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Veined chert is a sedimentary rock characterized by a matrix of chert, which is a hard, dense, cryptocrystalline form of silica, intersected by distinct veins of macrocrystalline quartz. The chert matrix can vary widely in color (grey, black, brown, red, green, white) and texture (smooth to slightly granular). The quartz veins are typically white or clear, but can be stained by impurities, and exhibit a more crystalline appearance, often showing individual quartz crystals (though sometimes massive). The veins represent healed fractures within the chert, indicating a history of brittle deformation and subsequent mineral infilling.
How to Identify
- Color
- Chert matrix can be various colors (grey, black, brown, red, green, white, tan). Quartz veins are typically white, clear, or milky, but can be stained by iron oxides (red/orange) or other impurities.
- Luster
- Chert matrix typically dull to waxy; quartz veins vitreous (glassy).
- Texture
- Chert matrix is very fine-grained, smooth to slightly granular. Quartz veins show crystalline texture, from fine-grained to coarse-grained interlocking crystals, often visible to the naked eye.
- Crystal Form
- Chert is cryptocrystalline (crystals too small to see without magnification). Quartz veins exhibit macrocrystalline quartz, often as euhedral to anhedral crystals filling fractures.
- Cleavage
- Neither chert nor quartz exhibits true cleavage. Both display conchoidal fracture.
- Geological Environment
- Common in sedimentary sequences, particularly within limestone, dolomite, and shale formations where chert beds or nodules are present. Also found in areas of hydrothermal activity or where silica-rich fluids have circulated through fractured chert.
Key Facts
- Hardness: 7 (Mohs scale) for both chert and quartz.
- Specific Gravity: 2.58 - 2.64 g/cm³ (for both chert and quartz).
- Crystal System: Trigonal (for macrocrystalline quartz in veins); Chert is cryptocrystalline, so no macroscopic crystal system.
- Color: Chert matrix highly variable; Quartz veins typically white, clear, or milky.
- Luster: Chert: dull to waxy; Quartz veins: vitreous.
- Transparency: Chert: opaque to translucent; Quartz veins: translucent to transparent.
- Fracture: Conchoidal (for both chert and quartz).
- Cleavage: None.
- Composition: Silicon dioxide (SiO2).
Quick Check
- Color: Variable chert matrix (grey, black, brown, etc.) with white/clear quartz veins.
- Luster: Chert: dull to waxy; Quartz veins: vitreous.
- Streak: White (for both chert and quartz).
Physical Characteristics
- Crystal Habit: Chert: cryptocrystalline, massive. Quartz veins: massive, granular, or as euhedral to anhedral crystals filling fractures.
- Cleavage Type: None.
- Fracture Type: Conchoidal.
- Tenacity: Brittle.
- Luster Type: Chert: dull to waxy. Quartz veins: vitreous.
Formation
Veined chert forms when pre-existing chert, a microcrystalline or cryptocrystalline variety of quartz, is fractured. These fractures then serve as conduits for silica-rich hydrothermal fluids or groundwater. As these fluids cool or undergo changes in pressure and chemistry, dissolved silica precipitates as macrocrystalline quartz within the fractures, forming veins. The chert itself typically forms from the accumulation and diagenesis of siliceous skeletal remains (e.g., radiolarians, diatoms, sponge spicules) or by direct precipitation from silica-rich waters.
Usage
Historically, chert (including veined varieties) was extensively used for tool-making due to its conchoidal fracture, which produces sharp edges. The quartz veins, being harder and more brittle, can sometimes complicate knapping but also add aesthetic appeal. Modern uses are limited, primarily as aggregate, road fill, or occasionally as decorative stone. Collectors value specimens for their aesthetic contrast between the chert matrix and the quartz veins.
Age Distribution
Found in chert formations of all geological ages, from Precambrian to Cenozoic, wherever chert has undergone fracturing and subsequent quartz precipitation.
Where to Find
Ohio, USA
Flint Ridge State Memorial is famous for its high-quality, colorful chert, often exhibiting quartz veins, used extensively by Native Americans.
Missouri, USA
Ozark region, abundant chert formations, including veined varieties, often found in stream beds and road cuts.
England
Flint nodules in chalk cliffs (e.g., Dover) can sometimes show quartz infilling of fractures.
Australia
Various chert deposits, particularly in ancient sedimentary basins, may contain quartz veins.
Worldwide
Chert is a common sedimentary rock, and veined chert can be found in any region with significant chert deposits that have undergone fracturing and subsequent silica precipitation.
Finding Tips
Look in Sedimentary Environments
Search in areas with exposed sedimentary rocks, especially limestone, dolomite, and shale formations, where chert nodules or beds are common. Riverbeds and gravel pits are also good places to find weathered chert fragments.
Identify Fractures
Look for rocks with distinct linear or irregular patterns of a different color or texture running through a chert matrix. The veins will typically be lighter (white, clear) than the surrounding chert.
Test Hardness
Both chert and quartz are hard (Mohs 7). They will scratch glass and steel. This helps distinguish them from softer rocks.
Observe Fracture Pattern
Both chert and quartz exhibit conchoidal fracture, producing sharp, curved surfaces. The veins might show a more crystalline break if individual quartz crystals are large enough.
Similar Rocks
Jasper
Opaque, microcrystalline Quartz with iron inclusions
Also known as: Red Chert
Flint
Dark grey to black cryptocrystalline Quartz, typically found in chalk
Also known as: Nodular Chert
Agate
Banded variety of chalcedony (microcrystalline quartz)
Also known as: Banded Chalcedony
Quartzite
Metamorphosed quartz sandstone
Also known as: Metamorphosed Sandstone
Scientific Classification
- Mineral Class
- Silicate (Tectosilicate)
- Group
- Quartz Group
- Crystal System
- Trigonal (for quartz); Chert is cryptocrystalline.
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide
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