Authigenesis of titanium minerals in two Proterozoic sedimentary rocks from southern and central Sweden.

Two Proterozoic sequences, the Visingo group and the Dala sandstone, were studied by petrography and microprobe. Leucoxene, sphene, rutile, anatase and brookite occur in these rocks as direct alteration products of detrital biotite and ilmenite, as pore fillings and as partial replacements of minerals such as quartz and feldspar. The poorly crystallized mixture known as leucoxene is abundant; euhedral authigenic Ti minerals are also common, especially in the Dala sandstones, which have undergone very low-grade metamorphism. It is noted that the diagenesis of the biotite and ilmenite would be expected to change the magnetic properties of these rocks. -H.R.B.