CONNECTED MAGNETITE AND OPAL STRUCTURES IN “GLACIAL SEDIMENTS” REVEAL CHEMICAL, NOT MECHANICAL, EROSION

Microscopic examination shows silica “sand” in the Juneau Alaska area is not abraded rock.

“Sands” from the beneath the Mendenhall glacier and local estuaries, show particles joined by delicate silica (and sometimes magnetite) connections to form masses 1cm in diameter and larger.

What appear to be poorly sorted “glacial outwash” deposits contain many interconnected gravel and sand-sized particles. Grain surfaces show textures, ridges and spines, that can only be formed in a mold. Interconnected grains usually have a common longitudinal axis.

Silica and magnetite grains are intergrown and connected in complicated chains. Clumps and chains of grains with very different mineral compositions are formed by the disintegration of granular rocks such as Eagle River tonalite. “Core stones” are covered with chains of grains arranged in regular grids of intersecting and radiating lines. Small dark grains in this “sand” are mostly magnetite. Most of the silica appears to be opal.

Grains often fit together like a plug into a socket. If the material remains damp, it tends to stay connected, like wet plastic buckets nested together. Individual grains have definite attachment structures and core rocks have corresponding surfaces and structures to receive them. Light pipes through the silica and emerges through pores that correspond with these attachment points.

Simultaneous deposition of magnetite and opal and the fibrous nature of the silica, suggests chemical weathering in situ, mediated by bacteria and fungi: biofilms. Core stones covered with attached sand usually have rounded surfaces, showing rounding is not caused by abrasion but by disintegration of rock from the outside inward.

Similar materials are found in Oregon, Maine and Tasmania.