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Most petrified wood collectors are familiar with the so-called “halite wood” from Sweet Home Oregon with quartz or chalcedony pseudomorphs of halite crystals that originally crystallized in the wood which was submerged for a long time in very saline waters of the Little Butte Formation sediments. Few however are as familiar with the same process resulting in quartz pseudomorphs after the mineral anhydrite. There is a very good reason for this unfamiliarity – this material is considerably more rare than halite wood. Why? They both began the same way, immersed in saline water as the wood began to deteriorate. When petrification began, the evaporite mineral was dissolved away and the resulting cubic cavities began to fill with quartz or chalcedony. However, the chemistry of anhydrite (anhydrous calcium sulfate) is much different than that of halite (sodium chloride). Under normal circumstances, the evaporite mineral will be combined with two molecules of water and precipitate as hydrous calcium sulfate – the mineral gypsum. But when the water temperature is above 40C (104F), and there is plenty of sodium or potassium chloride in the water, the calcium sulfate will precipitate as the anhydrous form – Anhydrite. Just think for a moment how still and hot it must have been for the water temperature to be above 40C. Not a typical condition, and when it would happen, it would likely have been a temporary condition. So that is the primary reason that the anhydrite pseudomorphs are so uncommon. Look at the photomicrographs. Anhydrite crystallizes in the orthorhombic system which is quite evident from the long laths you see in the photomicrographs. If these were the result of halite which crystallizes in the cubic system, the pseudomorphs would be square, not rectangular. The pseudomorphs generally measure in the range of 5 to 7 mm in length and 2 to 4 mm in width. Several of the pseudomorphs also show the termination face of the crystal! Additionally, the wood retains the pattern of the growth rings (however there is no remaining woody fine structure so a taxonomic designation is not possible). description from Jim Mills (California)

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