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Zeolites from Rock Island Dam, Upper Columbia River, Washington, USA

Rock Island is a hydrothermal locality situated in Tertiary volcanics at the base of the Rock Island Dam. It was unearthed when dredging for repair of the dam's footings was undertaken in the early 1950s. Some of this material is accessable in tips along the margin of terraces adjacent to the access road just to the south of the dam on the west bank of the Columbia, including the fore- and middle-ground of this photo. Specimens occur in small cobbles through small boulders of vessicular basalt, welded tuff and ignumbrite.
Material can also be collected on the east side of the river along the beach below the dam at low water. The SEMs come from material collected on the east bank. As part of repair and maintenance, cores were taken accross the reservoir just in back of the dam. No hydothermal deposits were encountered, so the actual source of the deposit remains a mystery. Right - arrow marks the collecting area on the west bank.
Left - Upper arrow is the downstream extent of material on west bank. Lower arrow suggests the location of the collecting beach on east bank when the water is lower.

Follow the image links for more SEMs

Erionite or Offretite (colorless)
    Hexagonal rods. Not striated
    Often bent or curved
    Usually associated with Heulandite

Erionite or Offretite (colorless)
    Striated rods
    Usually associated with Heulandite
Heulandite with Erionite or Offretite (colorless)
    Order is commonlly:
      Clay mineral
      Offretite or Erionite
      Spehres of unidentified mineral (pale brown to trnanslucent redish brown)
      Heulandite (either plates or coffins)
      Spheres of unidentified mineral
Heulendite without the fuss

    Interlocking domes as continuous cavity linings
    Brown to bistre in crossection
    Brown to velvet coffee black-brown on top
    May grow out of base of translucent brownish red resin
    Usually the first thing to crystalize
    Some spheres crystalizing at later stages could be Goethite

Unidentified Associated Minerals
    Tiny rosettes of interlocking fibers
    Small spheres, possibly of a clay mineral
    Acicular Pyrite

If you would like samples for identification or analysis, or want specimens for your micromount collection, drop me a line.