Cretaceous Coral Fossil?

[hcpiv13]

I was recently sent pictures of something that I am having a hard time identifying online.  It was found in a creek bed of Cretaceous age, shallow marine limestone and marl on the west side of San Antonio, TX. It has a section that looks like it was attached to something.  It does react to HCL and my initial thought was some sort of coral, but it has a very strange texture. I have seen a number of different types of coral, but never something like this and can't find anything similar. Any ideas? The closeup images of the texture were taken with a USB microscope.

[JohnJ]

I looks like tufa to me.

[abyssunder]

I think, JohnJ is right. 

[jpc]

I think tufa as well. 

[oyo]

I don't know what it`s but I'm sure it's not a coral.
 
 

[hcpiv13]

Thanks for the tips!

[BobWill]

I have trouble telling tufa from boxwork, mostly because I have no experience with either one outside of photographs and one specimen I found in North Sulphur River that was identified as boxwork. Can someone explain some good way to tell them apart?

[FranzBernhard]

3 hours ago, BobWill said:

I have trouble telling tufa from boxwork

These are the questions we need! Many might take the difference for granted, but its not!

Most often, it is clear from the context, what is what. But what is, if you have only an isolated specimen or even only a pic?

I don´t have a good way to tell context-less, isolated specimens apart, but lets try it anyways:

- Boxwork has some 2D-aspect (lamellae). Tufa often also some 1D (former pine needles etc.).

- Boxwork lamellae are made of quartz, sometimes dolomite or even rarer minerals. Tufa is made of calcium carbonate.

- The lamellae of boxwork are usually compact. Tufa is usually porous.

Franz Bernhard