FMF - Friends of Minerals Forum, discussion and message board

[Reef]

I would call this dendritic growth. But it is believed that quartz can not have it.
The sizes of the samples are mainly up to ~ 15 mm.
Locality: Alatau, Bashkortostan Republic, Southern Urals, Urals Region, Russia

[Niels Brouwer]

It doesn't really look like dendritic growth to me, as far as I can tell from these images. It reminds me more of the cracks you see for example in mud. I suspect a more likely explanation for the patterns on those crystals would be some sort of coating that covered the face of the quartz during its development. It looks like the coating developed cracks through which the quartz could continue to grow, whereas the remaining areas of coating inhibited the growth, resulting in the lower areas on the faces.

[Bob Harman]

I believe you can see this type of very superficial surface change in many types of crystals, whether they be resistant and stout, like quartz, or less resistant and fragile like calcite and fluorite. I believe this is a type of slow superficial surface etching with resorption. Remember, these crystals have existed for many millennia and been subjected to ambient fluids for the same time frame. Very slow surface etching and resorption might preferentially occur on some of the crystal surfaces. Much more rapid changes occur with other minerals such as calcite or fluorite leading to corrosion with some interesting etched forms. BOB

[Pierre Joubert]

I believe these crystals formed in association with another mineral species that 'weathered away, leaving these beautiful markings. What the other mineral was can possibly be established by studying the direct environment.

[Pierre Joubert]

I would like to add that the edges of these patterns will most likely be sharp and not rounded. If one made a cast of this pattern, it will help a lot to understand the other 'weathered' mineral/s.

[John Betts]

I think the quartz is twinned following the Dauphiné law and that is preferential etching of one of the quartz "hands" (i.e. left-hand quartz or right-hand quartz was etched, the other not etched.)

[Pierre Joubert]

[Reef]

I do not know how this is formed, but interesting and beautiful. In my opinion there is no etching here. Whims of skeletal growth. Host rocks: calcareous sandstone, marl, calcite, dolomite. The veins themselves: calcite and a little quartz.

[Reef]

A request, who has similar samples with a "dendritic" pattern, publish a photo.
A few more photos.

[Pierre Joubert]

Most unusual and quite unique!

[Bob Morgan]

The earlier pictures certainly look to me like etching, but restricted to areas where the crystal may have been in contact with something else in which there are etch channels forming the dendritic pattern. Carrollite has etch patterns from such contact with dolomite although not dendritic.