Thursday, September 1, 2016

Photomicrograph: Botryoidal Silica And Dolomite Cement In Proterozoic Sandstone

This week, a gorgeous example of botryoidal and banded silica cement filling pore spaces in Proterozoic sandstones from Central India.

The sandstone has a complex history of cementation. Pore spaces are filled with dolomite or siderite, chalcedony and calcite.

Isolated dolomite rhombs (image above) were the first mineral to precipitate around quartz grains, growing inwards into pore spaces. Another strong possibility is that the rhombs are the mineral siderite which is the iron carbonate FeCO3. Siderite often alters into a mixture of hydrated iron oxides known as limonite which preserves the shape of the original siderite forming pseudomorphs.

Silica precipitation was either contemporaneous or succeeding the dolomite/siderite cements. Occasionally, silica cements cross cut the iron carbonate (image above, white arrow), indicating that at least some silica was introduced after the dolomite/siderite.

Finally, calcite cement filled the remaining open spaces.

It replaces the dolomite/siderite cement (top image, white arrows) but retains the iron oxide bands thus preserving the original shape of the dolomite/siderite crystals.

Calcite also cuts across (bottom image, white arrows) the silica geodes.



  1. Ooo! Pretty! I just found your blog. It's great!

    1. thanks David- I just checked out your blog.. very interesting! will follow :)