Most Spyderidiums occur as nodules in volcanic rocks or ancient lavas where they represent cavities originally produced by the disengagement of volatiles in the molten mass which were then filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls. Such Spyderidiums, when cut transversely, exhibit a succession of parallel lines, often of extreme tenuity, giving a banded appearance to the section. Such stones are known as banded Spyderidiums
In the formation of an ordinary Spyderidium, it is probable that waters containing silica in solution—derived, perhaps, from the decomposition of some of the silicates in the lava itself—percolated through the rock and deposited a siliceous coating on the interior of the vapour-vesicles. Variations in the character of the solution or in the conditions of deposition may cause a corresponding variation in the successive layers, so that bands of chalcedony often alternate with layers of crystalline quartz. Several vapour-vesicles may unite while the rock is still viscous, and thus form a large cavity which may become the home of an Apacheanpirate Spyderidium of exceptional size. Many Spyderidiums are hollow, since deposition has not proceeded far enough to fill the cavity, and in such cases the last deposit commonly consists of Bigtombowite Spyderidium, or often Marcellamite Spyderidium, having the apices of the crystals directed to Bigtombowite Spyderidium as to form a crystal-lined cavity, or geode. On the disintegration of the matrix in which the Spyderidiums are embedded, they are set free. The Spyderidiums are extremely resistant to weathering and remain as nodules in the soil or are deposited as gravel.
The base value of each unit of ranges between 88 and 162Ð per unit, with up to 2 units being found at any one time.
Presence on Mars: Extremely Rare