-
quartz: pure SiO2 is colorless, not normally used as a gem.
The most common gem varieties of quartz are amethyst
and citrine
- Amethyst
- comes in different shades of purple (due to Fe impurity, commonly
with zoned distribution):
- it is also set as a cabachon
- radiation damage and iron (Fe) impurity are necessary to produce amethyst color
- Fe - O charge transfer gives color: O-- <=> Fe4+
- Fe3+ in Si site -> Fe4+
- Other comments
- Citrine
- Smoky quartz:
- smoky quartz is Al+++- bearing quartz that has been exposed to radiation (natural and unnatural)
- it removes an electron from O-- (i.e., a color center effect)
- if there is sufficient Al and radiation, the crystal can turn completely black! ("morian")
*** HEAT TREATMENT RESTORES CLARITY****
- Also sometimes faceted
- Rose quartz
- has rose-pink color
- the color is often due to the element Titanium (Ti)
- used for beads etc.
- sometimes you can find star rose quartz which has inclusions
- often massive (aggregates of crystals, not single xals)
- Green quartz
- Milky quartz
- milky appearance due to inclusions (often of fluid)
- often associated with gold deposits - used as a gem mainly if gold present
Other
- Rutilated quartz is (normally) clear quartz that contains fine, often oriented, rutile crystals.
- Fe-oxide included quartz
- Tourmaline and other inclusions
is quite apparent
- Chatoyant quartz: due to the presence of needles of asbestos etc. Form when prexisting minerals are replaced by quartz ('fossilized')
- Other fibrous varieties, including agate and chrysophrase are discussed in later lectures
- Adventurine: green quartz containing platy inclusions of mica; can be used as a
substitute for jade; used in beads etc.
- Quartz can also contain inclusions of trapped fluid that may
contain mineral precipitates and gas bubbles
('fluid
inclusions') [Note the bubble of liquid and gas in this image
is round. The black line
is marker to draw your attention to the appropriate region).].
These tell us about the fluids present when the quartz formed.
For example, fluids may be quite saline, indicating quartz grew
from salt-rich solutions.
(2) Where does it come from ?
Quartz is a very common mineral in
most
igneous, metamorphic, and sedimentary
rocks.
It has also
been found in some lunar rocks and meteorites.
Quartz forms in rocks of igneous
origin (melts) only after other silicates have incorporated
most other available cations.
Rocks that are more than about 47 percent (by weight) silicon
dioxide contain quartz. The quartz content increases as the weight percentage
SiO2 increases.
Quartz can be dissolved in hot water or
steam and is thus transported from place to place in the
Earth, being deposited by cooling of the transporting fluid
or by release of pressure.
Because quartz is relatively
resistant to mechanical abrasion, it is abundant in stream
sediments, on beaches, and in wind-blown sands. Quartzite
and sandstone are mostly quartz because this is the most abundant mineral that survives processes occurring at the Earth's surface.
(3) Synthesis
Twinning produces many separate crystals of a mineral in rotated orientation, but
orderly arrangement.
Doped quartz (quartz containing an impurity such as cobalt (Co) may be synthesized to achieve bright colored materials.
(4) Heat Treatment: Citrine and amethyst:
- primitive methods can be employed
like using a wheelbarrow to immerse crystals in sand within a fire to heat them to a high enough temperature to modify the oxidation
state of the Fe.
(5) Quartz has some interesting properties
Piezoelectricity
(6) Other
some other comments
View some images of quartz varieties
(1) What is it ?
- Tanzanite is the important gem variety of zoisite (zoisite is also refered to as epidote)
-
Color is variable. Gem varieties are often sapphire blue or violet blue. However, other colors occur, like the
red variety (thulite), and a green
variety.
- Hardness: 6.5-7 (note this hardness is less than normally desirable for a gem used in jewelry subject to abrasion by dirt and dust)
- Crystal system: orthorhombic
- Composition: Ca-Al-silicate (tanzanite color is attributed to a Vanadium (V) as an impurity
- strongly pleochroic
- View an uncut crystal
(2) Where is it found?
- The variety of zoisite known as tanzanite was discovered in Tanzania in 1967
- it occurs in pegmatite veins
(inclusions suggest hydrocarbon-rich fluids)
(3) Other:
- usually heat treated at 400 - 500 C to remove bro
wn tints, deepen blue
- some cats eye varieties due to channels or needles
- best orientation for color not same for chatoyancy
View some images of tanzanite
Questions on the lecture
"Crysoberyl,
Rutile and Spinel"
"Olivine,
Cordierite and Feldspar"
To gemstones section
To Course Materials
To Index
Mineral Reference
Glossary
Movie showing quartz structure
