I don't think it is Sabatia angularis. That do not means I can't mistake.
What make me to say that the flower, presented by Jay, it is Sabatia arenicola was to observe the calyx.
Calyx lobe is short if we compare to the calyx tube and have more a triangular shape, that, how I read, it is caracteristic for the S.arenicola. (But I think in different conditions can be more or less obvious triangular shape). And the sharp raised ridges on calyx tube.
S.angularis have calyx with sharply ridges while S.angularis lacks ridges or have low ridges.
A page with good pictures for S.angularis
http://www.missouriplants.com/...
I think Sabatia arenicola is more close to Sabatia campestris. They have the calyx tube very similar, but the calyx lobes is different special if we compare the length of lobe relative to calyx tube length.
All this three species can have white flower form. But it is a rare form.
Bibliography
[1] A field guide to Southwestern and Texas wildflowers - by Theodore F.Niehaus, Charles L.Ripper and Virginia Savage; - The Peterson Field Guide Series 1984.
[2] Plants of The Texas Coastal Bend - by Roy L.Lehman, Ruth O'Brien and Tammy White; - Texas A&M University Press College Station, second printing 2009
On WEB pages
[a] Here
http://www.biodiversitylibrary...
on page 1315-1317 description of all these three species.
[b] Here
http://www.biodiversitylibrary...
interesting key to delimited some species of Sabatia (zoom for can well read or if somebody is more interested it can download like PDF )
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Note
About image of S.arenicola presented here
http://www.wildflower.org/gall...
I don't know what to say about this picture. It is difficult to see the calyx. And the calyx lobes don't look for me like of S.arenicola.
But below on page it is mentioned that the image has not been verified.
And of course the plants depend very much of growing condition (A cultivated specie can look very different compare with really wild sisters)
If we look with attention to the picture on right the plant have a flower with 7 petals and in [1] (book mentioned above) it is write "always 5 petals" so it is possible the plant from wildflower.org it is not S.arenicola.
