The uncompahgrite, a coarse-grained melilite rock from Iron Hill, Colorado, offers an unusually good opportunity to follow a series of reactions and replacements of melilite and other minerals ranging from those caused by the residual liquor of the crystallizing magma before it separated from the crystal-mesh to those caused by hydrothermal solutions that gathered into fractures and were probably relatively dilute.

The first replacement formed a very small amount of light colored perofskite in narrow rims, associated with some phlogopite, about the darker colored magmatic perofskite and to a less extent about magnetite. This was followed by a larger scale replacement of melilite by a dark titaniferous garnet, that formed either irregular grains or more commonly, rims about perofskite, magnetite, and pyroxene; and to a less extent about melilite grains. In part the garnet replacement was concentrated in veinlike masses. The replacement of melilite by perofskite and probably much of that by garnet was brought about by the residual liquor of the magma before it had moved from the crystal-mesh. A part of the garnet was deposited by solutions moving in fractures.

The garnet became lighter colored and lower in titanium content toward the end of its period of deposition and it was joined by vesuvianite and this in turn by a colorless diopside. The replacement of melilite by vesuvianite and diopside took place along fractures in the rock, was unevenly distributed, and locally yielded large bodies.

The next alteration was to sheaves of a finely fibrous new mineral, called juanite. Juanite was followed by an undetermined fibrous mineral (mineral A) and this by cebollite. The cebollite, was, in turn, followed by a second undetermined mineral (mineral B).

Brugnatellite was found locally. Hastingsite replaced the pyroxene in small amount and soda tremolite and aegirite were formed in seams; they were probaby a result of the intrusion of the uncompahgrite by a pyroxenite.

In the replacement of melilite by vesuvianite and diopside, by juanite, and by cebollite, there was very little change in chemical composition.

Juanite occurs in fibrous growths. It is probably orthorhombic with Z parallel to the length. It appears to be optically positive with a moderate axial angle, α = 1.640; γ = 1.647. Fusibility about 3; Hardness 512. It has the composition 10CaO·4MgO · Al2O3 · 11 SiO2 · 4H2O.

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