Dunure 015

Dunure, Ayrshire, Scotland west coast. 30mm x 20mm

A small, but feature-packed agate from the west coast of Scotland. This specimen is a water worn beach agate eroded out of Early Devonian host rock, approximatel;y 380 million years old. It demonstrates the first chalcedony layer, hemiagates, asymmetric wall banding passing to symmetric wall banding, coarsely fibrous white bands, escape tubes with later inflow of iron minerals, cracks associated with sector boundary weaknesses and chromatography.
  • 1 There is a moderately wide, brown first chalcedony layer of even thickness, with no obvious structure or texture, extending around the margin of the entire agate.
  • 2. The first chalcedony layer is followed by a series of hemiagates characterised by a small initial opaque white hemisphere, followed by numerous fine bands of similar width. The hemiagates are present throughout, but are most frequent along the base

A small, but feature-packed agate from the west coast of Scotland. This specimen is a water worn beach agate eroded out of Early Devonian host rock, approximatel;y 380 million years old. It demonstrates the first chalcedony layer, hemiagates, asymmetric wall banding passing to symmetric wall banding, coarsely fibrous white bands, escape tubes with later inflow of iron minerals, cracks associated with sector boundary weaknesses and chromatography. In more detail:

  • 1. There is a moderately wide, brown first chalcedony layer of even thickness, with no obvious structure or texture, extending around the margin of the entire agate.
  • 2. The first chalcedony layer is followed by a series of hemiagates characterised by a small initial opaque white hemisphere, followed by numerous fine bands of similar width. The hemiagates are present throughout, but are most frequent along the base.
  • 3. A finely banded wall banding zone that is quite strongly asymmetric, broadest at the base (where the hemiagates are also best developed) but narrowing to virtually nothing in the north east.
  • 4. A very broad, white, coarsely fibrous band that is partly asymmetric, thinning to the north east but not disappearing.
  • 5. This marks a significant change, from asymmetric banding to symmetric banding with alternations of white and grey coarsely fibrous bands without mineral inclusions and pale lilac chalcedony bands with regularly distributed haematite inclusions.
  • 6. The alternations give way to a broad area of haematite speckled chalcedony, a darker (clear?) band with no inclusions, a white (but not obviously fibrous) band with sparse inclusions and, finally, a crystalline quartz fill.
  • 7. There are two very obvious escape tubes at the left side of the agate. Both reveal their true “inside to outside” flow nature by the deformation of the bands they have pushed through, bending them from inside to out. This can only have been produced by an inside to outside fluid movement. It also proves that, at the time of the laid movement, the bent and realigned bands of the agate were sufficiently plastic to undergo deformation without fracture.
  • 8. The lower escape tube has a small dilation, or swelling at the end. This formed when the escaping fluid met resistance from the already crystallised first chalcedony layer and spread sideways rather than forward.
  • 9. Having provided an exit route for internal fluid, both escape tubes allowed external, iron rich fluids to enter along the escape tube and deposit both goethite (yellow) and haematite (red) particles first along the tube and then laterally along the intersecting bands. It is obvious from the distribution of these particles that the flow of mobilised iron was from outside to inside. The separation of goethite and haematite particles is clear and they do not seem to mix. The hematite within the upper escape tube is formed as zoned clumps with darker centres, suggesting that the iron entered the agate as dissolved iron ions which then precipitated (as opposed to the transport of already formed iron oxide particles).
Detail of upper escape tube showing inside to outside band deformation and outside to inside iron mobilisation with separation of goethite and haematite and zoned haematite within the escape tube.

    Detail of upper escape tube showing inside to outside band deformation and outside to inside iron mobilisation with separation of goethite and haematite and zoned haematite within the escape tube.

    • 10. A bright red patch between white fibrous bands at the right hand side of the agate was also caused by mobilisation of iron. In this case, the iron was transported along a fine crack that can be seen extending from the quartz centre to the outer white bands. This crack follows the fortification angles of the agate and follows the boundary between growth sectors – where chalcedony growing in different directions meet. This is often a line of weakness in agate that allows fractures to develop.
    • 11. There is a chromatographic shift in the lower part of the agate, affecting primarily the asymmetric banded zone. It is white to the left and brown to the right. What this represents, chemically, is unclear.

    2 responses to “Dunure 015”

    1. chris.harlow.ch Avatar
      chris.harlow.ch

      You’re getting there. Big learning curve though.

    2. chris.harlow.ch Avatar
      chris.harlow.ch

      Still a ways to go.