Oregon sunstone

Copper inclusions

If you wait around long enough, even unusual things can happen, geologically speaking. Beginning about 17 million years ago in south-central Oregon USA, vast flows of basalt began to erupt, ultimately covering much of eastern Oregon and adjacent areas: The Columbia Plateau Basalt. In places eruptions continued until a few million years ago, and some of the volcanic rocks are rhyolitic rather than basaltic. It’s all pretty spectacular, but not incredibly unusual in earth’s history.

In vugs (open spaces that often were originally gas bubbles in the molten rock) in the basalt or rhyolite, late-stage minerals sometimes form. Lots of different minerals are possible, but feldspars, among the commonest minerals in the earth’s crust, are fairly common in those vugs. There are several different varieties of feldspar defined mostly by their composition, and in volcanic flows oligoclase, a specific sodium-calcium aluminosilicate, is typical, but in an area around Plush, Oregon, the mineral labradorite forms in the rocks. Labradorite is similar to oligoclase, but labradorite has more calcium than sodium while oligoclase has a greater proportion of sodium. Technically, oligoclase and labradorite are no longer discrete minerals, but are varieties of albite and anorthite and part of the albite-anorthite series comprising the plagioclase feldspars. Let me ignore that for this post, please.

OK, so volcanic flows with labradorite crystals in vugs isn’t a very big deal. Feldspars, especially the group called plagioclase which includes oligoclase and labradorite, often (even usually) crystallize in separate crystals oriented differently but in contact with each other – an arrangement called twinning. In plagioclase, repeated crystals in twin contacts on parallel planes make a version called “polysynthetic twinning” or albite twinning, and the thin parallel crystals are called twin lamellae. The parallel contact twin planes between each thin crystal are expressed as fine parallel lines in the mineral, visible especially in the left part of the crystal in the top photo. OK fine, and maybe that’s too much jargon – sorry.

But those twin planes between the thin lamellae are important, because they are zones of very slightly lesser integrity in the overall rock, places where it can be easier for external stuff to get inside the rock along those planes. So now we get to the pretty unusual part.

There were hot waters associated with the volcanic flows (no surprise), and from some unknown place they dissolved some copper, pure elemental copper. Those copper-bearing fluids came in contact with the labradorite in the vugs of the rocks, where they worked their way into the parallel twin planes of the labradorite, where tiny blebs of pure copper were deposited. The copper was not only deposited on those flat twin planes, but the bits of copper grew in flat plates parallel to the twin planes, and more or less parallel to each other. The consequence of that is that when you tilt a crystal that has these tiny (less than a tenth of a millimeter) copper blebs in it, they reflect the light similarly because they are all parallel to each other, like tiny little mirrors or solar panels in a solar-power field. That coppery reflection gives the clear labradorite a distinctive sheen, and the rock is called sunstone.

In my photos here, the entire specimen of labradorite is about 19 mm long, and I think you can see the parallel twin planes that are about a millimeter apart. It’s challenging to photograph the tiny blebs of copper on those planes because they are inside the rock, but I hope you can get an impression of how they are situated and oriented.

Similar rocks with flashy inclusions of mirror-bright steely or silvery black hematite (iron oxide) also are sometimes called sunstone. The copper-bearing sunstone of Oregon is close to unique in the world, as far as I can tell.

Not surprisingly, labradorite was named for occurrences in Labrador, but it’s found in many locations worldwide. Sometimes (and famously) labradorite has an iridescent purplish-blue sheen (called labradorescence, or schiller, German for shimmer) that results from light interference patterns (like an oil slick) caused by extremely thin intergrowths of slightly different composition, but the labradorite that makes Oregon sunstone doesn’t have that quality; the copper is deposited on twin planes.

Iridescent labradorite from the Archean Nain Complex, Tabor Island, Labrador.