On our wish list for a sample.
The following is excerpted from: [Source: http://academic.brooklyn.cuny.edu/geology ]
The red color is due to iron oxides. Freshly quarried sandstone tends to be yellow-brown due to the presence of limonte. Dehydration of the quarried rock due to extended exposure to the air, results in conversion of limonite (FeO[OH]) to hematite (Fe2O3). The distinctive red-brown color of brownstone is due to the presence of hematite.The Portland Brownstone is a coarse sandstone (feldspathic arenite), with feldspar content as high as 65%. Quartz and mica comprise most of the remainder of the rock. These detrital components were eroded from Ordovician gneisses and schists of Connecticut's adjacent Eastern Highlands. Albite cements the sand grains.
Brownstone was commonly face-bedded—that is, the rock was cut parallel to the bedding, and slabs were applied so that the bedding is vertical, and parallel to the wall. This means that the same bed is exposed across the whole block, thereby ensuring that the color and texture was uniform. Unfortunately, as soon as a decade or two after construction, many examples of face-bedded brownstone began flaking off in sheets. This common form of is due to the physical weathering process of spalling due to frost action. Water concentrates along bedding planes. When it freezes, water expands by approximately 10%. This expansion exerts a pressure of 2,000 psi, which pries off thin layers of rock.