Does the sample really display (A) a matrix of fine grains of an iron mineral (in which the coarser well rounded quartz grains are swimming) or did you rather observe (B) a crystalline cement with no indication of a sedimentary transport of iron mineral grains? Or both?

(A) If you think that there are fine iron mineral grains which have been transported and sedimented and if you observe that the coarser quartz grains are not contacting/supporting each other but swimming in between the iron mineral grains you may search for 'matrix-supported' fabrics and 'mass flow deposits'.

(B) Cements (e.g. of limonite, haematite) are usually the product of a precipitation process that occurred after sedimentation (e.g. during diagenesis or soil formation).

Another possibility: iron ooid formation (through a precipitation process before sedimentation). Search for 'iron ooid' or 'oolitic iron'.

Thanks Dr. Michael for your response. In my samples the second one is true. It is mainly hematite, subordinate limonite and minor amount of silica cement. It is true that cementing takes place during diagenesis and mainly open spaces are filled in. Then the mechanism that keeps the framework grains floating in cement is not clear to me. Please elaborate.

I don't think that grains 'floating' in cement can be readily explained through diagnesis. Perhaps it is an optical illusion: There is so much inter-grain space filled by cement that you percieve the quartz grains as floating but in fact they are supporting each other (grain-supported fabric with high a amount of cement).

Another idea: The quartz grains got a haematitic coating before their depostion and thus the percentage of iron oxides/hydroxides is particularly high because it comes from both, a pre-depostion coating of quartz grains and a diagenetic precipitation of cement.