Accretionary Wedge #42: Countertop Geology of two marbles

The Accretionary Wedge is a semi-regular collection of geoblog posts that follow a common theme. Ian Saginor is hosting AW #42 at his volcanoclast blog with the theme of Countertop Geology. For my first ever Wedge, Ian has tasked everyone to:

  • Find great countertops or decorative/building stone, as long as it’s been “separated by humans from it’s source”;
  • Post some pretty pictures;
  • …And maybe hazard an interpretation or two.

Good thing Ian expanded this topic to include decorative and building stone, because it opened up the opportunity to show off two awesome pieces from around the Notre Dame campus. First up is the Kugel Fountain in one of our student centers, the Coleman-Morse buildingThe Marble, as some call it, “contains a 30-inch solid granite sphere which weighs 1,300 pounds and floats on 7 lbs. of water pressure” (via the ND website).

Notre Dame’s Kugel Fountain. The granite sphere constantly rotates – ‘clockwise’ during this 1-second exposure – due to minor differences in supporting water pressure.

Take a closer look…

Unlike most granites, you have to hold this one steady to get a decent photo! Large ovoid alkali feldspars (pink) enclose opaques (dark minerals, mostly hornblende), and in some cases plagioclase feldspar (gray) form rims on alkali feldspars. Click any photo to enlarge.

The Marble was first described to me as an orbicular granite, which seemed to fit the rounded appearance of alkali feldspars (pink). However, in comparing it to other orbicular granites (thanks, Google!), I noted these feldspars did not exhibit the same patterns as other orbicules, namely crystals radiating outward and/or as concentric patterns. In addition, many of the rounded feldspars appear to be single crystals instead of layers. Fortunately, Ole Nielsen was there with an Olelog post on Orbicular Granites from Kuru, Finland (EDIT: Olelog blog is now defunct due to the closing of myOpera hosting), which led to this post on Rapakivi granite in the Sandatlas. Jackpot! (and thanks, guys)! Turns out The Marble is a Rapakivi granite orb, NOT an orbicular granite orb. Misconceptions: Busted! The term rapakivi is Finnish for crumbly rock after the tendency of the granite to weather rapidly when exposed.

This slightly lighter photo shows a bit more of the darker minerals and textures. The gray plagioclase rims (there are also some patchy areas) really stand out.
Coarse-grained, generally euhedral crystals (well-formed as a result of slow cooling and unrestricted growth) fill the space between ovoids. These look mostly like more feldspars, micas (black/gray, shiny and ‘platy’) and hornblende. Quartz (white and gray) in this image are small (<~0.05cm) and difficult to pick out. The large ovoid at top right is ~3cm.

PART TWO of this post focuses on a different sort of marble – the lithology instead of the shape. Walk into the Hesburgh Library and you get smacked in the eyeballs with meters and meters of marble slabs lining the concourse, administrative areas, and support columns.

The Hesburgh Library concourse rocks. It also marbles.

Confounded and intrigued by the expanse, I asked our library Help Center for additional information. They did a wonderful job in uncovering an October 1967 article from Stone Magazine by then-Associate Professor of Geology Erhard M. Winkler, who wrote:

“Medium to dark gray polished Lido limestone-marble from Morocco dominates the interior walls for both the [library] concourse and the reading rooms of the first floor and the second floor. Imported by the Carthage Marble Corporation… The fine-grained limestone-marble is cut by numerous white calcite veins, which indicate strong shattering and subsequent repair with calcite from circulating solutions in the geologic past. A few dark gray clumps of irregular shape appear to be insoluble residues dating from a time when some solution took place within the rock, also in the geologic past. The diamond match of this distinct pattern gives the appearance of fine boxwork.”

The country of Morocco is on the NW coast of Africa and is part of the south side of the Strait of Gibraltar. Unfortunately, the Lido limestone/marble is such a popular decorative stone that I did not feel like spending the time to sift through the multitude of manufacturing and foreign websites to unearth anything useful. So instead, let’s all make our own interpretations!

A sequence of larger sub-‘horizontal’ calcite veins (white) cross-cut this section of marble, which in turn are dissected by an overall perpendicular (more vertical) sequence of fractures. Center vein is ~1.5cm wide. Remember that these slabs are not in-situ, and we do not have any indicators of original horizontality. These could have been cut at any convenient angle.
My annotated version of previous image. Obvious vein displacement highlighted in green along fault plane. Black lines follow one side of some calcite veins to demonstrate overall displacement. It is possible the vein bounded by red dashes was originally connected to the center vein. More displacement means more void space to fill (if not a cohesive mass), so could that explain the higher degree of fracturing in the yellow highlighted area?

There are obvious large-scale differences (competence? pressure? mineralogy?) in affected regions of the marble.

Marble behind these plaques appears more highly fractured, and have more calcite veins than the lower section. Note the symmetry along the center vertical axis: These are sequential sections of the marble! Diagonal calcite veins in lower left center are once again ~1.5cm wide on average.

Calcite veins are neat and all, but what else is there?  How about those “insolubles” that Dr. Winkler mentioned?

Multiple calcite vein swarms and fractures cross-cut one another. A few large (~1 cm wide max), sub-parallel and sub-horizontal calcite veins cross-cut just about everything (‘dipping’ slightly to the right). More dark minerals than normal (higher abundance of silicates?), plus some orange phase (maybe calcite again, but more Fe or Mn impurities?). Looks like there was some major fluid movement along the diagonal from bottom left to top right. Field of view is ~0.5 meters.
As Dr. Winkler so eloquently wrote, here we have some “dark gray clumps of irregular shape”, the largest of which is cut by a ~1cm vein. Or is this leftover insoluble material surrounding the vein?

Thinking about this for too long marbles my brain, so I’ll leave it to y’all to quietly ponder amongst yourselves.

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1 thought on “Accretionary Wedge #42: Countertop Geology of two marbles”

  1. Fascinating post. That rapakivi granite in particular is known as Baltic Brown. Those Finnish rapakivis seemed to become quite popular in the 1980s. The majority formed between 1 and 1.75 billion years ago with an average age of 1.54 billion years. No other type of granite magmatism fits in such a tight time frame.
    Cheers,
    David B. Williams
    Stories in Stone: Travels Through Urban Geology

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