I couldn't resist posting about this lovely micrograph, even though I blogged about this mineral quite recently. This is the same floor sample from medieval Riga, it's absolutely chock full of these iron phosphate crystals (vivianite) in various forms. The last image I posted showed the typical blue amorphous mass, here is another view, this time of a collection of smaller 'rosette' crystals, showing the monoclinic form. You can see that there is a mix of blue and yellowish/grey crystals in these rosettes. Vivianite turns blue on exposure to air, suggesting that this part of the deposit remained waterlogged whilst other parts were partially exposed. Curious that these little crystals appear to be clustering around that white area - a 'crack' in the floor.