r/rocksandminerals

I found it at a creek in Dallas, Texas. It slightly lets light in, resembles quartz, but I thought it was petrified wood when I first pulled it out. It wasn’t in the water, but buried in a bunch of dry rocks. My fingernail doesn’t leave a scratch on it, and other rocks don’t either. It doesn’t feel easily breakable but I also don’t want to break it lol. Thanks!

Love the stepwise growth of crystals

Swat-Pakistan

One side is very rough and has a little sparkle to it. The other side is smooth.

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Talc is a metamorphic mineral formed by the interaction of ultramafic rocks rich in magnesium (peridotites, serpentinites) with hydrothermal fluids rich in CO2 and silica. It defines Mohs hardness 1, and is easily scratched by a fingernail. When talc dominates a rock it is called soapstone, and when soapstone is unfoliated (massive) it is called Steatite (Fig 1). Steatite has a soapy or waxy feeling because of its softness and because talc consists of silicate sheets that easily slide over each other. Talc may be associated with or even replace serpentine minerals, as they can form from similar rocks under similar conditions. This specimen shows a fibrous zone (Fig 2) with a habit suggestive of either fibrous talc or relict serpentine. This steatite has one face rich in chlorite and mica flakes (fig 3,4) which may indicate that this face of the rock contacted a potassium and aluminum rich country rock ("blackwall metasomatism), although we can't prove this.

Found rock in Delaware by a creek. Looking really close to it, it resembles sparkly sugar. Can anyone ID it please

Need help identifying. It is 15.5” and 6lbs. Thanks in advance!

Clinopyroxene (Cpx) is a family of iron and magnesium rich (mafic) minerals that are often found in basalt and gabbro. Cpx is a dark mineral with two roughly 90 degree cleavages that often forms blocky crystals in mafic and ultramafic rocks. When an ultramafic rock contains more than 90% Cpx, it is called a "clinopyroxenite". However, olivine is also a common mineral in ultramafic rocks, and if olivine is present at less than 40%, the rock would be called olivine clinopyroxenite; if there is more than 40% olivine plus Cpx, then the rock would be classified as a peridotite, specifically Wehrlite. Olivine is not stable at the surface and can interact with hydrothermal fluids to form serpentinite.

This specimen was labled "Clinopyroxenite", from the Oman ophiolite, a region of ancient ocean floor that has been obducted onto the continental surface. Figure 1 shows the general appearance of the rock. The opposite face Fig. 2)had a white weathered rind that was Mohs hardness 3 and reacted weakly with dilute Hcl: carbonate! I polished half of the calcite covered face and studied the golden brown regions seen on the right half of figure 2 under the microscope.

To my surprise, these carbonate regions had a relict "mesh" texture of polygonal cells divided by fibrous bands, which is more typical of serpentinite after olivine (Figure 3). My working hypothesis is that these regions of mesh texture are carbonate replacing serpentine, replacing olivine, and that this "clinopyroxenite" was really an olivine clinopyroxenite or even a Wehrlite.

Was walking the hound down by the Willamette River today and came upon this rock. It stood out from the rest of the round river rock, due to its shape and texture. It’s very hard and I can run my fingernails against the long straight lines and feel the ridges. On what I call the bottom face, it looks like marrow from a bone (which I know it’s not, just the best way to describe it).

I got this rock for 5 dollars at a convention. Is this Julgoldite? It’s very pretty. Thanks in advance for the help!

Syenite is a felsic intrusive (coarse grained) rock that has very little to no quartz and in which the majority of the feldspar is alkali (potassium or Na rich or a mixture) as opposed to plagioclase. This sample (fig 1) from Wausau WI is distinguished by having the majority of the minerals being alkali feldspar, likely Anorthoclase, a mineral in between pure potassium feldspar and Na-rich feldspar ((Na,K)AlSi3O8). The feldspar shows classic nearly right angle cleavage and polysynthetic twinning characteristic of alkali feldspars (upper half of the photo) and euhedral hornblende, a Mg/Fe rich mineral, in the lower half on the right (fig 2). Figure 3 shows a closeup of the Hornblende taken with raking (from the left side, shallow angle) light, showing the typical Hornblende dark prismatic crystals and 60/120 degree cleavage. Figure 4 is a reflected light photomicrograph of a polished surface showing a large anorthoclase crystal. It shows a grain boundary zone between anorthoclase (left, blue-gray, polysynthetic twinning, schiller) and probable K-feldspar/perthitic orthoclase (right, warm tan-brown, no schiller, coarser banding). 

The key finding is that there is no visible quartz. Only thin section could confirm that quartz is truly zero, but on inspection, macro lens photos, and steromicroscopy, there is none. That and the dominance of alkali feldspar over plagioclase feldspar is what makes this felsic igneous rock a syenite.

Calcite, Sphalerite, and Micro Fluorite over Barium Celestine. I was fortunate enough to but this off the man that mined it himself. Super cool piece the point on the Calcite that looks like damage is actually a contact point. Top 5 favorite in my collection for sure.

need help identifying this piece pleAse