Monday Mineral: Natrolite

Starting with a crop from the first picture in last week's Monday Mineral, you can see the acicular (needle-like) crystals of natrolite in amongst the blocky crystals of apophyllite. Natrolite is a member of the zeolite group of minerals. They are hydrated alumino-silicates, meaning that there is water bound onto an aluminum and silicon oxide framework, along with various metallic ions; in the case of natrolite, the major metal is sodium.The name "zeolite" comes from the Greek "zein," meaning "to boil," and "lithos," meaning "stone." As a group, heating them causes them to swell and give off water vapor. Natrolite doesn't show this as well as some others, but it does show it. There are two reasons to start with this particular zeolite: first, I've got a couple of decent samples. : ) Second, it's the most common zeolite around here. As a budding geo undergrad, it was the first mineral I could dependably identify. Like apophyllite, it is tetragonal, It is orthorhombic (I really should look this stuff up rather than counting on my memory) but only a few samples I've ever found allowed me to actually observe that symmetry. The crystals are normally in dense, radiating clusters. They seem to start at a point of nucleation, then grow outward en masse. So the color (white to light pastels) and the radiating bundles, along with occurence (mafic to intermediate igneous rocks, as a secondary mineral in fractures and voids, especially in pillow basalts), are the features I find particularly useful for ID. The above picture shows the surface of a goup of clusters; below is an oblique view to show the side and top together. And below is a side view of the clusters. This is the bottom of the same sample; this would have been the surface from which the whole mass grew.
Since it's a very showy mineral, and contrasts with its darker host material, people out on hikes tend to pick it up and carry it home. Eventually it makes its way to the garden, as with this piece I pried out of a neighbor's yard this morning. I put it back before I ID'd the glassy mineral on top of this sample; trying to tell from the picture, I suspect it's either calcite (but the cleavage doesn't look right) or analcime (another fairly common zeolite). The sample pictured in the middle four images is from a quarry on McCulloch Peak in McDonald State Forest. The forest is a Oregon State Research facility, so you can't drive here without permission and a key to the gates. But you can hike, bike, or ride a horse; the public is allowed on the forest lands, they just can't drive onto them. And the built-up area to the lower right (SE) is my lovely home town- or a small corner of it. Pin coordinates are 44.634523° -123.346144°