Burbankite is a moderately rare mineral at MSH, found only in the miarolitic cavities. Conversely, none of the other “burbankite family” minerals are found in this environment. So this is definitely burbankite.
While there are no essential REEs in burbankite, at MSH the mineral usually has a fair amount of Ce. Consequently it undergoes a noticeable color change depending on the type of lighting. These photos were taken using tungsten lighting. Under CFL, the crystal turns quite bright green. The crystal is fairly gemmy.
The typical size of burbankite crystals at MSH is about 1-3 mm. The visible part of this crystal is about 4.5 mm, so it is relatively large. (It is naked eye visible, but you will need 15X to see it as in the close-up photos.)
The crystal appears to have a pinacoidal termination. As can be seen in the second and third photos, the dark brown chlorite group (?) mineral in the background also partly covers the termination. (Cf. figure 27 in the 1990 Min Rec special issue on MSH for another example of the same sort of termination)
The fourth photo (FOV 6.8 x 4.5 mm) shows some of the natrolite crystals on the specimen. Most of these are still intact, but on the specimen as a whole, most of the natrolite crystals are broken. The interior of these crystals is still glassy (as can be seen from some of the broken crystals show in the center of the left hand cluster), but the exteriors have turned white. Probably they are now gonnardite. This is the common fate of much of the natrolite at MSH, especially specimens from the miarolitic cavities. These begin to turn white immediately upon exposure to the atmosphere and become very brittle. Many crystals exfoliate. Because of the extreme fragility of these crystals, I can not guarantee that they will arrive as shown in the photo. However, even broken, as most of the crystals already are, hand held they provide a very attractive contrast to the greenish yellow calcite and dark brown chlorite (?) group mineral in the cavity.
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