Plumose calcite

An unexpected texture

I picked this rock up in the foothills of the northern Tobacco Root Mountains, Montana, in 1969. What attracted me was the unusual texture, which I’d call feathery or plumose, like white and gray to almost black feathers on relatively planar surfaces in the rock. I initially thought it was some metamorphic mineral like tremolite, which often forms such textures on surfaces like joints or bedding or foliation planes. But it’s almost all calcite.

The rock is variably recrystallized by metamorphism, so it’s fair to call this marble, although it is mostly rather fine-grained. It’s the texture that’s unusual.

My label from 1969 says “rock at abandoned house, Sacry's Ranch, MT” and I no longer know exactly where that abandoned house was (I’m pretty sure it’s gone now), but even so I’m sure this is a metamorphosed limestone, probably the Mississippian Madison Lodgepole or possibly the Mission Canyon formation. The metamorphism came from an igneous sill injected into the 330-million-year-old limestone about 75 to 80 million years ago, or possibly from hot waters migrating along nearby fault zones. I’m sure the general location is within the red circle on the map (Vuke and others, 2014, MBMG OFR 648, Bozeman quad) in the gray, which is the Madison Lodgepole and Mission Canyon formations (Mmc).

Metamorphic rocks develop planar and linear fabrics in the minerals that grow by recrystallization under conditions of increased heat as well as pressure, usually directed pressure that aligns mineral grains like mica and others in preferred orientations. The every-which-way geometry of the calcite in this rock suggests that it was NOT formed in a setting with a strong, preferred pressure orientation. That’s pretty much what you’d expect from heating by an igneous intrusion – it just heated up the rock so the calcite recrystallized into larger crystals from the original very fine (sub-microscopic) material in the limestone. The pressure, such as it was, was not strongly oriented in a particular way, so these “plumes” of calcite have all sorts of orientations.

Although most evident on flat surfaces (which might or might not represent the original bedding in the limestone), these feathery textures pervade the rock’s interior as well. That may indicate the original structure (bedding etc.) in the rock, or it may represent the way calcite grew in a porous and permeable original, forming its own pathways as it developed, expanding along paths of least resistance (cracks, cleavages) inside the solid rock.

Also present are some tiny (mostly smaller than a tenth of a millimeter) metallic silvery grains that I’m about 60% sure are graphite. They might also just be reflective biotite, but I think there are surface textures similar to some that form on graphite crystals. Graphite, pure carbon, forms in metamorphic rocks like marble (the carbon comes from the limestone, calcium carbonate) and indicates rather high temperatures and pressures. High pressure is not incompatible with the non-oriented calcite in the rock; it just needed to be pressure that did not have a preferential direction. 

This is definitely an unusual texture for calcite, and my statement above that it reflects non-preferential pressure is (I think) true, but probably doesn’t tell the whole story. There were probably tiny variations in the metamorphic environment within the cracks in the original rock, such as geometry of the crack, temperature and trace elements in the hot fluids, and preferred nucleation points, that resulted in the fibrous or plumose habit for the calcite.

Cat. No. 977.