometer.

c) The contact goniometer is less accurate but useful to measure interfacial angles on relatively large specimens (sizes of several cm across or more). It will work even on a specimen with dull or slightly damaged faces that cannot reflect a sharp beam of light.

The reflecting goniometer requires a specimen with faces that reflect light but it is more accurate and works best on specimens that are only a few cm or mm across in size.

7. X-ray diffraction proved for the first time that the highly ordered internal arrangement of crystals existed at the atomic scale.

8. Dates, properties and instruments should be matched as follows:
 
Since the Stone Age colour human eye
Since 1780 interfacial angles contact goniometer
Since 1828 light polarization polarizing microscope
Since 1912 internal order X-ray diffractometer
Since early 1960s chemical composition electron microprobe
Since early 1970s defects electron microscope

9. a) Most elements are reactive in their pure state under most terrestrial conditions. All tend to adopt the electronic configuration of the closest inert gas, by either gaining electrons (thus becoming anions) or losing electrons (thus becoming cations).

b) Any five among these: Au, Pt, Ag, Cu, C, Sb, Ir, Fe, Pd, S.

c) You might have singled out halite (NaCl), diamond and graphite (C), as well as any mineral used as a gem (e.g., diamond, topaz, corundum). Metals, although they can occur fairly pure in their natural state, often form alloys with one another, and must frequently be separated for industrial use (e.g. gold-silver) .