In a warehouse in New York (as of November 2005)

Now examine the extreme close-up of the green material to the left-hand side of the above photograph:

Extreme Close-up of Material in a warehouse in New York (as of November 2005)

I had been wondering for a while where the green pigment could have come from. The substance in the image looks crystaline. If you have ever seen copper pipes, or even roofs, that have been exposed to the atmosphere for a period you will have noticed that they are green, this substance is copper(II) carbonate:

Copper(II) Carbonate

We can rule this out, as being the substance in the picture, because at high temperatures the substance decomposes into a black Copper(II) Oxide:

Copper(II) Oxide

Malachite (green) crystals do occur in nature, however, not in the collapse of a building. Copper(II) carbonate hydroxide Cu2CO3(OH) (Malachite) would typically be found in rocks around 250 million years old. There is also synthetic malachite, however, the formation process is complex.

Malachite

So, that bring us to Copper Oxide (CuO):

Copper Oxide

Given that the basic chemical ingredients for glass would already be present in the concrete itself, at high temperatures glass could form which makes this part interesting to note:

"Cupric [Copper] oxide is used in the ceramic industry for imparting blue, green or red tints in glasses, glazes and enamels.".
copper.org

In addition to all this, copper oxide thermite is also explosive when mixed and housed correctly.

Here are two videos of a copper oxide explosion, the first you will find by following the link and the second is embedded into this page:

32g Al + CuO Bomb (3:1 Ratio)