Name/Title
Petrified WoodEntry/Object ID
1984.62.34Description
Chemical Composition: SiO2
Crystal System: Triclinic System
Description: Common Name: Petrified Wood
Chemistry: SiO2
Class: Silicates
Location: Oregon
Description: (Butterfly) Brown and tans in an irregular banded pattern.
Physical Characteristics:
•Color is as variable as the spectrum, but clear quartz is by far the most common color followed by white or cloudy (milky quartz). Purple (Amethyst), pink (Rose Quartz), gray or brown to black (Smoky Quartz) are also common. Cryptocrystalline varieties can be multicolored.
•Luster is glassy to vitreous as crystals, while cryptocrystalline forms are usually waxy to dull but can be vitreous.
•Transparency: Crystals are transparent to translucent, cryptocrystalline forms can be translucent or opaque.
•Crystal System is trigonal; 3 2.
•Crystal Habits are again widely variable but the most common habit is hexagonal prisms terminated with a six sided pyramid (actually two rhombohedrons). Three of the six sides of the pyramid may dominate causing the pyramid to be or look three sided. Left and right handed crystals are possible and identifiable only if minor trigonal pyramidal faces are present. Druse forms (crystal lined rock with just the pyramids showing) are also common. Massive forms can be just about any type but common forms include botryoidal, globular, stalactitic, crusts of agate such as lining the interior of a geode and many many more.
•Cleavage is very weak in three directions (rhombohedral).
•Fracture is conchoidal.
•Hardness is 7, less in cryptocrystalline forms.
•Specific Gravity is 2.65 or less if cryptocrystalline. (average)
•Streak is white.
•Other Characteristics: Striations on prism faces run perpendicular to C axis, piezoelectric (see tourmaline) and index of refraction is 1.55.
•Associated Minerals are numerous and varied but here are some of the more classic associations of quartz (although any list of associated minerals of quartz is only a partial list): amazonite a variety of microcline, tourmalines especially elbaite, wolframite, pyrite, rutile, zeolites, fluorite, calcite, gold, muscovite, topaz, beryl, hematite and spodumene.
•Best Field Indicators are first the fact that it is very common (always assume transparent clear crystals may be quartz), crystal habit, hardness, striations, good conchoidal fracture and lack of good cleavage.
Fracture: Conchoidal
Hardness: 7 Quartz
Luster: Glassy
Occurrence: What is Petrified Wood?
Petrified wood is a fossil. It forms when plant material is buried by sediment and protected from decay by oxygen and organisms. Then, groundwater rich in dissolved solids flows through the sediment replacing the original plant material with silica, calcite, pyrite or another inorganic material such as opal. The result is a fossil of the original woody material that often exhibits preserved details of the bark, wood and cellular structures.
Some specimens of petrified wood are such accurate preservations that people do not realize they are fossils until they pick them up and are shocked by their weight. These specimens with near perfect preservation are unusual; however, specimens that exhibit clearly recognizable bark and woody structures are very common.
amethyst and citrine quartz
Some petrified logs contain a spectacular surprise. Cavities within them served as crystallization locations for quartz crystals such as the citrine (yellow, left) and amethyst (purple, right) shown here. Images by Petrified Forest National Park.
Petrified Forest National Park
The most famous locality for observing petrified wood is Petrified Forest National Park near the community of Holbrook in northeastern Arizona. About 225 million years ago this area was a lowland with a tropical climate and covered by a dense forest. Rivers flooded by tropical rain storms washed mud and other sediments into the lowlands. Enormous coniferous trees up to 9 feet in diameter and 200 feet tall lived and died in these lowlands. Fallen trees and broken branches were often buried by the river sediments. Nearby volcanoes erupted numerous times. These eruptions blanketed the area in volcanic ash with a high silica content.
Rapid burial allowed the plant debris to escape destruction by oxygen and insects. The soluble ash was dissolved by groundwater flowing through the sediments. The dissolved ash served as a source of silica that replaced the plant debris, creating petrified wood. Trace amounts of iron, manganese and other minerals were included in the silica and gave the petrified wood a variety of colors. These sediments, plant debris and volcanic ash became part of a rock unit known today as the Chinle Formation.
In the millions of years after the Chinle Formation was deposited the area was uplifted and the rocks deposited above the Chinle were been eroded away. The petrified wood is much harder and resistant to weathering than the mud rocks and ash deposits of the Chinle. Instead of eroding away the wood accumulated on the ground surface as the surrounding mud rocks and ash layers were eroded away. That is why areas of the Park are covered with a litter of petrified wood trunks, branches and fragments. Today, visitors to the park can observe the petrified wood and photograph it; however, collecting petrified wood in the park is prohibited.
Other Petrified Wood Localities
Petrified wood is not rare. It is found in volcanic deposits and sedimentary rocks at many of locations worldwide. It is sometimes found where volcanic activity covered plant material with ash, mudflows or pyroclastic debris. It is found where wood in sedimentary deposits was replaced by minerals precipitated from groundwater. It is especially abundant around coal seams, although many of the wood specimens in these locations are casts and molds rather than petrifications. One almost unbelievable material from Western Australia is known as "peanut wood" because of its ovoid markings but those markings are actually boreholes drilled by a clam!
In the United States, noteworthy locations where abundant fossilized wood can be seen include: •Petrified Forest National Park near Holbrook, Arizona
•Petrified Palm Deposits in the Catahoula Formation of Louisiana, Texas and Mississippi
•Ginkgo Petrified Forest near Wanapum Reservoir, Washington
•The Petrified Forest near Calistoga, California
•Mississippi Petrified Forest near Flora, Mississippi
•Gilboa Fossil Forest near Gilboa, New York
•Florissant Fossil Beds near Florissant, Colorado
•Gallatin Petrified Forest near Yellowstone, Wyoming
•Escalante Petrified Forest State Park near Escalante, Utah
•Petrified Wood Park near Lemmon, South Dakota (a rock sculpture park - some made of local petrified wood)
Rock Type: Sedimentary
Specific Gravity: 2.65
Streak: WhiteCollection
Leland "Cap" MentzerAcquisition
Accession
1984.62Source or Donor
Leland "Cap" MentzerAcquisition Method
Long-term LoanDimensions
Width
6-3/4 inDepth
1/2 inLength
8-1/2 inLocation
Location
Container
RightDrawer
BottomShelf
Center, CenterWall
NorthRoom
Mentzer HallBuilding
Mentzer Hall: "Caps Collectables" North case#9Category
PermanentMoved By
Steve MillerDate
March 23, 2011Location
Shelf
BottomCabinet
Case# 9Building
Mentzer Hall: "Caps Collectables"Category
Permanent
